[lttng-dev] [PATCH lttng-tools] Fix: consumer data pending for empty streams

[David Goulet]

Merged!

Mathieu Desnoyers:
> 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 at efficios.com>
> ---
> diff --git a/src/common/consumer.c b/src/common/consumer.c
> index 59207da..e64e2e2 100644
> --- 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 +3403,16 @@ 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 2003cbe..91f6b5c 100644
> --- 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;
>