Fix: relayd: ressource leaks on viewer_stream_create error

Observed issue
==============

When facing failure to open viewer stream chunks in the context of "Fix:
relayd: failure to open chunk files concurrently with session clear",
we observe that the relay daemon triggers an assertion due to a
non-empty session hash table on cleanup.

Cause
=====

viewer_stream_create() does a stream_get(), but without any matching
stream_put() on error. This in turn holds a reference on the ctf_trace,
which holds a reference on the session.

By inspecting the code, we notice that the following ressources can be
leaked on error:

- vstream->stream_file.handle,
- vstream->index_file,
- vstream->stream.

In non-error scenarios, viewer_stream_release() is responsible for
releasing references on the composite objects.

The vstream->stream_file.trace_chunk is not an issue because it is put
in the destroy handler (as well as within the release, before having its
vstream->stream_file.trace_chunk pointer set to NULL).

Solution
========

Properly put references on all objects which are contained by the viewer
stream on error by introducing
viewer_stream_release_composite_objects(), which is used both in the
error path of viewer_stream_create() and in viewer_stream_release().

Note
====

Why not move those "put" operations in viewer_stream_destroy ?

This is done in the release to ensure we put references on composite
objects immediately when our own reference reaches 0, rather than
waiting for a grace period through call_rcu, which could then cause
chained call_rcu callbacks and require multiple invocation of
rcu_barrier on relayd exit to guarantee that all callbacks have been
executed and all ressources properly freed.

Known drawbacks
===============

None.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I864b58eda94ebda5e6faea45c922d4e814a15daa

diff --git a/src/bin/lttng-relayd/viewer-stream.cpp b/src/bin/lttng-relayd/viewer-stream.cpp
index ca0be9a62d200136972ac6922a692a6bb5917526..69671bd7af4552ae6eb1803654692265a1c30cd3 100644 (file)
--- a/src/bin/lttng-relayd/viewer-stream.cpp
+++ b/src/bin/lttng-relayd/viewer-stream.cpp
@@ -20,9 +20,26 @@
 #include "lttng-relayd.h"
 #include "viewer-stream.h"
 
-static void viewer_stream_destroy(struct relay_viewer_stream *vstream)
+static void viewer_stream_release_composite_objects(struct relay_viewer_stream *vstream)
 {
+       if (vstream->stream_file.handle) {
+               fs_handle_close(vstream->stream_file.handle);
+               vstream->stream_file.handle = NULL;
+       }
+       if (vstream->index_file) {
+               lttng_index_file_put(vstream->index_file);
+               vstream->index_file = NULL;
+       }
+       if (vstream->stream) {
+               stream_put(vstream->stream);
+               vstream->stream = NULL;
+       }
        lttng_trace_chunk_put(vstream->stream_file.trace_chunk);
+       vstream->stream_file.trace_chunk = NULL;
+}
+
+static void viewer_stream_destroy(struct relay_viewer_stream *vstream)
+{
        free(vstream->path_name);
        free(vstream->channel_name);
        free(vstream);
@@ -198,6 +215,8 @@ struct relay_viewer_stream *viewer_stream_create(struct relay_stream *stream,
 
 error:
        if (vstream) {
+               /* Not using `put` since vstream is assumed to be published. */
+               viewer_stream_release_composite_objects(vstream);
                viewer_stream_destroy(vstream);
        }
        return NULL;
@@ -221,23 +240,8 @@ static void viewer_stream_release(struct urcu_ref *ref)
        if (vstream->stream->is_metadata) {
                rcu_assign_pointer(vstream->stream->trace->viewer_metadata_stream, NULL);
        }
-
        viewer_stream_unpublish(vstream);
-
-       if (vstream->stream_file.handle) {
-               fs_handle_close(vstream->stream_file.handle);
-               vstream->stream_file.handle = NULL;
-       }
-       if (vstream->index_file) {
-               lttng_index_file_put(vstream->index_file);
-               vstream->index_file = NULL;
-       }
-       if (vstream->stream) {
-               stream_put(vstream->stream);
-               vstream->stream = NULL;
-       }
-       lttng_trace_chunk_put(vstream->stream_file.trace_chunk);
-       vstream->stream_file.trace_chunk = NULL;
+       viewer_stream_release_composite_objects(vstream);
        call_rcu(&vstream->rcu_node, viewer_stream_destroy_rcu);
 }