Fix: consumer data pending for empty streams

We should at least output one packet before a stream can be considered
as readable. So far, for PID buffers, if an application exits at the
wrong timing before a stop waiting for data pending, empty streams could
be visible by a babeltrace executed after data pending incorrectly
returned false.

Fix it by considering a stream for which the consumerd has written 0
bytes to the output as having data pending.

This applies to 2.3-rc and stable-2.2.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>

diff --git a/src/common/consumer.c b/src/common/consumer.c
index 59207da2f2f99f03a27b8a014f682025b03e39f7..7acb8560ca4e07bfdccc89518477dad6a8d79051 100644 (file)
--- a/src/common/consumer.c
+++ b/src/common/consumer.c
@@ -498,6 +498,7 @@ struct lttng_consumer_stream *consumer_allocate_stream(uint64_t channel_key,
        stream->key = stream_key;
        stream->out_fd = -1;
        stream->out_fd_offset = 0;
+       stream->output_written = 0;
        stream->state = state;
        stream->uid = uid;
        stream->gid = gid;
@@ -1475,6 +1476,7 @@ ssize_t lttng_consumer_on_read_subbuffer_mmap(
                                        SYNC_FILE_RANGE_WRITE);
                        stream->out_fd_offset += ret;
                }
+               stream->output_written += ret;
                written += ret;
        }
        lttng_consumer_sync_trace_file(stream, orig_offset);
@@ -1688,6 +1690,7 @@ ssize_t lttng_consumer_on_read_subbuffer_splice(
                                        SYNC_FILE_RANGE_WRITE);
                        stream->out_fd_offset += ret_splice;
                }
+               stream->output_written += ret_splice;
                written += ret_splice;
        }
        lttng_consumer_sync_trace_file(stream, orig_offset);
@@ -3400,6 +3403,15 @@ int consumer_data_pending(uint64_t id)
                 */
                ret = cds_lfht_is_node_deleted(&stream->node.node);
                if (!ret) {
+                       /*
+                        * An empty output file is not valid. We need at least one packet
+                        * generated per stream, even if it contains no event, so it
+                        * contains at least one packet header.
+                        */
+                       if (stream->output_written == 0) {
+                               pthread_mutex_unlock(&stream->lock);
+                               goto data_pending;
+                       }
                        /* Check the stream if there is data in the buffers. */
                        ret = data_pending(stream);
                        if (ret == 1) {

diff --git a/src/common/consumer.h b/src/common/consumer.h
index 2003cbe43003f304c7bf60387416b9a5c8a1da76..91f6b5ca15a6ddbdcbc8f28e641f824e34a4c64d 100644 (file)
--- a/src/common/consumer.h
+++ b/src/common/consumer.h
@@ -221,6 +221,8 @@ struct lttng_consumer_stream {
        int out_fd; /* output file to write the data */
        /* Write position in the output file descriptor */
        off_t out_fd_offset;
+       /* Amount of bytes written to the output */
+       uint64_t output_written;
        enum lttng_consumer_stream_state state;
        int shm_fd_is_copy;
        int data_read;