Fix: notification thread: RCU-safe reclaim of hash table nodes

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

/*
 * Copyright (C) 2017 - Jérémie Galarneau <jeremie.galarneau@efficios.com>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License, version 2 only, as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc., 51
 * Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#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.h>
#include <lttng/notification/notification-internal.h>
#include <lttng/condition/condition-internal.h>
#include <lttng/condition/buffer-usage-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)

struct lttng_trigger_list_element {
        struct lttng_trigger *trigger;
        struct cds_list_head node;
};

struct lttng_channel_trigger_list {
        struct channel_key channel_key;
        struct cds_list_head list;
        struct cds_lfht_node channel_triggers_ht_node;
        /* call_rcu delayed 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;
};

struct notification_client_list {
        struct lttng_trigger *trigger;
        struct cds_list_head list;
        struct cds_lfht_node notification_trigger_ht_node;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

struct notification_client {
        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 {
                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;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

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

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

        client = caa_container_of(node, struct notification_client,
                        client_socket_ht_node);

        return !!(client->socket == socket);
}

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_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(struct cds_lfht_node *node, const void *key)
{
        struct lttng_trigger *trigger_key = (struct lttng_trigger *) key;
        struct notification_client_list *client_list;
        struct lttng_condition *condition;
        struct lttng_condition *condition_key = lttng_trigger_get_condition(
                        trigger_key);

        assert(condition_key);

        client_list = caa_container_of(node, struct notification_client_list,
                        notification_trigger_ht_node);
        condition = lttng_trigger_get_condition(client_list->trigger);

        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;
        struct lttng_condition *condition;

        assert(condition_key);

        client_list = caa_container_of(node, struct notification_client_list,
                        notification_trigger_ht_node);
        condition = lttng_trigger_get_condition(client_list->trigger);

        return !!lttng_condition_is_equal(condition_key, condition);
}

static
unsigned long lttng_condition_buffer_usage_hash(
        struct lttng_condition *_condition)
{
        unsigned long hash = 0;
        struct lttng_condition_buffer_usage *condition;

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

        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;
}

/*
 * 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(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);
        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
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_name) {
                free(channel_info->session_name);
        }
        if (channel_info->channel_name) {
                free(channel_info->channel_name);
        }
        call_rcu(&channel_info->rcu_node, free_channel_info_rcu);
}

static
struct channel_info *channel_info_copy(struct channel_info *channel_info)
{
        struct channel_info *copy = zmalloc(sizeof(*channel_info));

        assert(channel_info);
        assert(channel_info->session_name);
        assert(channel_info->channel_name);

        if (!copy) {
                goto end;
        }

        memcpy(copy, channel_info, sizeof(*channel_info));
        copy->session_name = NULL;
        copy->channel_name = NULL;

        copy->session_name = strdup(channel_info->session_name);
        if (!copy->session_name) {
                goto error;
        }
        copy->channel_name = strdup(channel_info->channel_name);
        if (!copy->channel_name) {
                goto error;
        }
        cds_lfht_node_init(&channel_info->channels_ht_node);
end:
        return copy;
error:
        channel_info_destroy(copy);
        return NULL;
}

/* This function must be called with the RCU read lock held. */
static
int evaluate_condition_for_client(struct lttng_trigger *trigger,
                struct lttng_condition *condition,
                struct notification_client *client,
                struct notification_thread_state *state)
{
        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;
        struct lttng_evaluation *evaluation = NULL;
        struct notification_client_list client_list = { 0 };
        struct notification_client_list_element client_list_element = { 0 };

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

        /* Find the channel associated with the trigger. */
        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) {
                        struct lttng_condition *current_condition =
                                lttng_trigger_get_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 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_condition(condition, &evaluation, state, NULL,
                        last_sample, channel_info->capacity);
        if (ret) {
                WARN("[notification-thread] Fatal error occurred while evaluating a newly subscribed-to condition");
                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_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, channel_info->uid, channel_info->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 cds_lfht_iter iter;
        struct cds_lfht_node *node;
        struct notification_client_list *client_list;
        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;
        }

        rcu_read_lock();

        /*
         * 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);

        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) {
                free(client_list_element);
                goto end_unlock;
        }

        client_list = caa_container_of(node, struct notification_client_list,
                        notification_trigger_ht_node);
        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_unlock;
        }

        /*
         * 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);
        cds_list_add(&client_list_element->node, &client_list->list);
end_unlock:
        rcu_read_unlock();
end:
        if (_status) {
                *_status = status;
        }
        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 cds_lfht_iter iter;
        struct cds_lfht_node *node;
        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.
         */
        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) {
                goto end_unlock;
        }

        client_list = caa_container_of(node, struct notification_client_list,
                        notification_trigger_ht_node);
        cds_list_for_each_entry_safe(client_list_element, client_tmp,
                        &client_list->list, node) {
                if (client_list_element->client->socket != client->socket) {
                        continue;
                }
                cds_list_del(&client_list_element->node);
                free(client_list_element);
                break;
        }
end_unlock:
        rcu_read_unlock();
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)
{
        struct lttng_condition_list_element *condition_list_element, *tmp;

        if (!client) {
                return;
        }

        /* 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);
        }

        if (client->socket >= 0) {
                (void) lttcomm_close_unix_sock(client->socket);
        }
        lttng_dynamic_buffer_reset(&client->communication.inbound.buffer);
        lttng_dynamic_buffer_reset(&client->communication.outbound.buffer);
        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_key_ulong((void *) (unsigned long) socket, lttng_ht_seed),
                        match_client,
                        (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 trigger_applies_to_channel(struct lttng_trigger *trigger,
                struct channel_info *info)
{
        enum lttng_condition_status status;
        struct lttng_condition *condition;
        const char *trigger_session_name = NULL;
        const char *trigger_channel_name = NULL;
        enum lttng_domain_type trigger_domain;

        condition = lttng_trigger_get_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:
                break;
        default:
                goto fail;
        }

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

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

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

        if (strcmp(info->session_name, trigger_session_name)) {
                goto fail;
        }
        if (strcmp(info->channel_name, trigger_channel_name)) {
                goto fail;
        }

        return true;
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;
}

static
int handle_notification_thread_command_add_channel(
        struct notification_thread_state *state,
        struct channel_info *channel_info,
        enum lttng_error_code *cmd_result)
{
        struct cds_list_head trigger_list;
        struct channel_info *new_channel_info;
        struct channel_key *channel_key;
        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;

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

        CDS_INIT_LIST_HEAD(&trigger_list);

        new_channel_info = channel_info_copy(channel_info);
        if (!new_channel_info) {
                goto error;
        }

        channel_key = &new_channel_info->key;

        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,
                                channel_info)) {
                        continue;
                }

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

        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_unlock;
        }
        channel_trigger_list->channel_key = *channel_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);

        /* Add channel to the channel_ht which owns the channel_infos. */
        cds_lfht_add(state->channels_ht,
                        hash_channel_key(channel_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(channel_key),
                        &channel_trigger_list->channel_triggers_ht_node);
        rcu_read_unlock();
        *cmd_result = LTTNG_OK;
        return 0;
error_unlock:
        rcu_read_unlock();
error:
        /* Empty trigger list */
        channel_info_destroy(new_channel_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 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(
                                kernel_tracer_fd);
                break;
        }
        default:
                ret = 1;
        }
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,
 *     - 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 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;
        struct channel_info *channel;
        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 = zmalloc(sizeof(*client_list));
        if (!client_list) {
                ret = -1;
                goto error_free_ht_element;
        }
        cds_lfht_node_init(&client_list->notification_trigger_ht_node);
        CDS_INIT_LIST_HEAD(&client_list->list);
        client_list->trigger = trigger;

        /* 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_free_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);
        }

        cds_lfht_add(state->notification_trigger_clients_ht,
                        lttng_condition_hash(condition),
                        &client_list->notification_trigger_ht_node);
        /*
         * Client list ownership transferred to the
         * notification_trigger_clients_ht.
         */
        client_list = NULL;

        /*
         * Add the trigger to list of triggers bound to the channels currently
         * known.
         */
        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 error_free_client_list;
                }
                CDS_INIT_LIST_HEAD(&trigger_list_element->node);
                trigger_list_element->trigger = trigger;
                cds_list_add(&trigger_list_element->node, &trigger_list->list);

                /* A trigger can only apply to one channel. */
                break;
        }

        *cmd_result = LTTNG_OK;
error_free_client_list:
        if (client_list) {
                cds_list_for_each_entry_safe(client_list_element, tmp,
                                &client_list->list, node) {
                        free(client_list_element);
                }
                free(client_list);
        }
error_free_ht_element:
        free(trigger_ht_element);
error:
        if (free_trigger) {
                struct lttng_action *action = lttng_trigger_get_action(trigger);

                lttng_condition_destroy(condition);
                lttng_action_destroy(action);
                lttng_trigger_destroy(trigger);
        }
        rcu_read_unlock();
        return ret;
}

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

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 *node, *triggers_ht_node;
        struct lttng_channel_trigger_list *trigger_list;
        struct notification_client_list *client_list;
        struct notification_client_list_element *client_list_element, *tmp;
        struct lttng_trigger_ht_element *trigger_ht_element = NULL;
        struct lttng_condition *condition = lttng_trigger_get_condition(
                        trigger);
        struct lttng_action *action;
        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) {
                        struct lttng_condition *current_condition =
                                        lttng_trigger_get_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.
         */
        cds_lfht_lookup(state->notification_trigger_clients_ht,
                        lttng_condition_hash(condition),
                        match_client_list,
                        trigger,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        assert(node);
        client_list = caa_container_of(node, struct notification_client_list,
                        notification_trigger_ht_node);
        cds_list_for_each_entry_safe(client_list_element, tmp,
                        &client_list->list, node) {
                free(client_list_element);
        }
        cds_lfht_del(state->notification_trigger_clients_ht, node);
        call_rcu(&client_list->rcu_node, free_notification_client_list_rcu);

        /* 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);

        condition = lttng_trigger_get_condition(trigger_ht_element->trigger);
        lttng_condition_destroy(condition);
        action = lttng_trigger_get_action(trigger_ht_element->trigger);
        lttng_action_destroy(action);
        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 event_fd to put it back into a quiescent state. */
        ret = read(lttng_pipe_get_readfd(handle->cmd_queue.event_pipe), &counter, sizeof(counter));
        if (ret == -1) {
                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);
        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,
                                &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_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:
        cds_list_del(&cmd->cmd_list_node);
        lttng_waiter_wake_up(&cmd->reply_waiter);
        pthread_mutex_unlock(&handle->cmd_queue.lock);
        return ret;
error_unlock:
        /* Wake-up and return a fatal error to the calling thread. */
        lttng_waiter_wake_up(&cmd->reply_waiter);
        pthread_mutex_unlock(&handle->cmd_queue.lock);
        cmd->reply_code = LTTNG_ERR_FATAL;
error:
        /* Indicate a fatal error to the caller. */
        return -1;
}

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

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;
}

static
int client_reset_inbound_state(struct notification_client *client)
{
        int ret;

        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;
        }
        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;
        ret = client_reset_inbound_state(client);
        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);
        rcu_read_unlock();

        return ret;
error:
        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;
        }

        ret = lttng_poll_del(&state->events, client_socket);
        if (ret) {
                ERR("[notification-thread] Failed to remove client socket from poll set");
        }
        cds_lfht_del(state->client_socket_ht,
                        &client->client_socket_ht_node);
        notification_client_destroy(client, state);
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;

                ret = handle_notification_thread_client_disconnect(
                                client->socket, state);
                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;

        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;
                }
        }
        return error_occurred ? -1 : 0;
}

static
int client_flush_outgoing_queue(struct notification_client *client,
                struct notification_thread_state *state)
{
        ssize_t ret;
        size_t to_send_count;

        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 && (errno == EAGAIN || errno == EWOULDBLOCK)) ||
                        (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) {
                        goto error;
                }

                /*
                 * 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 error;
                }
        } else if (ret < 0) {
                /* Generic error, disconnect the client. */
                ERR("[notification-thread] Failed to send flush outgoing queue, disconnecting client (socket fd = %i)",
                                client->socket);
                ret = handle_notification_thread_client_disconnect(
                                client->socket, state);
                if (ret) {
                        goto error;
                }
        } else {
                /* No error and flushed the queue completely. */
                ret = lttng_dynamic_buffer_set_size(
                                &client->communication.outbound.buffer, 0);
                if (ret) {
                        goto error;
                }
                ret = lttng_poll_mod(&state->events, client->socket,
                                CLIENT_POLL_MASK_IN);
                if (ret) {
                        goto error;
                }

                client->communication.outbound.queued_command_reply = false;
                client->communication.outbound.dropped_notification = false;
        }

        return 0;
error:
        return -1;
}

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)];

        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_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:
        {
                /*
                 * 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);
                if (ret) {
                        goto end;
                }
                break;
        }
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE:
        {
                struct lttng_notification_channel_command_handshake *handshake_client;
                struct lttng_notification_channel_command_handshake handshake_reply = {
                        .major = LTTNG_NOTIFICATION_CHANNEL_VERSION_MAJOR,
                        .minor = LTTNG_NOTIFICATION_CHANNEL_VERSION_MINOR,
                };
                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)];

                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;
                }

                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;
                }
                client->validated = true;
                break;
        }
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE:
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNSUBSCRIBE:
        {
                struct lttng_condition *condition;
                enum lttng_notification_channel_status status =
                                LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;
                const struct lttng_buffer_view condition_view =
                                lttng_buffer_view_from_dynamic_buffer(
                                        &client->communication.inbound.buffer,
                                        0, -1);
                size_t expected_condition_size =
                                client->communication.inbound.buffer.size;

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

                if (client->communication.inbound.msg_type ==
                                LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE) {
                        /*
                         * FIXME The current state should be evaluated on
                         * subscription.
                         */
                        ret = notification_thread_client_subscribe(client,
                                        condition, state, &status);
                } else {
                        ret = notification_thread_client_unsubscribe(client,
                                        condition, state, &status);
                }
                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;
                }
                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;

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

        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) {
                goto error_disconnect_client;
        }

        client->communication.inbound.bytes_to_receive -= recv_ret;
        if (client->communication.inbound.bytes_to_receive == 0) {
                ret = client_dispatch_message(client, state);
                if (ret) {
                        /*
                         * Only returns an error if this client must be
                         * disconnected.
                         */
                        goto error_disconnect_client;
                }
        } else {
                goto end;
        }
end:
        return ret;
error_disconnect_client:
        ret = handle_notification_thread_client_disconnect(socket, state);
        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;
        }

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

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

        if (!sample) {
                goto end;
        }

        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 accomodate 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;
                }
        }
end:
        return result;
}

static
int evaluate_condition(struct lttng_condition *condition,
                struct lttng_evaluation **evaluation,
                struct notification_thread_state *state,
                struct channel_state_sample *previous_sample,
                struct channel_state_sample *latest_sample,
                uint64_t buffer_capacity)
{
        int ret = 0;
        enum lttng_condition_type condition_type;
        bool previous_sample_result;
        bool latest_sample_result;

        condition_type = lttng_condition_get_type(condition);
        /* No other condition type supported for the moment. */
        assert(condition_type == LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW ||
                        condition_type == LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);

        previous_sample_result = evaluate_buffer_usage_condition(condition,
                        previous_sample, buffer_capacity);
        latest_sample_result = evaluate_buffer_usage_condition(condition,
                        latest_sample, buffer_capacity);

        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) {
                *evaluation = lttng_evaluation_buffer_usage_create(
                                condition_type,
                                latest_sample->highest_usage,
                                buffer_capacity);
                if (!*evaluation) {
                        ret = -1;
                        goto end;
                }
        }
end:
        return ret;
}

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

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

static
int send_evaluation_to_clients(struct lttng_trigger *trigger,
                struct lttng_evaluation *evaluation,
                struct notification_client_list* client_list,
                struct notification_thread_state *state,
                uid_t channel_uid, gid_t channel_gid)
{
        int ret = 0;
        struct lttng_dynamic_buffer msg_buffer;
        struct notification_client_list_element *client_list_element, *tmp;
        struct lttng_notification *notification;
        struct lttng_condition *condition;
        ssize_t expected_notification_size, notification_size;
        struct lttng_notification_channel_message msg;

        lttng_dynamic_buffer_init(&msg_buffer);

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

        notification = lttng_notification_create(condition, evaluation);
        if (!notification) {
                ret = -1;
                goto end;
        }

        expected_notification_size = lttng_notification_serialize(notification,
                        NULL);
        if (expected_notification_size < 0) {
                ERR("[notification-thread] Failed to get size of serialized notification");
                ret = -1;
                goto end;
        }

        msg.type = (int8_t) LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_NOTIFICATION;
        msg.size = (uint32_t) expected_notification_size;
        ret = lttng_dynamic_buffer_append(&msg_buffer, &msg, sizeof(msg));
        if (ret) {
                goto end;
        }

        ret = lttng_dynamic_buffer_set_size(&msg_buffer,
                        msg_buffer.size + expected_notification_size);
        if (ret) {
                goto end;
        }

        notification_size = lttng_notification_serialize(notification,
                        msg_buffer.data + sizeof(msg));
        if (notification_size != expected_notification_size) {
                ERR("[notification-thread] Failed to serialize notification");
                ret = -1;
                goto end;
        }

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

                if (client->uid != channel_uid && client->gid != channel_gid &&
                                client->uid != 0) {
                        /* Client is not allowed to monitor this channel. */
                        DBG("[notification-thread] Skipping client at it does not have the permission to receive notification for this channel");
                        continue;
                }

                DBG("[notification-thread] Sending notification to client (fd = %i, %zu bytes)",
                                client->socket, msg_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, state);
                                if (ret) {
                                        goto end;
                                }
                        }
                        continue;
                }

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

                ret = client_flush_outgoing_queue(client, state);
                if (ret) {
                        goto end;
                }
        }
        ret = 0;
end:
        lttng_notification_destroy(notification);
        lttng_dynamic_buffer_reset(&msg_buffer);
        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_state_sample previous_sample, latest_sample;
        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;

        /*
         * 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;

        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 (!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")",
                        channel_info->channel_name,
                        latest_sample.key.key,
                        channel_info->session_name,
                        latest_sample.highest_usage,
                        latest_sample.lowest_usage);

        /* 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 (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;
                previous_sample_available = true;
        } 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);
        }

        /* 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 (!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) {
                struct lttng_condition *condition;
                struct lttng_action *action;
                struct lttng_trigger *trigger;
                struct notification_client_list *client_list;
                struct lttng_evaluation *evaluation = NULL;

                trigger = trigger_list_element->trigger;
                condition = lttng_trigger_get_condition(trigger);
                assert(condition);
                action = lttng_trigger_get_action(trigger);

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

                /*
                 * Check if any client is subscribed to the result of this
                 * evaluation.
                 */
                cds_lfht_lookup(state->notification_trigger_clients_ht,
                                lttng_condition_hash(condition),
                                match_client_list,
                                trigger,
                                &iter);
                node = cds_lfht_iter_get_node(&iter);
                assert(node);

                client_list = caa_container_of(node,
                                struct notification_client_list,
                                notification_trigger_ht_node);
                if (cds_list_empty(&client_list->list)) {
                        /*
                         * No clients interested in the evaluation's result,
                         * skip it.
                         */
                        continue;
                }

                ret = evaluate_condition(condition, &evaluation, state,
                                previous_sample_available ? &previous_sample : NULL,
                                &latest_sample, channel_info->capacity);
                if (ret) {
                        goto end_unlock;
                }

                if (!evaluation) {
                        continue;
                }

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