Fix: notification: deadlock on cmd_queue.lock and client->lock

[lttng-tools.git] / src / bin / lttng-sessiond / notification-thread-events.c

/*
 * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 */

#define _LGPL_SOURCE
#include <urcu.h>
#include <urcu/rculfhash.h>

#include <common/defaults.h>
#include <common/error.h>
#include <common/futex.h>
#include <common/unix.h>
#include <common/dynamic-buffer.h>
#include <common/hashtable/utils.h>
#include <common/sessiond-comm/sessiond-comm.h>
#include <common/macros.h>
#include <lttng/condition/condition.h>
#include <lttng/action/action-internal.h>
#include <lttng/notification/notification-internal.h>
#include <lttng/condition/condition-internal.h>
#include <lttng/condition/buffer-usage-internal.h>
#include <lttng/condition/session-consumed-size-internal.h>
#include <lttng/condition/session-rotation-internal.h>
#include <lttng/notification/channel-internal.h>

#include <time.h>
#include <unistd.h>
#include <assert.h>
#include <inttypes.h>
#include <fcntl.h>

#include "notification-thread.h"
#include "notification-thread-events.h"
#include "notification-thread-commands.h"
#include "lttng-sessiond.h"
#include "kernel.h"

#define CLIENT_POLL_MASK_IN (LPOLLIN | LPOLLERR | LPOLLHUP | LPOLLRDHUP)
#define CLIENT_POLL_MASK_IN_OUT (CLIENT_POLL_MASK_IN | LPOLLOUT)

enum lttng_object_type {
        LTTNG_OBJECT_TYPE_UNKNOWN,
        LTTNG_OBJECT_TYPE_NONE,
        LTTNG_OBJECT_TYPE_CHANNEL,
        LTTNG_OBJECT_TYPE_SESSION,
};

struct lttng_trigger_list_element {
        /* No ownership of the trigger object is assumed. */
        const struct lttng_trigger *trigger;
        struct cds_list_head node;
};

struct lttng_channel_trigger_list {
        struct channel_key channel_key;
        /* List of struct lttng_trigger_list_element. */
        struct cds_list_head list;
        /* Node in the channel_triggers_ht */
        struct cds_lfht_node channel_triggers_ht_node;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

/*
 * List of triggers applying to a given session.
 *
 * See:
 *   - lttng_session_trigger_list_create()
 *   - lttng_session_trigger_list_build()
 *   - lttng_session_trigger_list_destroy()
 *   - lttng_session_trigger_list_add()
 */
struct lttng_session_trigger_list {
        /*
         * Not owned by this; points to the session_info structure's
         * session name.
         */
        const char *session_name;
        /* List of struct lttng_trigger_list_element. */
        struct cds_list_head list;
        /* Node in the session_triggers_ht */
        struct cds_lfht_node session_triggers_ht_node;
        /*
         * Weak reference to the notification system's session triggers
         * hashtable.
         *
         * The session trigger list structure structure is owned by
         * the session's session_info.
         *
         * The session_info is kept alive the the channel_infos holding a
         * reference to it (reference counting). When those channels are
         * destroyed (at runtime or on teardown), the reference they hold
         * to the session_info are released. On destruction of session_info,
         * session_info_destroy() will remove the list of triggers applying
         * to this session from the notification system's state.
         *
         * This implies that the session_triggers_ht must be destroyed
         * after the channels.
         */
        struct cds_lfht *session_triggers_ht;
        /* Used for delayed RCU reclaim. */
        struct rcu_head rcu_node;
};

struct lttng_trigger_ht_element {
        struct lttng_trigger *trigger;
        struct cds_lfht_node node;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

struct lttng_condition_list_element {
        struct lttng_condition *condition;
        struct cds_list_head node;
};

struct notification_client_list_element {
        struct notification_client *client;
        struct cds_list_head node;
};

/*
 * Thread safety of notification_client and notification_client_list.
 *
 * The notification thread (main thread) and the action executor
 * interact through client lists. Hence, when the action executor
 * thread looks-up the list of clients subscribed to a given
 * condition, it will acquire a reference to the list and lock it
 * while attempting to communicate with the various clients.
 *
 * It is not necessary to reference-count clients as they are guaranteed
 * to be 'alive' if they are present in a list and that list is locked. Indeed,
 * removing references to the client from those subscription lists is part of
 * the work performed on destruction of a client.
 *
 * No provision for other access scenarios are taken into account;
 * this is the bare minimum to make these accesses safe and the
 * notification thread's state is _not_ "thread-safe" in any general
 * sense.
 */
struct notification_client_list {
        pthread_mutex_t lock;
        struct urcu_ref ref;
        const struct lttng_trigger *trigger;
        struct cds_list_head list;
        /* Weak reference to container. */
        struct cds_lfht *notification_trigger_clients_ht;
        struct cds_lfht_node notification_trigger_clients_ht_node;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

struct notification_client {
        /* Nests within the notification_client_list lock. */
        pthread_mutex_t lock;
        notification_client_id id;
        int socket;
        /* Client protocol version. */
        uint8_t major, minor;
        uid_t uid;
        gid_t gid;
        /*
         * Indicates if the credentials and versions of the client have been
         * checked.
         */
        bool validated;
        /*
         * Conditions to which the client's notification channel is subscribed.
         * List of struct lttng_condition_list_node. The condition member is
         * owned by the client.
         */
        struct cds_list_head condition_list;
        struct cds_lfht_node client_socket_ht_node;
        struct cds_lfht_node client_id_ht_node;
        struct {
                /*
                 * If a client's communication is inactive, it means that a
                 * fatal error has occurred (could be either a protocol error or
                 * the socket API returned a fatal error). No further
                 * communication should be attempted; the client is queued for
                 * clean-up.
                 */
                bool active;
                struct {
                        /*
                         * During the reception of a message, the reception
                         * buffers' "size" is set to contain the current
                         * message's complete payload.
                         */
                        struct lttng_dynamic_buffer buffer;
                        /* Bytes left to receive for the current message. */
                        size_t bytes_to_receive;
                        /* Type of the message being received. */
                        enum lttng_notification_channel_message_type msg_type;
                        /*
                         * Indicates whether or not credentials are expected
                         * from the client.
                         */
                        bool expect_creds;
                        /*
                         * Indicates whether or not credentials were received
                         * from the client.
                         */
                        bool creds_received;
                        /* Only used during credentials reception. */
                        lttng_sock_cred creds;
                } inbound;
                struct {
                        /*
                         * Indicates whether or not a notification addressed to
                         * this client was dropped because a command reply was
                         * already buffered.
                         *
                         * A notification is dropped whenever the buffer is not
                         * empty.
                         */
                        bool dropped_notification;
                        /*
                         * Indicates whether or not a command reply is already
                         * buffered. In this case, it means that the client is
                         * not consuming command replies before emitting a new
                         * one. This could be caused by a protocol error or a
                         * misbehaving/malicious client.
                         */
                        bool queued_command_reply;
                        struct lttng_dynamic_buffer buffer;
                } outbound;
        } communication;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

struct channel_state_sample {
        struct channel_key key;
        struct cds_lfht_node channel_state_ht_node;
        uint64_t highest_usage;
        uint64_t lowest_usage;
        uint64_t channel_total_consumed;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

static unsigned long hash_channel_key(struct channel_key *key);
static int evaluate_buffer_condition(const struct lttng_condition *condition,
                struct lttng_evaluation **evaluation,
                const struct notification_thread_state *state,
                const struct channel_state_sample *previous_sample,
                const struct channel_state_sample *latest_sample,
                uint64_t previous_session_consumed_total,
                uint64_t latest_session_consumed_total,
                struct channel_info *channel_info);
static
int send_evaluation_to_clients(const struct lttng_trigger *trigger,
                const struct lttng_evaluation *evaluation,
                struct notification_client_list *client_list,
                struct notification_thread_state *state,
                uid_t channel_uid, gid_t channel_gid);


/* session_info API */
static
void session_info_destroy(void *_data);
static
void session_info_get(struct session_info *session_info);
static
void session_info_put(struct session_info *session_info);
static
struct session_info *session_info_create(const char *name,
                uid_t uid, gid_t gid,
                struct lttng_session_trigger_list *trigger_list,
                struct cds_lfht *sessions_ht);
static
void session_info_add_channel(struct session_info *session_info,
                struct channel_info *channel_info);
static
void session_info_remove_channel(struct session_info *session_info,
                struct channel_info *channel_info);

/* lttng_session_trigger_list API */
static
struct lttng_session_trigger_list *lttng_session_trigger_list_create(
                const char *session_name,
                struct cds_lfht *session_triggers_ht);
static
struct lttng_session_trigger_list *lttng_session_trigger_list_build(
                const struct notification_thread_state *state,
                const char *session_name);
static
void lttng_session_trigger_list_destroy(
                struct lttng_session_trigger_list *list);
static
int lttng_session_trigger_list_add(struct lttng_session_trigger_list *list,
                const struct lttng_trigger *trigger);


static
int match_client_socket(struct cds_lfht_node *node, const void *key)
{
        /* This double-cast is intended to supress pointer-to-cast warning. */
        const int socket = (int) (intptr_t) key;
        const struct notification_client *client = caa_container_of(node,
                        struct notification_client, client_socket_ht_node);

        return client->socket == socket;
}

static
int match_client_id(struct cds_lfht_node *node, const void *key)
{
        /* This double-cast is intended to supress pointer-to-cast warning. */
        const notification_client_id id = *((notification_client_id *) key);
        const struct notification_client *client = caa_container_of(
                        node, struct notification_client, client_id_ht_node);

        return client->id == id;
}

static
int match_channel_trigger_list(struct cds_lfht_node *node, const void *key)
{
        struct channel_key *channel_key = (struct channel_key *) key;
        struct lttng_channel_trigger_list *trigger_list;

        trigger_list = caa_container_of(node, struct lttng_channel_trigger_list,
                        channel_triggers_ht_node);

        return !!((channel_key->key == trigger_list->channel_key.key) &&
                        (channel_key->domain == trigger_list->channel_key.domain));
}

static
int match_session_trigger_list(struct cds_lfht_node *node, const void *key)
{
        const char *session_name = (const char *) key;
        struct lttng_session_trigger_list *trigger_list;

        trigger_list = caa_container_of(node, struct lttng_session_trigger_list,
                        session_triggers_ht_node);

        return !!(strcmp(trigger_list->session_name, session_name) == 0);
}

static
int match_channel_state_sample(struct cds_lfht_node *node, const void *key)
{
        struct channel_key *channel_key = (struct channel_key *) key;
        struct channel_state_sample *sample;

        sample = caa_container_of(node, struct channel_state_sample,
                        channel_state_ht_node);

        return !!((channel_key->key == sample->key.key) &&
                        (channel_key->domain == sample->key.domain));
}

static
int match_channel_info(struct cds_lfht_node *node, const void *key)
{
        struct channel_key *channel_key = (struct channel_key *) key;
        struct channel_info *channel_info;

        channel_info = caa_container_of(node, struct channel_info,
                        channels_ht_node);

        return !!((channel_key->key == channel_info->key.key) &&
                        (channel_key->domain == channel_info->key.domain));
}

static
int match_condition(struct cds_lfht_node *node, const void *key)
{
        struct lttng_condition *condition_key = (struct lttng_condition *) key;
        struct lttng_trigger_ht_element *trigger;
        struct lttng_condition *condition;

        trigger = caa_container_of(node, struct lttng_trigger_ht_element,
                        node);
        condition = lttng_trigger_get_condition(trigger->trigger);
        assert(condition);

        return !!lttng_condition_is_equal(condition_key, condition);
}

static
int match_client_list_condition(struct cds_lfht_node *node, const void *key)
{
        struct lttng_condition *condition_key = (struct lttng_condition *) key;
        struct notification_client_list *client_list;
        const struct lttng_condition *condition;

        assert(condition_key);

        client_list = caa_container_of(node, struct notification_client_list,
                        notification_trigger_clients_ht_node);
        condition = lttng_trigger_get_const_condition(client_list->trigger);

        return !!lttng_condition_is_equal(condition_key, condition);
}

static
int match_session(struct cds_lfht_node *node, const void *key)
{
        const char *name = key;
        struct session_info *session_info = caa_container_of(
                node, struct session_info, sessions_ht_node);

        return !strcmp(session_info->name, name);
}

static
unsigned long lttng_condition_buffer_usage_hash(
        const struct lttng_condition *_condition)
{
        unsigned long hash;
        unsigned long condition_type;
        struct lttng_condition_buffer_usage *condition;

        condition = container_of(_condition,
                        struct lttng_condition_buffer_usage, parent);

        condition_type = (unsigned long) condition->parent.type;
        hash = hash_key_ulong((void *) condition_type, lttng_ht_seed);
        if (condition->session_name) {
                hash ^= hash_key_str(condition->session_name, lttng_ht_seed);
        }
        if (condition->channel_name) {
                hash ^= hash_key_str(condition->channel_name, lttng_ht_seed);
        }
        if (condition->domain.set) {
                hash ^= hash_key_ulong(
                                (void *) condition->domain.type,
                                lttng_ht_seed);
        }
        if (condition->threshold_ratio.set) {
                uint64_t val;

                val = condition->threshold_ratio.value * (double) UINT32_MAX;
                hash ^= hash_key_u64(&val, lttng_ht_seed);
        } else if (condition->threshold_bytes.set) {
                uint64_t val;

                val = condition->threshold_bytes.value;
                hash ^= hash_key_u64(&val, lttng_ht_seed);
        }
        return hash;
}

static
unsigned long lttng_condition_session_consumed_size_hash(
        const struct lttng_condition *_condition)
{
        unsigned long hash;
        unsigned long condition_type =
                        (unsigned long) LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE;
        struct lttng_condition_session_consumed_size *condition;
        uint64_t val;

        condition = container_of(_condition,
                        struct lttng_condition_session_consumed_size, parent);

        hash = hash_key_ulong((void *) condition_type, lttng_ht_seed);
        if (condition->session_name) {
                hash ^= hash_key_str(condition->session_name, lttng_ht_seed);
        }
        val = condition->consumed_threshold_bytes.value;
        hash ^= hash_key_u64(&val, lttng_ht_seed);
        return hash;
}

static
unsigned long lttng_condition_session_rotation_hash(
        const struct lttng_condition *_condition)
{
        unsigned long hash, condition_type;
        struct lttng_condition_session_rotation *condition;

        condition = container_of(_condition,
                        struct lttng_condition_session_rotation, parent);
        condition_type = (unsigned long) condition->parent.type;
        hash = hash_key_ulong((void *) condition_type, lttng_ht_seed);
        assert(condition->session_name);
        hash ^= hash_key_str(condition->session_name, lttng_ht_seed);
        return hash;
}

/*
 * The lttng_condition hashing code is kept in this file (rather than
 * condition.c) since it makes use of GPLv2 code (hashtable utils), which we
 * don't want to link in liblttng-ctl.
 */
static
unsigned long lttng_condition_hash(const struct lttng_condition *condition)
{
        switch (condition->type) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                return lttng_condition_buffer_usage_hash(condition);
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                return lttng_condition_session_consumed_size_hash(condition);
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
                return lttng_condition_session_rotation_hash(condition);
        default:
                ERR("[notification-thread] Unexpected condition type caught");
                abort();
        }
}

static
unsigned long hash_channel_key(struct channel_key *key)
{
        unsigned long key_hash = hash_key_u64(&key->key, lttng_ht_seed);
        unsigned long domain_hash = hash_key_ulong(
                (void *) (unsigned long) key->domain, lttng_ht_seed);

        return key_hash ^ domain_hash;
}

static
unsigned long hash_client_socket(int socket)
{
        return hash_key_ulong((void *) (unsigned long) socket, lttng_ht_seed);
}

static
unsigned long hash_client_id(notification_client_id id)
{
        return hash_key_u64(&id, lttng_ht_seed);
}

/*
 * Get the type of object to which a given condition applies. Bindings let
 * the notification system evaluate a trigger's condition when a given
 * object's state is updated.
 *
 * For instance, a condition bound to a channel will be evaluated everytime
 * the channel's state is changed by a channel monitoring sample.
 */
static
enum lttng_object_type get_condition_binding_object(
                const struct lttng_condition *condition)
{
        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                return LTTNG_OBJECT_TYPE_CHANNEL;
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
                return LTTNG_OBJECT_TYPE_SESSION;
        default:
                return LTTNG_OBJECT_TYPE_UNKNOWN;
        }
}

static
void free_channel_info_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct channel_info, rcu_node));
}

static
void channel_info_destroy(struct channel_info *channel_info)
{
        if (!channel_info) {
                return;
        }

        if (channel_info->session_info) {
                session_info_remove_channel(channel_info->session_info,
                                channel_info);
                session_info_put(channel_info->session_info);
        }
        if (channel_info->name) {
                free(channel_info->name);
        }
        call_rcu(&channel_info->rcu_node, free_channel_info_rcu);
}

static
void free_session_info_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct session_info, rcu_node));
}

/* Don't call directly, use the ref-counting mechanism. */
static
void session_info_destroy(void *_data)
{
        struct session_info *session_info = _data;
        int ret;

        assert(session_info);
        if (session_info->channel_infos_ht) {
                ret = cds_lfht_destroy(session_info->channel_infos_ht, NULL);
                if (ret) {
                        ERR("[notification-thread] Failed to destroy channel information hash table");
                }
        }
        lttng_session_trigger_list_destroy(session_info->trigger_list);

        rcu_read_lock();
        cds_lfht_del(session_info->sessions_ht,
                        &session_info->sessions_ht_node);
        rcu_read_unlock();
        free(session_info->name);
        call_rcu(&session_info->rcu_node, free_session_info_rcu);
}

static
void session_info_get(struct session_info *session_info)
{
        if (!session_info) {
                return;
        }
        lttng_ref_get(&session_info->ref);
}

static
void session_info_put(struct session_info *session_info)
{
        if (!session_info) {
                return;
        }
        lttng_ref_put(&session_info->ref);
}

static
struct session_info *session_info_create(const char *name, uid_t uid, gid_t gid,
                struct lttng_session_trigger_list *trigger_list,
                struct cds_lfht *sessions_ht)
{
        struct session_info *session_info;

        assert(name);

        session_info = zmalloc(sizeof(*session_info));
        if (!session_info) {
                goto end;
        }
        lttng_ref_init(&session_info->ref, session_info_destroy);

        session_info->channel_infos_ht = cds_lfht_new(DEFAULT_HT_SIZE,
                        1, 0, CDS_LFHT_AUTO_RESIZE | CDS_LFHT_ACCOUNTING, NULL);
        if (!session_info->channel_infos_ht) {
                goto error;
        }

        cds_lfht_node_init(&session_info->sessions_ht_node);
        session_info->name = strdup(name);
        if (!session_info->name) {
                goto error;
        }
        session_info->uid = uid;
        session_info->gid = gid;
        session_info->trigger_list = trigger_list;
        session_info->sessions_ht = sessions_ht;
end:
        return session_info;
error:
        session_info_put(session_info);
        return NULL;
}

static
void session_info_add_channel(struct session_info *session_info,
                struct channel_info *channel_info)
{
        rcu_read_lock();
        cds_lfht_add(session_info->channel_infos_ht,
                        hash_channel_key(&channel_info->key),
                        &channel_info->session_info_channels_ht_node);
        rcu_read_unlock();
}

static
void session_info_remove_channel(struct session_info *session_info,
                struct channel_info *channel_info)
{
        rcu_read_lock();
        cds_lfht_del(session_info->channel_infos_ht,
                        &channel_info->session_info_channels_ht_node);
        rcu_read_unlock();
}

static
struct channel_info *channel_info_create(const char *channel_name,
                struct channel_key *channel_key, uint64_t channel_capacity,
                struct session_info *session_info)
{
        struct channel_info *channel_info = zmalloc(sizeof(*channel_info));

        if (!channel_info) {
                goto end;
        }

        cds_lfht_node_init(&channel_info->channels_ht_node);
        cds_lfht_node_init(&channel_info->session_info_channels_ht_node);
        memcpy(&channel_info->key, channel_key, sizeof(*channel_key));
        channel_info->capacity = channel_capacity;

        channel_info->name = strdup(channel_name);
        if (!channel_info->name) {
                goto error;
        }

        /*
         * Set the references between session and channel infos:
         *   - channel_info holds a strong reference to session_info
         *   - session_info holds a weak reference to channel_info
         */
        session_info_get(session_info);
        session_info_add_channel(session_info, channel_info);
        channel_info->session_info = session_info;
end:
        return channel_info;
error:
        channel_info_destroy(channel_info);
        return NULL;
}

static
bool notification_client_list_get(struct notification_client_list *list)
{
        return urcu_ref_get_unless_zero(&list->ref);
}

static
void free_notification_client_list_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct notification_client_list,
                        rcu_node));
}

static
void notification_client_list_release(struct urcu_ref *list_ref)
{
        struct notification_client_list *list =
                        container_of(list_ref, typeof(*list), ref);
        struct notification_client_list_element *client_list_element, *tmp;

        if (list->notification_trigger_clients_ht) {
                rcu_read_lock();
                cds_lfht_del(list->notification_trigger_clients_ht,
                                &list->notification_trigger_clients_ht_node);
                rcu_read_unlock();
                list->notification_trigger_clients_ht = NULL;
        }
        cds_list_for_each_entry_safe(client_list_element, tmp,
                                     &list->list, node) {
                free(client_list_element);
        }
        pthread_mutex_destroy(&list->lock);
        call_rcu(&list->rcu_node, free_notification_client_list_rcu);
}

static
struct notification_client_list *notification_client_list_create(
                const struct lttng_trigger *trigger)
{
        struct notification_client_list *client_list =
                        zmalloc(sizeof(*client_list));

        if (!client_list) {
                goto error;
        }
        pthread_mutex_init(&client_list->lock, NULL);
        urcu_ref_init(&client_list->ref);
        cds_lfht_node_init(&client_list->notification_trigger_clients_ht_node);
        CDS_INIT_LIST_HEAD(&client_list->list);
        client_list->trigger = trigger;
error:
        return client_list;
}

static
void publish_notification_client_list(
                struct notification_thread_state *state,
                struct notification_client_list *list)
{
        const struct lttng_condition *condition =
                        lttng_trigger_get_const_condition(list->trigger);

        assert(!list->notification_trigger_clients_ht);

        list->notification_trigger_clients_ht =
                        state->notification_trigger_clients_ht;

        rcu_read_lock();
        cds_lfht_add(state->notification_trigger_clients_ht,
                        lttng_condition_hash(condition),
                        &list->notification_trigger_clients_ht_node);
        rcu_read_unlock();
}

static
void notification_client_list_put(struct notification_client_list *list)
{
        if (!list) {
                return;
        }
        return urcu_ref_put(&list->ref, notification_client_list_release);
}

/* Provides a reference to the returned list. */
static
struct notification_client_list *get_client_list_from_condition(
        struct notification_thread_state *state,
        const struct lttng_condition *condition)
{
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct notification_client_list *list = NULL;

        rcu_read_lock();
        cds_lfht_lookup(state->notification_trigger_clients_ht,
                        lttng_condition_hash(condition),
                        match_client_list_condition,
                        condition,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (node) {
                list = container_of(node, struct notification_client_list,
                                notification_trigger_clients_ht_node);
                list = notification_client_list_get(list) ? list : NULL;
        }

        rcu_read_unlock();
        return list;
}

static
int evaluate_channel_condition_for_client(
                const struct lttng_condition *condition,
                struct notification_thread_state *state,
                struct lttng_evaluation **evaluation,
                uid_t *session_uid, gid_t *session_gid)
{
        int ret;
        struct cds_lfht_iter iter;
        struct cds_lfht_node *node;
        struct channel_info *channel_info = NULL;
        struct channel_key *channel_key = NULL;
        struct channel_state_sample *last_sample = NULL;
        struct lttng_channel_trigger_list *channel_trigger_list = NULL;

        rcu_read_lock();

        /* Find the channel associated with the condition. */
        cds_lfht_for_each_entry(state->channel_triggers_ht, &iter,
                        channel_trigger_list, channel_triggers_ht_node) {
                struct lttng_trigger_list_element *element;

                cds_list_for_each_entry(element, &channel_trigger_list->list, node) {
                        const struct lttng_condition *current_condition =
                                        lttng_trigger_get_const_condition(
                                                element->trigger);

                        assert(current_condition);
                        if (!lttng_condition_is_equal(condition,
                                        current_condition)) {
                                continue;
                        }

                        /* Found the trigger, save the channel key. */
                        channel_key = &channel_trigger_list->channel_key;
                        break;
                }
                if (channel_key) {
                        /* The channel key was found stop iteration. */
                        break;
                }
        }

        if (!channel_key){
                /* No channel found; normal exit. */
                DBG("[notification-thread] No known channel associated with newly subscribed-to condition");
                ret = 0;
                goto end;
        }

        /* Fetch channel info for the matching channel. */
        cds_lfht_lookup(state->channels_ht,
                        hash_channel_key(channel_key),
                        match_channel_info,
                        channel_key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        assert(node);
        channel_info = caa_container_of(node, struct channel_info,
                        channels_ht_node);

        /* Retrieve the channel's last sample, if it exists. */
        cds_lfht_lookup(state->channel_state_ht,
                        hash_channel_key(channel_key),
                        match_channel_state_sample,
                        channel_key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (node) {
                last_sample = caa_container_of(node,
                                struct channel_state_sample,
                                channel_state_ht_node);
        } else {
                /* Nothing to evaluate, no sample was ever taken. Normal exit */
                DBG("[notification-thread] No channel sample associated with newly subscribed-to condition");
                ret = 0;
                goto end;
        }

        ret = evaluate_buffer_condition(condition, evaluation, state,
                        NULL, last_sample,
                        0, channel_info->session_info->consumed_data_size,
                        channel_info);
        if (ret) {
                WARN("[notification-thread] Fatal error occurred while evaluating a newly subscribed-to condition");
                goto end;
        }

        *session_uid = channel_info->session_info->uid;
        *session_gid = channel_info->session_info->gid;
end:
        rcu_read_unlock();
        return ret;
}

static
const char *get_condition_session_name(const struct lttng_condition *condition)
{
        const char *session_name = NULL;
        enum lttng_condition_status status;

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                status = lttng_condition_buffer_usage_get_session_name(
                                condition, &session_name);
                break;
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                status = lttng_condition_session_consumed_size_get_session_name(
                                condition, &session_name);
                break;
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
                status = lttng_condition_session_rotation_get_session_name(
                                condition, &session_name);
                break;
        default:
                abort();
        }
        if (status != LTTNG_CONDITION_STATUS_OK) {
                ERR("[notification-thread] Failed to retrieve session rotation condition's session name");
                goto end;
        }
end:
        return session_name;
}

static
int evaluate_session_condition_for_client(
                const struct lttng_condition *condition,
                struct notification_thread_state *state,
                struct lttng_evaluation **evaluation,
                uid_t *session_uid, gid_t *session_gid)
{
        int ret;
        struct cds_lfht_iter iter;
        struct cds_lfht_node *node;
        const char *session_name;
        struct session_info *session_info = NULL;

        rcu_read_lock();
        session_name = get_condition_session_name(condition);

        /* Find the session associated with the trigger. */
        cds_lfht_lookup(state->sessions_ht,
                        hash_key_str(session_name, lttng_ht_seed),
                        match_session,
                        session_name,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (!node) {
                DBG("[notification-thread] No known session matching name \"%s\"",
                                session_name);
                ret = 0;
                goto end;
        }

        session_info = caa_container_of(node, struct session_info,
                        sessions_ht_node);
        session_info_get(session_info);

        /*
         * Evaluation is performed in-line here since only one type of
         * session-bound condition is handled for the moment.
         */
        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
                if (!session_info->rotation.ongoing) {
                        ret = 0;
                        goto end_session_put;
                }

                *evaluation = lttng_evaluation_session_rotation_ongoing_create(
                                session_info->rotation.id);
                if (!*evaluation) {
                        /* Fatal error. */
                        ERR("[notification-thread] Failed to create session rotation ongoing evaluation for session \"%s\"",
                                        session_info->name);
                        ret = -1;
                        goto end_session_put;
                }
                ret = 0;
                break;
        default:
                ret = 0;
                goto end_session_put;
        }

        *session_uid = session_info->uid;
        *session_gid = session_info->gid;

end_session_put:
        session_info_put(session_info);
end:
        rcu_read_unlock();
        return ret;
}

static
int evaluate_condition_for_client(const struct lttng_trigger *trigger,
                const struct lttng_condition *condition,
                struct notification_client *client,
                struct notification_thread_state *state)
{
        int ret;
        struct lttng_evaluation *evaluation = NULL;
        struct notification_client_list client_list = {
                .lock = PTHREAD_MUTEX_INITIALIZER,
        };
        struct notification_client_list_element client_list_element = { 0 };
        uid_t object_uid = 0;
        gid_t object_gid = 0;

        assert(trigger);
        assert(condition);
        assert(client);
        assert(state);

        switch (get_condition_binding_object(condition)) {
        case LTTNG_OBJECT_TYPE_SESSION:
                ret = evaluate_session_condition_for_client(condition, state,
                                &evaluation, &object_uid, &object_gid);
                break;
        case LTTNG_OBJECT_TYPE_CHANNEL:
                ret = evaluate_channel_condition_for_client(condition, state,
                                &evaluation, &object_uid, &object_gid);
                break;
        case LTTNG_OBJECT_TYPE_NONE:
                ret = 0;
                goto end;
        case LTTNG_OBJECT_TYPE_UNKNOWN:
        default:
                ret = -1;
                goto end;
        }
        if (ret) {
                /* Fatal error. */
                goto end;
        }
        if (!evaluation) {
                /* Evaluation yielded nothing. Normal exit. */
                DBG("[notification-thread] Newly subscribed-to condition evaluated to false, nothing to report to client");
                ret = 0;
                goto end;
        }

        /*
         * Create a temporary client list with the client currently
         * subscribing.
         */
        cds_lfht_node_init(&client_list.notification_trigger_clients_ht_node);
        CDS_INIT_LIST_HEAD(&client_list.list);
        client_list.trigger = trigger;

        CDS_INIT_LIST_HEAD(&client_list_element.node);
        client_list_element.client = client;
        cds_list_add(&client_list_element.node, &client_list.list);

        /* Send evaluation result to the newly-subscribed client. */
        DBG("[notification-thread] Newly subscribed-to condition evaluated to true, notifying client");
        ret = send_evaluation_to_clients(trigger, evaluation, &client_list,
                        state, object_uid, object_gid);

end:
        return ret;
}

static
int notification_thread_client_subscribe(struct notification_client *client,
                struct lttng_condition *condition,
                struct notification_thread_state *state,
                enum lttng_notification_channel_status *_status)
{
        int ret = 0;
        struct notification_client_list *client_list = NULL;
        struct lttng_condition_list_element *condition_list_element = NULL;
        struct notification_client_list_element *client_list_element = NULL;
        enum lttng_notification_channel_status status =
                        LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;

        /*
         * Ensure that the client has not already subscribed to this condition
         * before.
         */
        cds_list_for_each_entry(condition_list_element, &client->condition_list, node) {
                if (lttng_condition_is_equal(condition_list_element->condition,
                                condition)) {
                        status = LTTNG_NOTIFICATION_CHANNEL_STATUS_ALREADY_SUBSCRIBED;
                        goto end;
                }
        }

        condition_list_element = zmalloc(sizeof(*condition_list_element));
        if (!condition_list_element) {
                ret = -1;
                goto error;
        }
        client_list_element = zmalloc(sizeof(*client_list_element));
        if (!client_list_element) {
                ret = -1;
                goto error;
        }

        /*
         * Add the newly-subscribed condition to the client's subscription list.
         */
        CDS_INIT_LIST_HEAD(&condition_list_element->node);
        condition_list_element->condition = condition;
        cds_list_add(&condition_list_element->node, &client->condition_list);

        client_list = get_client_list_from_condition(state, condition);
        if (!client_list) {
                /*
                 * No notification-emiting trigger registered with this
                 * condition. We don't evaluate the condition right away
                 * since this trigger is not registered yet.
                 */
                free(client_list_element);
                goto end;
        }

        /*
         * The condition to which the client just subscribed is evaluated
         * at this point so that conditions that are already TRUE result
         * in a notification being sent out.
         *
         * The client_list's trigger is used without locking the list itself.
         * This is correct since the list doesn't own the trigger and the
         * object is immutable.
         */
        if (evaluate_condition_for_client(client_list->trigger, condition,
                        client, state)) {
                WARN("[notification-thread] Evaluation of a condition on client subscription failed, aborting.");
                ret = -1;
                free(client_list_element);
                goto end;
        }

        /*
         * Add the client to the list of clients interested in a given trigger
         * if a "notification" trigger with a corresponding condition was
         * added prior.
         */
        client_list_element->client = client;
        CDS_INIT_LIST_HEAD(&client_list_element->node);

        pthread_mutex_lock(&client_list->lock);
        cds_list_add(&client_list_element->node, &client_list->list);
        pthread_mutex_unlock(&client_list->lock);
end:
        if (_status) {
                *_status = status;
        }
        if (client_list) {
                notification_client_list_put(client_list);
        }
        return ret;
error:
        free(condition_list_element);
        free(client_list_element);
        return ret;
}

static
int notification_thread_client_unsubscribe(
                struct notification_client *client,
                struct lttng_condition *condition,
                struct notification_thread_state *state,
                enum lttng_notification_channel_status *_status)
{
        struct notification_client_list *client_list;
        struct lttng_condition_list_element *condition_list_element,
                        *condition_tmp;
        struct notification_client_list_element *client_list_element,
                        *client_tmp;
        bool condition_found = false;
        enum lttng_notification_channel_status status =
                        LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;

        /* Remove the condition from the client's condition list. */
        cds_list_for_each_entry_safe(condition_list_element, condition_tmp,
                        &client->condition_list, node) {
                if (!lttng_condition_is_equal(condition_list_element->condition,
                                condition)) {
                        continue;
                }

                cds_list_del(&condition_list_element->node);
                /*
                 * The caller may be iterating on the client's conditions to
                 * tear down a client's connection. In this case, the condition
                 * will be destroyed at the end.
                 */
                if (condition != condition_list_element->condition) {
                        lttng_condition_destroy(
                                        condition_list_element->condition);
                }
                free(condition_list_element);
                condition_found = true;
                break;
        }

        if (!condition_found) {
                status = LTTNG_NOTIFICATION_CHANNEL_STATUS_UNKNOWN_CONDITION;
                goto end;
        }

        /*
         * Remove the client from the list of clients interested the trigger
         * matching the condition.
         */
        client_list = get_client_list_from_condition(state, condition);
        if (!client_list) {
                goto end;
        }

        pthread_mutex_lock(&client_list->lock);
        cds_list_for_each_entry_safe(client_list_element, client_tmp,
                        &client_list->list, node) {
                if (client_list_element->client->id != client->id) {
                        continue;
                }
                cds_list_del(&client_list_element->node);
                free(client_list_element);
                break;
        }
        pthread_mutex_unlock(&client_list->lock);
        notification_client_list_put(client_list);
        client_list = NULL;
end:
        lttng_condition_destroy(condition);
        if (_status) {
                *_status = status;
        }
        return 0;
}

static
void free_notification_client_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct notification_client, rcu_node));
}

static
void notification_client_destroy(struct notification_client *client,
                struct notification_thread_state *state)
{
        if (!client) {
                return;
        }

        /*
         * The client object is not reachable by other threads, no need to lock
         * the client here.
         */
        if (client->socket >= 0) {
                (void) lttcomm_close_unix_sock(client->socket);
                client->socket = -1;
        }
        client->communication.active = false;
        lttng_dynamic_buffer_reset(&client->communication.inbound.buffer);
        lttng_dynamic_buffer_reset(&client->communication.outbound.buffer);
        pthread_mutex_destroy(&client->lock);
        call_rcu(&client->rcu_node, free_notification_client_rcu);
}

/*
 * Call with rcu_read_lock held (and hold for the lifetime of the returned
 * client pointer).
 */
static
struct notification_client *get_client_from_socket(int socket,
                struct notification_thread_state *state)
{
        struct cds_lfht_iter iter;
        struct cds_lfht_node *node;
        struct notification_client *client = NULL;

        cds_lfht_lookup(state->client_socket_ht,
                        hash_client_socket(socket),
                        match_client_socket,
                        (void *) (unsigned long) socket,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (!node) {
                goto end;
        }

        client = caa_container_of(node, struct notification_client,
                        client_socket_ht_node);
end:
        return client;
}

static
bool buffer_usage_condition_applies_to_channel(
                const struct lttng_condition *condition,
                const struct channel_info *channel_info)
{
        enum lttng_condition_status status;
        enum lttng_domain_type condition_domain;
        const char *condition_session_name = NULL;
        const char *condition_channel_name = NULL;

        status = lttng_condition_buffer_usage_get_domain_type(condition,
                        &condition_domain);
        assert(status == LTTNG_CONDITION_STATUS_OK);
        if (channel_info->key.domain != condition_domain) {
                goto fail;
        }

        status = lttng_condition_buffer_usage_get_session_name(
                        condition, &condition_session_name);
        assert((status == LTTNG_CONDITION_STATUS_OK) && condition_session_name);

        status = lttng_condition_buffer_usage_get_channel_name(
                        condition, &condition_channel_name);
        assert((status == LTTNG_CONDITION_STATUS_OK) && condition_channel_name);

        if (strcmp(channel_info->session_info->name, condition_session_name)) {
                goto fail;
        }
        if (strcmp(channel_info->name, condition_channel_name)) {
                goto fail;
        }

        return true;
fail:
        return false;
}

static
bool session_consumed_size_condition_applies_to_channel(
                const struct lttng_condition *condition,
                const struct channel_info *channel_info)
{
        enum lttng_condition_status status;
        const char *condition_session_name = NULL;

        status = lttng_condition_session_consumed_size_get_session_name(
                        condition, &condition_session_name);
        assert((status == LTTNG_CONDITION_STATUS_OK) && condition_session_name);

        if (strcmp(channel_info->session_info->name, condition_session_name)) {
                goto fail;
        }

        return true;
fail:
        return false;
}

static
bool trigger_applies_to_channel(const struct lttng_trigger *trigger,
                const struct channel_info *channel_info)
{
        const struct lttng_condition *condition;
        bool trigger_applies;

        condition = lttng_trigger_get_const_condition(trigger);
        if (!condition) {
                goto fail;
        }

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                trigger_applies = buffer_usage_condition_applies_to_channel(
                                condition, channel_info);
                break;
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                trigger_applies = session_consumed_size_condition_applies_to_channel(
                                condition, channel_info);
                break;
        default:
                goto fail;
        }

        return trigger_applies;
fail:
        return false;
}

static
bool trigger_applies_to_client(struct lttng_trigger *trigger,
                struct notification_client *client)
{
        bool applies = false;
        struct lttng_condition_list_element *condition_list_element;

        cds_list_for_each_entry(condition_list_element, &client->condition_list,
                        node) {
                applies = lttng_condition_is_equal(
                                condition_list_element->condition,
                                lttng_trigger_get_condition(trigger));
                if (applies) {
                        break;
                }
        }
        return applies;
}

/* Must be called with RCU read lock held. */
static
struct lttng_session_trigger_list *get_session_trigger_list(
                struct notification_thread_state *state,
                const char *session_name)
{
        struct lttng_session_trigger_list *list = NULL;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;

        cds_lfht_lookup(state->session_triggers_ht,
                        hash_key_str(session_name, lttng_ht_seed),
                        match_session_trigger_list,
                        session_name,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (!node) {
                /*
                 * Not an error, the list of triggers applying to that session
                 * will be initialized when the session is created.
                 */
                DBG("[notification-thread] No trigger list found for session \"%s\" as it is not yet known to the notification system",
                                session_name);
                goto end;
        }

        list = caa_container_of(node,
                        struct lttng_session_trigger_list,
                        session_triggers_ht_node);
end:
        return list;
}

/*
 * Allocate an empty lttng_session_trigger_list for the session named
 * 'session_name'.
 *
 * No ownership of 'session_name' is assumed by the session trigger list.
 * It is the caller's responsability to ensure the session name is alive
 * for as long as this list is.
 */
static
struct lttng_session_trigger_list *lttng_session_trigger_list_create(
                const char *session_name,
                struct cds_lfht *session_triggers_ht)
{
        struct lttng_session_trigger_list *list;

        list = zmalloc(sizeof(*list));
        if (!list) {
                goto end;
        }
        list->session_name = session_name;
        CDS_INIT_LIST_HEAD(&list->list);
        cds_lfht_node_init(&list->session_triggers_ht_node);
        list->session_triggers_ht = session_triggers_ht;

        rcu_read_lock();
        /* Publish the list through the session_triggers_ht. */
        cds_lfht_add(session_triggers_ht,
                        hash_key_str(session_name, lttng_ht_seed),
                        &list->session_triggers_ht_node);
        rcu_read_unlock();
end:
        return list;
}

static
void free_session_trigger_list_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct lttng_session_trigger_list,
                        rcu_node));
}

static
void lttng_session_trigger_list_destroy(struct lttng_session_trigger_list *list)
{
        struct lttng_trigger_list_element *trigger_list_element, *tmp;

        /* Empty the list element by element, and then free the list itself. */
        cds_list_for_each_entry_safe(trigger_list_element, tmp,
                        &list->list, node) {
                cds_list_del(&trigger_list_element->node);
                free(trigger_list_element);
        }
        rcu_read_lock();
        /* Unpublish the list from the session_triggers_ht. */
        cds_lfht_del(list->session_triggers_ht,
                        &list->session_triggers_ht_node);
        rcu_read_unlock();
        call_rcu(&list->rcu_node, free_session_trigger_list_rcu);
}

static
int lttng_session_trigger_list_add(struct lttng_session_trigger_list *list,
                const struct lttng_trigger *trigger)
{
        int ret = 0;
        struct lttng_trigger_list_element *new_element =
                        zmalloc(sizeof(*new_element));

        if (!new_element) {
                ret = -1;
                goto end;
        }
        CDS_INIT_LIST_HEAD(&new_element->node);
        new_element->trigger = trigger;
        cds_list_add(&new_element->node, &list->list);
end:
        return ret;
}

static
bool trigger_applies_to_session(const struct lttng_trigger *trigger,
                const char *session_name)
{
        bool applies = false;
        const struct lttng_condition *condition;

        condition = lttng_trigger_get_const_condition(trigger);
        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
        {
                enum lttng_condition_status condition_status;
                const char *condition_session_name;

                condition_status = lttng_condition_session_rotation_get_session_name(
                        condition, &condition_session_name);
                if (condition_status != LTTNG_CONDITION_STATUS_OK) {
                        ERR("[notification-thread] Failed to retrieve session rotation condition's session name");
                        goto end;
                }

                assert(condition_session_name);
                applies = !strcmp(condition_session_name, session_name);
                break;
        }
        default:
                goto end;
        }
end:
        return applies;
}

/*
 * Allocate and initialize an lttng_session_trigger_list which contains
 * all triggers that apply to the session named 'session_name'.
 *
 * No ownership of 'session_name' is assumed by the session trigger list.
 * It is the caller's responsability to ensure the session name is alive
 * for as long as this list is.
 */
static
struct lttng_session_trigger_list *lttng_session_trigger_list_build(
                const struct notification_thread_state *state,
                const char *session_name)
{
        int trigger_count = 0;
        struct lttng_session_trigger_list *session_trigger_list = NULL;
        struct lttng_trigger_ht_element *trigger_ht_element = NULL;
        struct cds_lfht_iter iter;

        session_trigger_list = lttng_session_trigger_list_create(session_name,
                        state->session_triggers_ht);

        /* Add all triggers applying to the session named 'session_name'. */
        cds_lfht_for_each_entry(state->triggers_ht, &iter, trigger_ht_element,
                        node) {
                int ret;

                if (!trigger_applies_to_session(trigger_ht_element->trigger,
                                session_name)) {
                        continue;
                }

                ret = lttng_session_trigger_list_add(session_trigger_list,
                                trigger_ht_element->trigger);
                if (ret) {
                        goto error;
                }

                trigger_count++;
        }

        DBG("[notification-thread] Found %i triggers that apply to newly created session",
                        trigger_count);
        return session_trigger_list;
error:
        lttng_session_trigger_list_destroy(session_trigger_list);
        return NULL;
}

static
struct session_info *find_or_create_session_info(
                struct notification_thread_state *state,
                const char *name, uid_t uid, gid_t gid)
{
        struct session_info *session = NULL;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct lttng_session_trigger_list *trigger_list;

        rcu_read_lock();
        cds_lfht_lookup(state->sessions_ht,
                        hash_key_str(name, lttng_ht_seed),
                        match_session,
                        name,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (node) {
                DBG("[notification-thread] Found session info of session \"%s\" (uid = %i, gid = %i)",
                                name, uid, gid);
                session = caa_container_of(node, struct session_info,
                                sessions_ht_node);
                assert(session->uid == uid);
                assert(session->gid == gid);
                session_info_get(session);
                goto end;
        }

        trigger_list = lttng_session_trigger_list_build(state, name);
        if (!trigger_list) {
                goto error;
        }

        session = session_info_create(name, uid, gid, trigger_list,
                        state->sessions_ht);
        if (!session) {
                ERR("[notification-thread] Failed to allocation session info for session \"%s\" (uid = %i, gid = %i)",
                                name, uid, gid);
                lttng_session_trigger_list_destroy(trigger_list);
                goto error;
        }
        trigger_list = NULL;

        cds_lfht_add(state->sessions_ht, hash_key_str(name, lttng_ht_seed),
                        &session->sessions_ht_node);
end:
        rcu_read_unlock();
        return session;
error:
        rcu_read_unlock();
        session_info_put(session);
        return NULL;
}

static
int handle_notification_thread_command_add_channel(
                struct notification_thread_state *state,
                const char *session_name, uid_t session_uid, gid_t session_gid,
                const char *channel_name, enum lttng_domain_type channel_domain,
                uint64_t channel_key_int, uint64_t channel_capacity,
                enum lttng_error_code *cmd_result)
{
        struct cds_list_head trigger_list;
        struct channel_info *new_channel_info = NULL;
        struct channel_key channel_key = {
                .key = channel_key_int,
                .domain = channel_domain,
        };
        struct lttng_channel_trigger_list *channel_trigger_list = NULL;
        struct lttng_trigger_ht_element *trigger_ht_element = NULL;
        int trigger_count = 0;
        struct cds_lfht_iter iter;
        struct session_info *session_info = NULL;

        DBG("[notification-thread] Adding channel %s from session %s, channel key = %" PRIu64 " in %s domain",
                        channel_name, session_name, channel_key_int,
                        channel_domain == LTTNG_DOMAIN_KERNEL ? "kernel" : "user space");

        CDS_INIT_LIST_HEAD(&trigger_list);

        session_info = find_or_create_session_info(state, session_name,
                        session_uid, session_gid);
        if (!session_info) {
                /* Allocation error or an internal error occurred. */
                goto error;
        }

        new_channel_info = channel_info_create(channel_name, &channel_key,
                        channel_capacity, session_info);
        if (!new_channel_info) {
                goto error;
        }

        rcu_read_lock();
        /* Build a list of all triggers applying to the new channel. */
        cds_lfht_for_each_entry(state->triggers_ht, &iter, trigger_ht_element,
                        node) {
                struct lttng_trigger_list_element *new_element;

                if (!trigger_applies_to_channel(trigger_ht_element->trigger,
                                new_channel_info)) {
                        continue;
                }

                new_element = zmalloc(sizeof(*new_element));
                if (!new_element) {
                        rcu_read_unlock();
                        goto error;
                }
                CDS_INIT_LIST_HEAD(&new_element->node);
                new_element->trigger = trigger_ht_element->trigger;
                cds_list_add(&new_element->node, &trigger_list);
                trigger_count++;
        }
        rcu_read_unlock();

        DBG("[notification-thread] Found %i triggers that apply to newly added channel",
                        trigger_count);
        channel_trigger_list = zmalloc(sizeof(*channel_trigger_list));
        if (!channel_trigger_list) {
                goto error;
        }
        channel_trigger_list->channel_key = new_channel_info->key;
        CDS_INIT_LIST_HEAD(&channel_trigger_list->list);
        cds_lfht_node_init(&channel_trigger_list->channel_triggers_ht_node);
        cds_list_splice(&trigger_list, &channel_trigger_list->list);

        rcu_read_lock();
        /* Add channel to the channel_ht which owns the channel_infos. */
        cds_lfht_add(state->channels_ht,
                        hash_channel_key(&new_channel_info->key),
                        &new_channel_info->channels_ht_node);
        /*
         * Add the list of triggers associated with this channel to the
         * channel_triggers_ht.
         */
        cds_lfht_add(state->channel_triggers_ht,
                        hash_channel_key(&new_channel_info->key),
                        &channel_trigger_list->channel_triggers_ht_node);
        rcu_read_unlock();
        session_info_put(session_info);
        *cmd_result = LTTNG_OK;
        return 0;
error:
        channel_info_destroy(new_channel_info);
        session_info_put(session_info);
        return 1;
}

static
void free_channel_trigger_list_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct lttng_channel_trigger_list,
                        rcu_node));
}

static
void free_channel_state_sample_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct channel_state_sample,
                        rcu_node));
}

static
int handle_notification_thread_command_remove_channel(
        struct notification_thread_state *state,
        uint64_t channel_key, enum lttng_domain_type domain,
        enum lttng_error_code *cmd_result)
{
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct lttng_channel_trigger_list *trigger_list;
        struct lttng_trigger_list_element *trigger_list_element, *tmp;
        struct channel_key key = { .key = channel_key, .domain = domain };
        struct channel_info *channel_info;

        DBG("[notification-thread] Removing channel key = %" PRIu64 " in %s domain",
                        channel_key, domain == LTTNG_DOMAIN_KERNEL ? "kernel" : "user space");

        rcu_read_lock();

        cds_lfht_lookup(state->channel_triggers_ht,
                        hash_channel_key(&key),
                        match_channel_trigger_list,
                        &key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        /*
         * There is a severe internal error if we are being asked to remove a
         * channel that doesn't exist.
         */
        if (!node) {
                ERR("[notification-thread] Channel being removed is unknown to the notification thread");
                goto end;
        }

        /* Free the list of triggers associated with this channel. */
        trigger_list = caa_container_of(node, struct lttng_channel_trigger_list,
                        channel_triggers_ht_node);
        cds_list_for_each_entry_safe(trigger_list_element, tmp,
                        &trigger_list->list, node) {
                cds_list_del(&trigger_list_element->node);
                free(trigger_list_element);
        }
        cds_lfht_del(state->channel_triggers_ht, node);
        call_rcu(&trigger_list->rcu_node, free_channel_trigger_list_rcu);

        /* Free sampled channel state. */
        cds_lfht_lookup(state->channel_state_ht,
                        hash_channel_key(&key),
                        match_channel_state_sample,
                        &key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        /*
         * This is expected to be NULL if the channel is destroyed before we
         * received a sample.
         */
        if (node) {
                struct channel_state_sample *sample = caa_container_of(node,
                                struct channel_state_sample,
                                channel_state_ht_node);

                cds_lfht_del(state->channel_state_ht, node);
                call_rcu(&sample->rcu_node, free_channel_state_sample_rcu);
        }

        /* Remove the channel from the channels_ht and free it. */
        cds_lfht_lookup(state->channels_ht,
                        hash_channel_key(&key),
                        match_channel_info,
                        &key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        assert(node);
        channel_info = caa_container_of(node, struct channel_info,
                        channels_ht_node);
        cds_lfht_del(state->channels_ht, node);
        channel_info_destroy(channel_info);
end:
        rcu_read_unlock();
        *cmd_result = LTTNG_OK;
        return 0;
}

static
int handle_notification_thread_command_session_rotation(
        struct notification_thread_state *state,
        enum notification_thread_command_type cmd_type,
        const char *session_name, uid_t session_uid, gid_t session_gid,
        uint64_t trace_archive_chunk_id,
        struct lttng_trace_archive_location *location,
        enum lttng_error_code *_cmd_result)
{
        int ret = 0;
        enum lttng_error_code cmd_result = LTTNG_OK;
        struct lttng_session_trigger_list *trigger_list;
        struct lttng_trigger_list_element *trigger_list_element;
        struct session_info *session_info;

        rcu_read_lock();

        session_info = find_or_create_session_info(state, session_name,
                        session_uid, session_gid);
        if (!session_info) {
                /* Allocation error or an internal error occurred. */
                ret = -1;
                cmd_result = LTTNG_ERR_NOMEM;
                goto end;
        }

        session_info->rotation.ongoing =
                        cmd_type == NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING;
        session_info->rotation.id = trace_archive_chunk_id;
        trigger_list = get_session_trigger_list(state, session_name);
        if (!trigger_list) {
                DBG("[notification-thread] No triggers applying to session \"%s\" found",
                                session_name);
                goto end;
        }

        cds_list_for_each_entry(trigger_list_element, &trigger_list->list,
                        node) {
                const struct lttng_condition *condition;
                const struct lttng_action *action;
                const struct lttng_trigger *trigger;
                struct notification_client_list *client_list;
                struct lttng_evaluation *evaluation = NULL;
                enum lttng_condition_type condition_type;
                bool client_list_is_empty;

                trigger = trigger_list_element->trigger;
                condition = lttng_trigger_get_const_condition(trigger);
                assert(condition);
                condition_type = lttng_condition_get_type(condition);

                if (condition_type == LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING &&
                                cmd_type != NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING) {
                        continue;
                } else if (condition_type == LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED &&
                                cmd_type != NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_COMPLETED) {
                        continue;
                }

                action = lttng_trigger_get_const_action(trigger);

                /* Notify actions are the only type currently supported. */
                assert(lttng_action_get_type_const(action) ==
                                LTTNG_ACTION_TYPE_NOTIFY);

                client_list = get_client_list_from_condition(state, condition);
                assert(client_list);

                pthread_mutex_lock(&client_list->lock);
                client_list_is_empty = cds_list_empty(&client_list->list);
                pthread_mutex_unlock(&client_list->lock);
                if (client_list_is_empty) {
                        /*
                         * No clients interested in the evaluation's result,
                         * skip it.
                         */
                        continue;
                }

                if (cmd_type == NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING) {
                        evaluation = lttng_evaluation_session_rotation_ongoing_create(
                                        trace_archive_chunk_id);
                } else {
                        evaluation = lttng_evaluation_session_rotation_completed_create(
                                        trace_archive_chunk_id, location);
                }

                if (!evaluation) {
                        /* Internal error */
                        ret = -1;
                        cmd_result = LTTNG_ERR_UNK;
                        goto put_list;
                }

                /* Dispatch evaluation result to all clients. */
                ret = send_evaluation_to_clients(trigger_list_element->trigger,
                                evaluation, client_list, state,
                                session_info->uid,
                                session_info->gid);
                lttng_evaluation_destroy(evaluation);
put_list:
                notification_client_list_put(client_list);
                if (caa_unlikely(ret)) {
                        break;
                }
        }
end:
        session_info_put(session_info);
        *_cmd_result = cmd_result;
        rcu_read_unlock();
        return ret;
}

static
int condition_is_supported(struct lttng_condition *condition)
{
        int ret;

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
        {
                enum lttng_domain_type domain;

                ret = lttng_condition_buffer_usage_get_domain_type(condition,
                                &domain);
                if (ret) {
                        ret = -1;
                        goto end;
                }

                if (domain != LTTNG_DOMAIN_KERNEL) {
                        ret = 1;
                        goto end;
                }

                /*
                 * Older kernel tracers don't expose the API to monitor their
                 * buffers. Therefore, we reject triggers that require that
                 * mechanism to be available to be evaluated.
                 */
                ret = kernel_supports_ring_buffer_snapshot_sample_positions();
                break;
        }
        default:
                ret = 1;
        }
end:
        return ret;
}

/* Must be called with RCU read lock held. */
static
int bind_trigger_to_matching_session(const struct lttng_trigger *trigger,
                struct notification_thread_state *state)
{
        int ret = 0;
        const struct lttng_condition *condition;
        const char *session_name;
        struct lttng_session_trigger_list *trigger_list;

        condition = lttng_trigger_get_const_condition(trigger);
        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
        {
                enum lttng_condition_status status;

                status = lttng_condition_session_rotation_get_session_name(
                                condition, &session_name);
                if (status != LTTNG_CONDITION_STATUS_OK) {
                        ERR("[notification-thread] Failed to bind trigger to session: unable to get 'session_rotation' condition's session name");
                        ret = -1;
                        goto end;
                }
                break;
        }
        default:
                ret = -1;
                goto end;
        }

        trigger_list = get_session_trigger_list(state, session_name);
        if (!trigger_list) {
                DBG("[notification-thread] Unable to bind trigger applying to session \"%s\" as it is not yet known to the notification system",
                                session_name);
                goto end;

        }

        DBG("[notification-thread] Newly registered trigger bound to session \"%s\"",
                        session_name);
        ret = lttng_session_trigger_list_add(trigger_list, trigger);
end:
        return ret;
}

/* Must be called with RCU read lock held. */
static
int bind_trigger_to_matching_channels(const struct lttng_trigger *trigger,
                struct notification_thread_state *state)
{
        int ret = 0;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct channel_info *channel;

        cds_lfht_for_each_entry(state->channels_ht, &iter, channel,
                        channels_ht_node) {
                struct lttng_trigger_list_element *trigger_list_element;
                struct lttng_channel_trigger_list *trigger_list;
                struct cds_lfht_iter lookup_iter;

                if (!trigger_applies_to_channel(trigger, channel)) {
                        continue;
                }

                cds_lfht_lookup(state->channel_triggers_ht,
                                hash_channel_key(&channel->key),
                                match_channel_trigger_list,
                                &channel->key,
                                &lookup_iter);
                node = cds_lfht_iter_get_node(&lookup_iter);
                assert(node);
                trigger_list = caa_container_of(node,
                                struct lttng_channel_trigger_list,
                                channel_triggers_ht_node);

                trigger_list_element = zmalloc(sizeof(*trigger_list_element));
                if (!trigger_list_element) {
                        ret = -1;
                        goto end;
                }
                CDS_INIT_LIST_HEAD(&trigger_list_element->node);
                trigger_list_element->trigger = trigger;
                cds_list_add(&trigger_list_element->node, &trigger_list->list);
                DBG("[notification-thread] Newly registered trigger bound to channel \"%s\"",
                                channel->name);
        }
end:
        return ret;
}

/*
 * FIXME A client's credentials are not checked when registering a trigger, nor
 *       are they stored alongside with the trigger.
 *
 * The effects of this are benign since:
 *     - The client will succeed in registering the trigger, as it is valid,
 *     - The trigger will, internally, be bound to the channel/session,
 *     - The notifications will not be sent since the client's credentials
 *       are checked against the channel at that moment.
 *
 * If this function returns a non-zero value, it means something is
 * fundamentally broken and the whole subsystem/thread will be torn down.
 *
 * If a non-fatal error occurs, just set the cmd_result to the appropriate
 * error code.
 */
static
int handle_notification_thread_command_register_trigger(
                struct notification_thread_state *state,
                struct lttng_trigger *trigger,
                enum lttng_error_code *cmd_result)
{
        int ret = 0;
        struct lttng_condition *condition;
        struct notification_client *client;
        struct notification_client_list *client_list = NULL;
        struct lttng_trigger_ht_element *trigger_ht_element = NULL;
        struct notification_client_list_element *client_list_element, *tmp;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        bool free_trigger = true;

        rcu_read_lock();

        condition = lttng_trigger_get_condition(trigger);
        assert(condition);

        ret = condition_is_supported(condition);
        if (ret < 0) {
                goto error;
        } else if (ret == 0) {
                *cmd_result = LTTNG_ERR_NOT_SUPPORTED;
                goto error;
        } else {
                /* Feature is supported, continue. */
                ret = 0;
        }

        trigger_ht_element = zmalloc(sizeof(*trigger_ht_element));
        if (!trigger_ht_element) {
                ret = -1;
                goto error;
        }

        /* Add trigger to the trigger_ht. */
        cds_lfht_node_init(&trigger_ht_element->node);
        trigger_ht_element->trigger = trigger;

        node = cds_lfht_add_unique(state->triggers_ht,
                        lttng_condition_hash(condition),
                        match_condition,
                        condition,
                        &trigger_ht_element->node);
        if (node != &trigger_ht_element->node) {
                /* Not a fatal error, simply report it to the client. */
                *cmd_result = LTTNG_ERR_TRIGGER_EXISTS;
                goto error_free_ht_element;
        }

        /*
         * Ownership of the trigger and of its wrapper was transfered to
         * the triggers_ht.
         */
        trigger_ht_element = NULL;
        free_trigger = false;

        /*
         * The rest only applies to triggers that have a "notify" action.
         * It is not skipped as this is the only action type currently
         * supported.
         */
        client_list = notification_client_list_create(trigger);
        if (!client_list) {
                ret = -1;
                goto error_free_ht_element;
        }

        /* Build a list of clients to which this new trigger applies. */
        cds_lfht_for_each_entry(state->client_socket_ht, &iter, client,
                        client_socket_ht_node) {
                if (!trigger_applies_to_client(trigger, client)) {
                        continue;
                }

                client_list_element = zmalloc(sizeof(*client_list_element));
                if (!client_list_element) {
                        ret = -1;
                        goto error_put_client_list;
                }
                CDS_INIT_LIST_HEAD(&client_list_element->node);
                client_list_element->client = client;
                cds_list_add(&client_list_element->node, &client_list->list);
        }

        switch (get_condition_binding_object(condition)) {
        case LTTNG_OBJECT_TYPE_SESSION:
                /* Add the trigger to the list if it matches a known session. */
                ret = bind_trigger_to_matching_session(trigger, state);
                if (ret) {
                        goto error_put_client_list;
                }
                break;
        case LTTNG_OBJECT_TYPE_CHANNEL:
                /*
                 * Add the trigger to list of triggers bound to the channels
                 * currently known.
                 */
                ret = bind_trigger_to_matching_channels(trigger, state);
                if (ret) {
                        goto error_put_client_list;
                }
                break;
        case LTTNG_OBJECT_TYPE_NONE:
                break;
        default:
                ERR("[notification-thread] Unknown object type on which to bind a newly registered trigger was encountered");
                ret = -1;
                goto error_put_client_list;
        }

        /*
         * Since there is nothing preventing clients from subscribing to a
         * condition before the corresponding trigger is registered, we have
         * to evaluate this new condition right away.
         *
         * At some point, we were waiting for the next "evaluation" (e.g. on
         * reception of a channel sample) to evaluate this new condition, but
         * that was broken.
         *
         * The reason it was broken is that waiting for the next sample
         * does not allow us to properly handle transitions for edge-triggered
         * conditions.
         *
         * Consider this example: when we handle a new channel sample, we
         * evaluate each conditions twice: once with the previous state, and
         * again with the newest state. We then use those two results to
         * determine whether a state change happened: a condition was false and
         * became true. If a state change happened, we have to notify clients.
         *
         * Now, if a client subscribes to a given notification and registers
         * a trigger *after* that subscription, we have to make sure the
         * condition is evaluated at this point while considering only the
         * current state. Otherwise, the next evaluation cycle may only see
         * that the evaluations remain the same (true for samples n-1 and n) and
         * the client will never know that the condition has been met.
         *
         * No need to lock the list here as it has not been published yet.
         */
        cds_list_for_each_entry_safe(client_list_element, tmp,
                        &client_list->list, node) {
                ret = evaluate_condition_for_client(trigger, condition,
                                client_list_element->client, state);
                if (ret) {
                        goto error_put_client_list;
                }
        }

        /*
         * Client list ownership transferred to the
         * notification_trigger_clients_ht.
         */
        publish_notification_client_list(state, client_list);
        client_list = NULL;

        *cmd_result = LTTNG_OK;

error_put_client_list:
        notification_client_list_put(client_list);

error_free_ht_element:
        free(trigger_ht_element);
error:
        if (free_trigger) {
                lttng_trigger_destroy(trigger);
        }
        rcu_read_unlock();
        return ret;
}

static
void free_lttng_trigger_ht_element_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct lttng_trigger_ht_element,
                        rcu_node));
}

static
int handle_notification_thread_command_unregister_trigger(
                struct notification_thread_state *state,
                struct lttng_trigger *trigger,
                enum lttng_error_code *_cmd_reply)
{
        struct cds_lfht_iter iter;
        struct cds_lfht_node *triggers_ht_node;
        struct lttng_channel_trigger_list *trigger_list;
        struct notification_client_list *client_list;
        struct lttng_trigger_ht_element *trigger_ht_element = NULL;
        struct lttng_condition *condition = lttng_trigger_get_condition(
                        trigger);
        enum lttng_error_code cmd_reply;

        rcu_read_lock();

        cds_lfht_lookup(state->triggers_ht,
                        lttng_condition_hash(condition),
                        match_condition,
                        condition,
                        &iter);
        triggers_ht_node = cds_lfht_iter_get_node(&iter);
        if (!triggers_ht_node) {
                cmd_reply = LTTNG_ERR_TRIGGER_NOT_FOUND;
                goto end;
        } else {
                cmd_reply = LTTNG_OK;
        }

        /* Remove trigger from channel_triggers_ht. */
        cds_lfht_for_each_entry(state->channel_triggers_ht, &iter, trigger_list,
                        channel_triggers_ht_node) {
                struct lttng_trigger_list_element *trigger_element, *tmp;

                cds_list_for_each_entry_safe(trigger_element, tmp,
                                &trigger_list->list, node) {
                        const struct lttng_condition *current_condition =
                                        lttng_trigger_get_const_condition(
                                                trigger_element->trigger);

                        assert(current_condition);
                        if (!lttng_condition_is_equal(condition,
                                        current_condition)) {
                                continue;
                        }

                        DBG("[notification-thread] Removed trigger from channel_triggers_ht");
                        cds_list_del(&trigger_element->node);
                        /* A trigger can only appear once per channel */
                        break;
                }
        }

        /*
         * Remove and release the client list from
         * notification_trigger_clients_ht.
         */
        client_list = get_client_list_from_condition(state, condition);
        assert(client_list);

        /* Put new reference and the hashtable's reference. */
        notification_client_list_put(client_list);
        notification_client_list_put(client_list);
        client_list = NULL;

        /* Remove trigger from triggers_ht. */
        trigger_ht_element = caa_container_of(triggers_ht_node,
                        struct lttng_trigger_ht_element, node);
        cds_lfht_del(state->triggers_ht, triggers_ht_node);

        /* Release the ownership of the trigger. */
        lttng_trigger_destroy(trigger_ht_element->trigger);
        call_rcu(&trigger_ht_element->rcu_node, free_lttng_trigger_ht_element_rcu);
end:
        rcu_read_unlock();
        if (_cmd_reply) {
                *_cmd_reply = cmd_reply;
        }
        return 0;
}

/* Returns 0 on success, 1 on exit requested, negative value on error. */
int handle_notification_thread_command(
                struct notification_thread_handle *handle,
                struct notification_thread_state *state)
{
        int ret;
        uint64_t counter;
        struct notification_thread_command *cmd;

        /* Read the event pipe to put it back into a quiescent state. */
        ret = lttng_read(lttng_pipe_get_readfd(handle->cmd_queue.event_pipe), &counter,
                        sizeof(counter));
        if (ret != sizeof(counter)) {
                goto error;
        }

        pthread_mutex_lock(&handle->cmd_queue.lock);
        cmd = cds_list_first_entry(&handle->cmd_queue.list,
                        struct notification_thread_command, cmd_list_node);
        cds_list_del(&cmd->cmd_list_node);
        pthread_mutex_unlock(&handle->cmd_queue.lock);
        switch (cmd->type) {
        case NOTIFICATION_COMMAND_TYPE_REGISTER_TRIGGER:
                DBG("[notification-thread] Received register trigger command");
                ret = handle_notification_thread_command_register_trigger(
                                state, cmd->parameters.trigger,
                                &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_UNREGISTER_TRIGGER:
                DBG("[notification-thread] Received unregister trigger command");
                ret = handle_notification_thread_command_unregister_trigger(
                                state, cmd->parameters.trigger,
                                &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_ADD_CHANNEL:
                DBG("[notification-thread] Received add channel command");
                ret = handle_notification_thread_command_add_channel(
                                state,
                                cmd->parameters.add_channel.session.name,
                                cmd->parameters.add_channel.session.uid,
                                cmd->parameters.add_channel.session.gid,
                                cmd->parameters.add_channel.channel.name,
                                cmd->parameters.add_channel.channel.domain,
                                cmd->parameters.add_channel.channel.key,
                                cmd->parameters.add_channel.channel.capacity,
                                &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_REMOVE_CHANNEL:
                DBG("[notification-thread] Received remove channel command");
                ret = handle_notification_thread_command_remove_channel(
                                state, cmd->parameters.remove_channel.key,
                                cmd->parameters.remove_channel.domain,
                                &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING:
        case NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_COMPLETED:
                DBG("[notification-thread] Received session rotation %s command",
                                cmd->type == NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING ?
                                "ongoing" : "completed");
                ret = handle_notification_thread_command_session_rotation(
                                state,
                                cmd->type,
                                cmd->parameters.session_rotation.session_name,
                                cmd->parameters.session_rotation.uid,
                                cmd->parameters.session_rotation.gid,
                                cmd->parameters.session_rotation.trace_archive_chunk_id,
                                cmd->parameters.session_rotation.location,
                                &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_QUIT:
                DBG("[notification-thread] Received quit command");
                cmd->reply_code = LTTNG_OK;
                ret = 1;
                goto end;
        default:
                ERR("[notification-thread] Unknown internal command received");
                goto error_unlock;
        }

        if (ret) {
                goto error_unlock;
        }
end:
        if (cmd->is_async) {
                free(cmd);
                cmd = NULL;
        } else {
                lttng_waiter_wake_up(&cmd->reply_waiter);
        }
        return ret;
error_unlock:
        /* Wake-up and return a fatal error to the calling thread. */
        lttng_waiter_wake_up(&cmd->reply_waiter);
        cmd->reply_code = LTTNG_ERR_FATAL;
error:
        /* Indicate a fatal error to the caller. */
        return -1;
}

static
int socket_set_non_blocking(int socket)
{
        int ret, flags;

        /* Set the pipe as non-blocking. */
        ret = fcntl(socket, F_GETFL, 0);
        if (ret == -1) {
                PERROR("fcntl get socket flags");
                goto end;
        }
        flags = ret;

        ret = fcntl(socket, F_SETFL, flags | O_NONBLOCK);
        if (ret == -1) {
                PERROR("fcntl set O_NONBLOCK socket flag");
                goto end;
        }
        DBG("Client socket (fd = %i) set as non-blocking", socket);
end:
        return ret;
}

/* Client lock must be acquired by caller. */
static
int client_reset_inbound_state(struct notification_client *client)
{
        int ret;

        ASSERT_LOCKED(client->lock);

        ret = lttng_dynamic_buffer_set_size(
                        &client->communication.inbound.buffer, 0);
        assert(!ret);

        client->communication.inbound.bytes_to_receive =
                        sizeof(struct lttng_notification_channel_message);
        client->communication.inbound.msg_type =
                        LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNKNOWN;
        LTTNG_SOCK_SET_UID_CRED(&client->communication.inbound.creds, -1);
        LTTNG_SOCK_SET_GID_CRED(&client->communication.inbound.creds, -1);
        ret = lttng_dynamic_buffer_set_size(
                        &client->communication.inbound.buffer,
                        client->communication.inbound.bytes_to_receive);
        return ret;
}

int handle_notification_thread_client_connect(
                struct notification_thread_state *state)
{
        int ret;
        struct notification_client *client;

        DBG("[notification-thread] Handling new notification channel client connection");

        client = zmalloc(sizeof(*client));
        if (!client) {
                /* Fatal error. */
                ret = -1;
                goto error;
        }
        pthread_mutex_init(&client->lock, NULL);
        client->id = state->next_notification_client_id++;
        CDS_INIT_LIST_HEAD(&client->condition_list);
        lttng_dynamic_buffer_init(&client->communication.inbound.buffer);
        lttng_dynamic_buffer_init(&client->communication.outbound.buffer);
        client->communication.inbound.expect_creds = true;

        pthread_mutex_lock(&client->lock);
        ret = client_reset_inbound_state(client);
        pthread_mutex_unlock(&client->lock);
        if (ret) {
                ERR("[notification-thread] Failed to reset client communication's inbound state");
                ret = 0;
                goto error;
        }

        ret = lttcomm_accept_unix_sock(state->notification_channel_socket);
        if (ret < 0) {
                ERR("[notification-thread] Failed to accept new notification channel client connection");
                ret = 0;
                goto error;
        }

        client->socket = ret;

        ret = socket_set_non_blocking(client->socket);
        if (ret) {
                ERR("[notification-thread] Failed to set new notification channel client connection socket as non-blocking");
                goto error;
        }

        ret = lttcomm_setsockopt_creds_unix_sock(client->socket);
        if (ret < 0) {
                ERR("[notification-thread] Failed to set socket options on new notification channel client socket");
                ret = 0;
                goto error;
        }

        ret = lttng_poll_add(&state->events, client->socket,
                        LPOLLIN | LPOLLERR |
                        LPOLLHUP | LPOLLRDHUP);
        if (ret < 0) {
                ERR("[notification-thread] Failed to add notification channel client socket to poll set");
                ret = 0;
                goto error;
        }
        DBG("[notification-thread] Added new notification channel client socket (%i) to poll set",
                        client->socket);

        rcu_read_lock();
        cds_lfht_add(state->client_socket_ht,
                        hash_client_socket(client->socket),
                        &client->client_socket_ht_node);
        cds_lfht_add(state->client_id_ht,
                        hash_client_id(client->id),
                        &client->client_id_ht_node);
        rcu_read_unlock();

        return ret;
error:
        notification_client_destroy(client, state);
        return ret;
}

/* RCU read-lock must be held by the caller. */
/* Client lock must be held by the caller */
static
int notification_thread_client_disconnect(
                struct notification_client *client,
                struct notification_thread_state *state)
{
        int ret;
        struct lttng_condition_list_element *condition_list_element, *tmp;

        /* Acquire the client lock to disable its communication atomically. */
        client->communication.active = false;
        ret = lttng_poll_del(&state->events, client->socket);
        if (ret) {
                ERR("[notification-thread] Failed to remove client socket %d from poll set",
                                client->socket);
        }

        cds_lfht_del(state->client_socket_ht, &client->client_socket_ht_node);
        cds_lfht_del(state->client_id_ht, &client->client_id_ht_node);

        /* Release all conditions to which the client was subscribed. */
        cds_list_for_each_entry_safe(condition_list_element, tmp,
                        &client->condition_list, node) {
                (void) notification_thread_client_unsubscribe(client,
                                condition_list_element->condition, state, NULL);
        }

        /*
         * Client no longer accessible to other threads (through the
         * client lists).
         */
        notification_client_destroy(client, state);
        return ret;
}

int handle_notification_thread_client_disconnect(
                int client_socket, struct notification_thread_state *state)
{
        int ret = 0;
        struct notification_client *client;

        rcu_read_lock();
        DBG("[notification-thread] Closing client connection (socket fd = %i)",
                        client_socket);
        client = get_client_from_socket(client_socket, state);
        if (!client) {
                /* Internal state corruption, fatal error. */
                ERR("[notification-thread] Unable to find client (socket fd = %i)",
                                client_socket);
                ret = -1;
                goto end;
        }

        pthread_mutex_lock(&client->lock);
        ret = notification_thread_client_disconnect(client, state);
        pthread_mutex_unlock(&client->lock);
end:
        rcu_read_unlock();
        return ret;
}

int handle_notification_thread_client_disconnect_all(
                struct notification_thread_state *state)
{
        struct cds_lfht_iter iter;
        struct notification_client *client;
        bool error_encoutered = false;

        rcu_read_lock();
        DBG("[notification-thread] Closing all client connections");
        cds_lfht_for_each_entry(state->client_socket_ht, &iter, client,
                        client_socket_ht_node) {
                int ret;

                pthread_mutex_lock(&client->lock);
                ret = notification_thread_client_disconnect(
                                client, state);
                pthread_mutex_unlock(&client->lock);
                if (ret) {
                        error_encoutered = true;
                }
        }
        rcu_read_unlock();
        return error_encoutered ? 1 : 0;
}

int handle_notification_thread_trigger_unregister_all(
                struct notification_thread_state *state)
{
        bool error_occurred = false;
        struct cds_lfht_iter iter;
        struct lttng_trigger_ht_element *trigger_ht_element;

        rcu_read_lock();
        cds_lfht_for_each_entry(state->triggers_ht, &iter, trigger_ht_element,
                        node) {
                int ret = handle_notification_thread_command_unregister_trigger(
                                state, trigger_ht_element->trigger, NULL);
                if (ret) {
                        error_occurred = true;
                }
        }
        rcu_read_unlock();
        return error_occurred ? -1 : 0;
}

static
int client_handle_transmission_status(
                struct notification_client *client,
                enum client_transmission_status transmission_status,
                struct notification_thread_state *state)
{
        int ret = 0;

        switch (transmission_status) {
        case CLIENT_TRANSMISSION_STATUS_COMPLETE:
                ret = lttng_poll_mod(&state->events, client->socket,
                                CLIENT_POLL_MASK_IN);
                if (ret) {
                        goto end;
                }

                client->communication.outbound.queued_command_reply = false;
                client->communication.outbound.dropped_notification = false;
                break;
        case CLIENT_TRANSMISSION_STATUS_QUEUED:
                /*
                 * We want to be notified whenever there is buffer space
                 * available to send the rest of the payload.
                 */
                ret = lttng_poll_mod(&state->events, client->socket,
                                CLIENT_POLL_MASK_IN_OUT);
                if (ret) {
                        goto end;
                }
                break;
        case CLIENT_TRANSMISSION_STATUS_FAIL:
                ret = notification_thread_client_disconnect(client, state);
                if (ret) {
                        goto end;
                }
                break;
        case CLIENT_TRANSMISSION_STATUS_ERROR:
                ret = -1;
                goto end;
        default:
                abort();
        }
end:
        return ret;
}

/* Client lock must be acquired by caller. */
static
enum client_transmission_status client_flush_outgoing_queue(
                struct notification_client *client,
                struct notification_thread_state *state)
{
        ssize_t ret;
        size_t to_send_count;
        enum client_transmission_status status;

        ASSERT_LOCKED(client->lock);

        assert(client->communication.outbound.buffer.size != 0);
        to_send_count = client->communication.outbound.buffer.size;
        DBG("[notification-thread] Flushing client (socket fd = %i) outgoing queue",
                        client->socket);

        ret = lttcomm_send_unix_sock_non_block(client->socket,
                        client->communication.outbound.buffer.data,
                        to_send_count);
        if ((ret >= 0 && ret < to_send_count)) {
                DBG("[notification-thread] Client (socket fd = %i) outgoing queue could not be completely flushed",
                                client->socket);
                to_send_count -= max(ret, 0);

                memcpy(client->communication.outbound.buffer.data,
                                client->communication.outbound.buffer.data +
                                client->communication.outbound.buffer.size - to_send_count,
                                to_send_count);
                ret = lttng_dynamic_buffer_set_size(
                                &client->communication.outbound.buffer,
                                to_send_count);
                if (ret) {
                        status = CLIENT_TRANSMISSION_STATUS_ERROR;
                        goto error;
                }
                status = CLIENT_TRANSMISSION_STATUS_QUEUED;
        } else if (ret < 0) {
                /* Generic error, disconnect the client. */
                ERR("[notification-thread] Failed to flush outgoing queue, disconnecting client (socket fd = %i)",
                                client->socket);
                status = CLIENT_TRANSMISSION_STATUS_FAIL;
        } else {
                /* No error and flushed the queue completely. */
                ret = lttng_dynamic_buffer_set_size(
                                &client->communication.outbound.buffer, 0);
                if (ret) {
                        status = CLIENT_TRANSMISSION_STATUS_ERROR;
                        goto error;
                }
                status = CLIENT_TRANSMISSION_STATUS_COMPLETE;
        }

        ret = client_handle_transmission_status(client, status, state);
        if (ret) {
                goto error;
        }

        return 0;
error:
        return -1;
}

/* Client lock must be acquired by caller. */
static
int client_send_command_reply(struct notification_client *client,
                struct notification_thread_state *state,
                enum lttng_notification_channel_status status)
{
        int ret;
        struct lttng_notification_channel_command_reply reply = {
                .status = (int8_t) status,
        };
        struct lttng_notification_channel_message msg = {
                .type = (int8_t) LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_COMMAND_REPLY,
                .size = sizeof(reply),
        };
        char buffer[sizeof(msg) + sizeof(reply)];

        ASSERT_LOCKED(client->lock);

        if (client->communication.outbound.queued_command_reply) {
                /* Protocol error. */
                goto error;
        }

        memcpy(buffer, &msg, sizeof(msg));
        memcpy(buffer + sizeof(msg), &reply, sizeof(reply));
        DBG("[notification-thread] Send command reply (%i)", (int) status);

        /* Enqueue buffer to outgoing queue and flush it. */
        ret = lttng_dynamic_buffer_append(
                        &client->communication.outbound.buffer,
                        buffer, sizeof(buffer));
        if (ret) {
                goto error;
        }

        ret = client_flush_outgoing_queue(client, state);
        if (ret) {
                goto error;
        }

        if (client->communication.outbound.buffer.size != 0) {
                /* Queue could not be emptied. */
                client->communication.outbound.queued_command_reply = true;
        }

        return 0;
error:
        return -1;
}

static
int client_handle_message_unknown(struct notification_client *client,
                struct notification_thread_state *state)
{
        int ret;

        pthread_mutex_lock(&client->lock);

        /*
         * Receiving message header. The function will be called again
         * once the rest of the message as been received and can be
         * interpreted.
         */
        const struct lttng_notification_channel_message *msg;

        assert(sizeof(*msg) == client->communication.inbound.buffer.size);
        msg = (const struct lttng_notification_channel_message *)
                              client->communication.inbound.buffer.data;

        if (msg->size == 0 ||
                        msg->size > DEFAULT_MAX_NOTIFICATION_CLIENT_MESSAGE_PAYLOAD_SIZE) {
                ERR("[notification-thread] Invalid notification channel message: length = %u",
                                msg->size);
                ret = -1;
                goto end;
        }

        switch (msg->type) {
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE:
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNSUBSCRIBE:
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE:
                break;
        default:
                ret = -1;
                ERR("[notification-thread] Invalid notification channel message: unexpected message type");
                goto end;
        }

        client->communication.inbound.bytes_to_receive = msg->size;
        client->communication.inbound.msg_type =
                        (enum lttng_notification_channel_message_type) msg->type;
        ret = lttng_dynamic_buffer_set_size(
                        &client->communication.inbound.buffer, msg->size);
end:
        pthread_mutex_unlock(&client->lock);
        return ret;
}

static
int client_handle_message_handshake(struct notification_client *client,
                struct notification_thread_state *state)
{
        int ret;
        struct lttng_notification_channel_command_handshake *handshake_client;
        const struct lttng_notification_channel_command_handshake handshake_reply = {
                        .major = LTTNG_NOTIFICATION_CHANNEL_VERSION_MAJOR,
                        .minor = LTTNG_NOTIFICATION_CHANNEL_VERSION_MINOR,
        };
        const struct lttng_notification_channel_message msg_header = {
                        .type = LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE,
                        .size = sizeof(handshake_reply),
        };
        enum lttng_notification_channel_status status =
                        LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;
        char send_buffer[sizeof(msg_header) + sizeof(handshake_reply)];

        pthread_mutex_lock(&client->lock);

        memcpy(send_buffer, &msg_header, sizeof(msg_header));
        memcpy(send_buffer + sizeof(msg_header), &handshake_reply,
                        sizeof(handshake_reply));

        handshake_client =
                        (struct lttng_notification_channel_command_handshake *)
                                        client->communication.inbound.buffer
                                                        .data;
        client->major = handshake_client->major;
        client->minor = handshake_client->minor;
        if (!client->communication.inbound.creds_received) {
                ERR("[notification-thread] No credentials received from client");
                ret = -1;
                goto end;
        }

        client->uid = LTTNG_SOCK_GET_UID_CRED(
                        &client->communication.inbound.creds);
        client->gid = LTTNG_SOCK_GET_GID_CRED(
                        &client->communication.inbound.creds);
        DBG("[notification-thread] Received handshake from client (uid = %u, gid = %u) with version %i.%i",
                        client->uid, client->gid, (int) client->major,
                        (int) client->minor);

        if (handshake_client->major !=
                        LTTNG_NOTIFICATION_CHANNEL_VERSION_MAJOR) {
                status = LTTNG_NOTIFICATION_CHANNEL_STATUS_UNSUPPORTED_VERSION;
        }

        ret = lttng_dynamic_buffer_append(
                        &client->communication.outbound.buffer, send_buffer,
                        sizeof(send_buffer));
        if (ret) {
                ERR("[notification-thread] Failed to send protocol version to notification channel client");
                goto end;
        }

        client->validated = true;
        client->communication.active = true;

        ret = client_flush_outgoing_queue(client, state);
        if (ret) {
                goto end;
        }

        ret = client_send_command_reply(client, state, status);
        if (ret) {
                ERR("[notification-thread] Failed to send reply to notification channel client");
                goto end;
        }

        /* Set reception state to receive the next message header. */
        ret = client_reset_inbound_state(client);
        if (ret) {
                ERR("[notification-thread] Failed to reset client communication's inbound state");
                goto end;
        }

end:
        pthread_mutex_unlock(&client->lock);
        return ret;
}

static
int client_handle_message_subscription(
                struct notification_client *client,
                enum lttng_notification_channel_message_type msg_type,
                struct notification_thread_state *state)
{
        int ret;
        struct lttng_condition *condition;
        enum lttng_notification_channel_status status =
                        LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;
        struct lttng_payload_view condition_view =
                        lttng_payload_view_from_dynamic_buffer(
                                        &client->communication.inbound.buffer,
                                        0, -1);
        size_t expected_condition_size;

        pthread_mutex_lock(&client->lock);
        expected_condition_size = client->communication.inbound.buffer.size;
        pthread_mutex_unlock(&client->lock);

        ret = lttng_condition_create_from_payload(&condition_view, &condition);
        if (ret != expected_condition_size) {
                ERR("[notification-thread] Malformed condition received from client");
                goto end;
        }

        if (msg_type == LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE) {
                ret = notification_thread_client_subscribe(
                                client, condition, state, &status);
        } else {
                ret = notification_thread_client_unsubscribe(
                                client, condition, state, &status);
        }
        if (ret) {
                goto end;
        }

        pthread_mutex_lock(&client->lock);
        ret = client_send_command_reply(client, state, status);
        if (ret) {
                ERR("[notification-thread] Failed to send reply to notification channel client");
                goto end_unlock;
        }

        /* Set reception state to receive the next message header. */
        ret = client_reset_inbound_state(client);
        if (ret) {
                ERR("[notification-thread] Failed to reset client communication's inbound state");
                goto end_unlock;
        }

end_unlock:
        pthread_mutex_unlock(&client->lock);
end:
        return ret;
}

static
int client_dispatch_message(struct notification_client *client,
                struct notification_thread_state *state)
{
        int ret = 0;

        if (client->communication.inbound.msg_type !=
                        LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE &&
                        client->communication.inbound.msg_type !=
                                LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNKNOWN &&
                        !client->validated) {
                WARN("[notification-thread] client attempted a command before handshake");
                ret = -1;
                goto end;
        }

        switch (client->communication.inbound.msg_type) {
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNKNOWN:
        {
                ret = client_handle_message_unknown(client, state);
                break;
        }
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE:
        {
                ret = client_handle_message_handshake(client, state);
                break;
        }
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE:
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNSUBSCRIBE:
        {
                ret = client_handle_message_subscription(client,
                                client->communication.inbound.msg_type, state);
                break;
        }
        default:
                abort();
        }
end:
        return ret;
}

/* Incoming data from client. */
int handle_notification_thread_client_in(
                struct notification_thread_state *state, int socket)
{
        int ret = 0;
        struct notification_client *client;
        ssize_t recv_ret;
        size_t offset;
        bool message_is_complete = false;

        client = get_client_from_socket(socket, state);
        if (!client) {
                /* Internal error, abort. */
                ret = -1;
                goto end;
        }

        pthread_mutex_lock(&client->lock);
        offset = client->communication.inbound.buffer.size -
                        client->communication.inbound.bytes_to_receive;
        if (client->communication.inbound.expect_creds) {
                recv_ret = lttcomm_recv_creds_unix_sock(socket,
                                client->communication.inbound.buffer.data + offset,
                                client->communication.inbound.bytes_to_receive,
                                &client->communication.inbound.creds);
                if (recv_ret > 0) {
                        client->communication.inbound.expect_creds = false;
                        client->communication.inbound.creds_received = true;
                }
        } else {
                recv_ret = lttcomm_recv_unix_sock_non_block(socket,
                                client->communication.inbound.buffer.data + offset,
                                client->communication.inbound.bytes_to_receive);
        }
        if (recv_ret >= 0) {
                client->communication.inbound.bytes_to_receive -= recv_ret;
                message_is_complete = client->communication.inbound
                                                      .bytes_to_receive == 0;
        }
        pthread_mutex_unlock(&client->lock);
        if (recv_ret < 0) {
                goto error_disconnect_client;
        }

        if (message_is_complete) {
                ret = client_dispatch_message(client, state);
                if (ret) {
                        /*
                         * Only returns an error if this client must be
                         * disconnected.
                         */
                        goto error_disconnect_client;
                }
        }
end:
        return ret;
error_disconnect_client:
        pthread_mutex_lock(&client->lock);
        ret = notification_thread_client_disconnect(client, state);
        pthread_mutex_unlock(&client->lock);
        return ret;
}

/* Client ready to receive outgoing data. */
int handle_notification_thread_client_out(
                struct notification_thread_state *state, int socket)
{
        int ret;
        struct notification_client *client;

        client = get_client_from_socket(socket, state);
        if (!client) {
                /* Internal error, abort. */
                ret = -1;
                goto end;
        }

        pthread_mutex_lock(&client->lock);
        ret = client_flush_outgoing_queue(client, state);
        pthread_mutex_unlock(&client->lock);
        if (ret) {
                goto end;
        }
end:
        return ret;
}

static
bool evaluate_buffer_usage_condition(const struct lttng_condition *condition,
                const struct channel_state_sample *sample,
                uint64_t buffer_capacity)
{
        bool result = false;
        uint64_t threshold;
        enum lttng_condition_type condition_type;
        const struct lttng_condition_buffer_usage *use_condition = container_of(
                        condition, struct lttng_condition_buffer_usage,
                        parent);

        if (use_condition->threshold_bytes.set) {
                threshold = use_condition->threshold_bytes.value;
        } else {
                /*
                 * Threshold was expressed as a ratio.
                 *
                 * TODO the threshold (in bytes) of conditions expressed
                 * as a ratio of total buffer size could be cached to
                 * forego this double-multiplication or it could be performed
                 * as fixed-point math.
                 *
                 * Note that caching should accommodates the case where the
                 * condition applies to multiple channels (i.e. don't assume
                 * that all channels matching my_chann* have the same size...)
                 */
                threshold = (uint64_t) (use_condition->threshold_ratio.value *
                                (double) buffer_capacity);
        }

        condition_type = lttng_condition_get_type(condition);
        if (condition_type == LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW) {
                DBG("[notification-thread] Low buffer usage condition being evaluated: threshold = %" PRIu64 ", highest usage = %" PRIu64,
                                threshold, sample->highest_usage);

                /*
                 * The low condition should only be triggered once _all_ of the
                 * streams in a channel have gone below the "low" threshold.
                 */
                if (sample->highest_usage <= threshold) {
                        result = true;
                }
        } else {
                DBG("[notification-thread] High buffer usage condition being evaluated: threshold = %" PRIu64 ", highest usage = %" PRIu64,
                                threshold, sample->highest_usage);

                /*
                 * For high buffer usage scenarios, we want to trigger whenever
                 * _any_ of the streams has reached the "high" threshold.
                 */
                if (sample->highest_usage >= threshold) {
                        result = true;
                }
        }

        return result;
}

static
bool evaluate_session_consumed_size_condition(
                const struct lttng_condition *condition,
                uint64_t session_consumed_size)
{
        uint64_t threshold;
        const struct lttng_condition_session_consumed_size *size_condition =
                        container_of(condition,
                                struct lttng_condition_session_consumed_size,
                                parent);

        threshold = size_condition->consumed_threshold_bytes.value;
        DBG("[notification-thread] Session consumed size condition being evaluated: threshold = %" PRIu64 ", current size = %" PRIu64,
                        threshold, session_consumed_size);
        return session_consumed_size >= threshold;
}

static
int evaluate_buffer_condition(const struct lttng_condition *condition,
                struct lttng_evaluation **evaluation,
                const struct notification_thread_state *state,
                const struct channel_state_sample *previous_sample,
                const struct channel_state_sample *latest_sample,
                uint64_t previous_session_consumed_total,
                uint64_t latest_session_consumed_total,
                struct channel_info *channel_info)
{
        int ret = 0;
        enum lttng_condition_type condition_type;
        const bool previous_sample_available = !!previous_sample;
        bool previous_sample_result = false;
        bool latest_sample_result;

        condition_type = lttng_condition_get_type(condition);

        switch (condition_type) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                if (caa_likely(previous_sample_available)) {
                        previous_sample_result =
                                evaluate_buffer_usage_condition(condition,
                                        previous_sample, channel_info->capacity);
                }
                latest_sample_result = evaluate_buffer_usage_condition(
                                condition, latest_sample,
                                channel_info->capacity);
                break;
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                if (caa_likely(previous_sample_available)) {
                        previous_sample_result =
                                evaluate_session_consumed_size_condition(
                                        condition,
                                        previous_session_consumed_total);
                }
                latest_sample_result =
                                evaluate_session_consumed_size_condition(
                                        condition,
                                        latest_session_consumed_total);
                break;
        default:
                /* Unknown condition type; internal error. */
                abort();
        }

        if (!latest_sample_result ||
                        (previous_sample_result == latest_sample_result)) {
                /*
                 * Only trigger on a condition evaluation transition.
                 *
                 * NOTE: This edge-triggered logic may not be appropriate for
                 * future condition types.
                 */
                goto end;
        }

        if (!evaluation || !latest_sample_result) {
                goto end;
        }

        switch (condition_type) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                *evaluation = lttng_evaluation_buffer_usage_create(
                                condition_type,
                                latest_sample->highest_usage,
                                channel_info->capacity);
                break;
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                *evaluation = lttng_evaluation_session_consumed_size_create(
                                latest_session_consumed_total);
                break;
        default:
                abort();
        }

        if (!*evaluation) {
                ret = -1;
                goto end;
        }
end:
        return ret;
}

static
int client_enqueue_dropped_notification(struct notification_client *client)
{
        int ret;
        struct lttng_notification_channel_message msg = {
                .type = (int8_t) LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_NOTIFICATION_DROPPED,
                .size = 0,
        };

        ASSERT_LOCKED(client->lock);

        ret = lttng_dynamic_buffer_append(
                        &client->communication.outbound.buffer, &msg,
                        sizeof(msg));
        return ret;
}

/*
 * Permission checks relative to notification channel clients are performed
 * here. Notice how object, client, and trigger credentials are involved in
 * this check.
 *
 * The `object` credentials are the credentials associated with the "subject"
 * of a condition. For instance, a `rotation completed` condition applies
 * to a session. When that condition is met, it will produce an evaluation
 * against a session. Hence, in this case, the `object` credentials are the
 * credentials of the "subject" session.
 *
 * The `trigger` credentials are the credentials of the user that registered the
 * trigger.
 *
 * The `client` credentials are the credentials of the user that created a given
 * notification channel.
 *
 * In terms of visibility, it is expected that non-privilieged users can only
 * register triggers against "their" objects (their own sessions and
 * applications they are allowed to interact with). They can then open a
 * notification channel and subscribe to notifications associated with those
 * triggers.
 *
 * As for privilieged users, they can register triggers against the objects of
 * other users. They can then subscribe to the notifications associated to their
 * triggers. Privilieged users _can't_ subscribe to the notifications of
 * triggers owned by other users; they must create their own triggers.
 *
 * This is more a concern of usability than security. It would be difficult for
 * a root user reliably subscribe to a specific set of conditions without
 * interference from external users (those could, for instance, unregister
 * their triggers).
 */
static
int send_evaluation_to_clients(const struct lttng_trigger *trigger,
                const struct lttng_evaluation *evaluation,
                struct notification_client_list* client_list,
                struct notification_thread_state *state,
                uid_t object_uid, gid_t object_gid)
{
        int ret = 0;
        struct lttng_payload msg_payload;
        struct notification_client_list_element *client_list_element, *tmp;
        const struct lttng_notification notification = {
                .condition = (struct lttng_condition *) lttng_trigger_get_const_condition(trigger),
                .evaluation = (struct lttng_evaluation *) evaluation,
        };
        struct lttng_notification_channel_message msg_header = {
                .type = (int8_t) LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_NOTIFICATION,
        };
        const struct lttng_credentials *trigger_creds = lttng_trigger_get_credentials(trigger);

        lttng_payload_init(&msg_payload);

        ret = lttng_dynamic_buffer_append(&msg_payload.buffer, &msg_header,
                        sizeof(msg_header));
        if (ret) {
                goto end;
        }

        ret = lttng_notification_serialize(&notification, &msg_payload);
        if (ret) {
                ERR("[notification-thread] Failed to serialize notification");
                ret = -1;
                goto end;
        }

        /* Update payload size. */
        ((struct lttng_notification_channel_message *) msg_payload.buffer.data)
                        ->size = (uint32_t)(
                        msg_payload.buffer.size - sizeof(msg_header));

        pthread_mutex_lock(&client_list->lock);
        cds_list_for_each_entry_safe(client_list_element, tmp,
                        &client_list->list, node) {
                struct notification_client *client =
                                client_list_element->client;

                ret = 0;
                pthread_mutex_lock(&client->lock);
                if (client->uid != object_uid && client->gid != object_gid &&
                                client->uid != 0) {
                        /* Client is not allowed to monitor this channel. */
                        DBG("[notification-thread] Skipping client at it does not have the object permission to receive notification for this trigger");
                        goto unlock_client;
                }

                if (client->uid != trigger_creds->uid && client->gid != trigger_creds->gid) {
                        DBG("[notification-thread] Skipping client at it does not have the permission to receive notification for this trigger");
                        goto unlock_client;
                }

                DBG("[notification-thread] Sending notification to client (fd = %i, %zu bytes)",
                                client->socket, msg_payload.buffer.size);
                if (client->communication.outbound.buffer.size) {
                        /*
                         * Outgoing data is already buffered for this client;
                         * drop the notification and enqueue a "dropped
                         * notification" message if this is the first dropped
                         * notification since the socket spilled-over to the
                         * queue.
                         */
                        DBG("[notification-thread] Dropping notification addressed to client (socket fd = %i)",
                                        client->socket);
                        if (!client->communication.outbound.dropped_notification) {
                                client->communication.outbound.dropped_notification = true;
                                ret = client_enqueue_dropped_notification(
                                                client);
                                if (ret) {
                                        goto unlock_client;
                                }
                        }
                        goto unlock_client;
                }

                ret = lttng_dynamic_buffer_append_buffer(
                                &client->communication.outbound.buffer,
                                &msg_payload.buffer);
                if (ret) {
                        goto unlock_client;
                }

                ret = client_flush_outgoing_queue(client, state);
                if (ret) {
                        goto unlock_client;
                }
unlock_client:
                pthread_mutex_unlock(&client->lock);
                if (ret) {
                        goto end_unlock_list;
                }
        }
        ret = 0;

end_unlock_list:
        pthread_mutex_unlock(&client_list->lock);
end:
        lttng_payload_reset(&msg_payload);
        return ret;
}

int handle_notification_thread_channel_sample(
                struct notification_thread_state *state, int pipe,
                enum lttng_domain_type domain)
{
        int ret = 0;
        struct lttcomm_consumer_channel_monitor_msg sample_msg;
        struct channel_info *channel_info;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct lttng_channel_trigger_list *trigger_list;
        struct lttng_trigger_list_element *trigger_list_element;
        bool previous_sample_available = false;
        struct channel_state_sample previous_sample, latest_sample;
        uint64_t previous_session_consumed_total, latest_session_consumed_total;

        /*
         * The monitoring pipe only holds messages smaller than PIPE_BUF,
         * ensuring that read/write of sampling messages are atomic.
         */
        ret = lttng_read(pipe, &sample_msg, sizeof(sample_msg));
        if (ret != sizeof(sample_msg)) {
                ERR("[notification-thread] Failed to read from monitoring pipe (fd = %i)",
                                pipe);
                ret = -1;
                goto end;
        }

        ret = 0;
        latest_sample.key.key = sample_msg.key;
        latest_sample.key.domain = domain;
        latest_sample.highest_usage = sample_msg.highest;
        latest_sample.lowest_usage = sample_msg.lowest;
        latest_sample.channel_total_consumed = sample_msg.total_consumed;

        rcu_read_lock();

        /* Retrieve the channel's informations */
        cds_lfht_lookup(state->channels_ht,
                        hash_channel_key(&latest_sample.key),
                        match_channel_info,
                        &latest_sample.key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (caa_unlikely(!node)) {
                /*
                 * Not an error since the consumer can push a sample to the pipe
                 * and the rest of the session daemon could notify us of the
                 * channel's destruction before we get a chance to process that
                 * sample.
                 */
                DBG("[notification-thread] Received a sample for an unknown channel from consumerd, key = %" PRIu64 " in %s domain",
                                latest_sample.key.key,
                                domain == LTTNG_DOMAIN_KERNEL ? "kernel" :
                                        "user space");
                goto end_unlock;
        }
        channel_info = caa_container_of(node, struct channel_info,
                        channels_ht_node);
        DBG("[notification-thread] Handling channel sample for channel %s (key = %" PRIu64 ") in session %s (highest usage = %" PRIu64 ", lowest usage = %" PRIu64", total consumed = %" PRIu64")",
                        channel_info->name,
                        latest_sample.key.key,
                        channel_info->session_info->name,
                        latest_sample.highest_usage,
                        latest_sample.lowest_usage,
                        latest_sample.channel_total_consumed);

        previous_session_consumed_total =
                        channel_info->session_info->consumed_data_size;

        /* Retrieve the channel's last sample, if it exists, and update it. */
        cds_lfht_lookup(state->channel_state_ht,
                        hash_channel_key(&latest_sample.key),
                        match_channel_state_sample,
                        &latest_sample.key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (caa_likely(node)) {
                struct channel_state_sample *stored_sample;

                /* Update the sample stored. */
                stored_sample = caa_container_of(node,
                                struct channel_state_sample,
                                channel_state_ht_node);

                memcpy(&previous_sample, stored_sample,
                                sizeof(previous_sample));
                stored_sample->highest_usage = latest_sample.highest_usage;
                stored_sample->lowest_usage = latest_sample.lowest_usage;
                stored_sample->channel_total_consumed = latest_sample.channel_total_consumed;
                previous_sample_available = true;

                latest_session_consumed_total =
                                previous_session_consumed_total +
                                (latest_sample.channel_total_consumed - previous_sample.channel_total_consumed);
        } else {
                /*
                 * This is the channel's first sample, allocate space for and
                 * store the new sample.
                 */
                struct channel_state_sample *stored_sample;

                stored_sample = zmalloc(sizeof(*stored_sample));
                if (!stored_sample) {
                        ret = -1;
                        goto end_unlock;
                }

                memcpy(stored_sample, &latest_sample, sizeof(*stored_sample));
                cds_lfht_node_init(&stored_sample->channel_state_ht_node);
                cds_lfht_add(state->channel_state_ht,
                                hash_channel_key(&stored_sample->key),
                                &stored_sample->channel_state_ht_node);

                latest_session_consumed_total =
                                previous_session_consumed_total +
                                latest_sample.channel_total_consumed;
        }

        channel_info->session_info->consumed_data_size =
                        latest_session_consumed_total;

        /* Find triggers associated with this channel. */
        cds_lfht_lookup(state->channel_triggers_ht,
                        hash_channel_key(&latest_sample.key),
                        match_channel_trigger_list,
                        &latest_sample.key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (caa_likely(!node)) {
                goto end_unlock;
        }

        trigger_list = caa_container_of(node, struct lttng_channel_trigger_list,
                        channel_triggers_ht_node);
        cds_list_for_each_entry(trigger_list_element, &trigger_list->list,
                        node) {
                const struct lttng_condition *condition;
                const struct lttng_action *action;
                const struct lttng_trigger *trigger;
                struct notification_client_list *client_list = NULL;
                struct lttng_evaluation *evaluation = NULL;
                bool client_list_is_empty;

                ret = 0;
                trigger = trigger_list_element->trigger;
                condition = lttng_trigger_get_const_condition(trigger);
                assert(condition);
                action = lttng_trigger_get_const_action(trigger);

                /* Notify actions are the only type currently supported. */
                assert(lttng_action_get_type_const(action) ==
                                LTTNG_ACTION_TYPE_NOTIFY);

                /*
                 * Check if any client is subscribed to the result of this
                 * evaluation.
                 */
                client_list = get_client_list_from_condition(state, condition);
                assert(client_list);
                client_list_is_empty = cds_list_empty(&client_list->list);
                if (client_list_is_empty) {
                        /*
                         * No clients interested in the evaluation's result,
                         * skip it.
                         */
                        goto put_list;
                }

                ret = evaluate_buffer_condition(condition, &evaluation, state,
                                previous_sample_available ? &previous_sample : NULL,
                                &latest_sample,
                                previous_session_consumed_total,
                                latest_session_consumed_total,
                                channel_info);
                if (caa_unlikely(ret)) {
                        goto put_list;
                }

                if (caa_likely(!evaluation)) {
                        goto put_list;
                }

                /* Dispatch evaluation result to all clients. */
                ret = send_evaluation_to_clients(trigger_list_element->trigger,
                                evaluation, client_list, state,
                                channel_info->session_info->uid,
                                channel_info->session_info->gid);
                lttng_evaluation_destroy(evaluation);
put_list:
                notification_client_list_put(client_list);
                if (caa_unlikely(ret)) {
                        break;
                }
        }
end_unlock:
        rcu_read_unlock();
end:
        return ret;
}