gstreamer: Fix race conditions in task pause/resume

The task run function races with two other threads that want to resume
the task: the requestCompleted() handler and the buffer-notify signal
handler. If the former queues completed requests or the latter queues
back buffers to the pool, and then resume the task, after the task run
handler checks the queues but before it attemps to pause the task, then
the task may be paused without noticing that more work is available.

The most immediate way to fix this is to take the stream_lock in the
requestCompleted() and buffer-notify signal handlers, or cover the whole
task run handler with the GstLibcameraSrcState lock. This could cause
long delays in the requestCompleted() handler, so that's not a good
option.

Instead, pause the task unconditionally at the beginning of its run
function, and track while processing buffers and requests if the task
needs to be resumed. It may also get resumed externally by the
buffer-notify signal handler or the request completion handler, which
are guaranteed not to race due to the lock taken by the gst_task_pause()
and gst_task_resume() functions.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>

src/gstreamer/gstlibcamerasrc.cpp

@@ -259,6 +259,7 @@ GstLibcameraSrcState::requestCompleted(Request *request)
 int GstLibcameraSrcState::processRequest()
 {
 	std::unique_ptr<RequestWrap> wrap;
+	int err = 0;
 
 	{
 		MutexLocker locker(lock_);
@@ -267,10 +268,13 @@ int GstLibcameraSrcState::processRequest()
 			wrap = std::move(completedRequests_.front());
 			completedRequests_.pop();
 		}
+
+		if (completedRequests_.empty())
+			err = -ENOBUFS;
 	}
 
 	if (!wrap)
-		return -ENODATA;
+		return -ENOBUFS;
 
 	GstFlowReturn ret = GST_FLOW_OK;
 	gst_flow_combiner_reset(src_->flow_combiner);
@@ -310,7 +314,7 @@ int GstLibcameraSrcState::processRequest()
 		return -EPIPE;
 	}
 
-	return 0;
+	return err;
 }
 
 static bool
@@ -380,47 +384,72 @@ gst_libcamera_src_task_run(gpointer user_data)
 	GstLibcameraSrc *self = GST_LIBCAMERA_SRC(user_data);
 	GstLibcameraSrcState *state = self->state;
 
+	/*
+	 * Start by pausing the task. The task may also get resumed by the
+	 * buffer-notify signal when new buffers are queued back to the pool,
+	 * or by the request completion handler when a new request has
+	 * completed.  Both will resume the task after adding the buffers or
+	 * request to their respective lists, which are checked below to decide
+	 * if the task needs to be resumed for another iteration. This is thus
+	 * guaranteed to be race-free, the lock taken by gst_task_pause() and
+	 * gst_task_resume() serves as a memory barrier.
+	 */
+	gst_task_pause(self->task);
+
+	bool doResume = false;
+
 	/*
 	 * Create and queue one request. If no buffers are available the
 	 * function returns -ENOBUFS, which we ignore here as that's not a
 	 * fatal error.
 	 */
 	int ret = state->queueRequest();
-	if (ret == -ENOMEM) {
+	switch (ret) {
+	case 0:
+		/*
+		 * The request was successfully queued, there may be enough
+		 * buffers to create a new one. Don't pause the task to give it
+		 * another try.
+		 */
+		doResume = true;
+		break;
+
+	case -ENOMEM:
 		GST_ELEMENT_ERROR(self, RESOURCE, NO_SPACE_LEFT,
 				  ("Failed to allocate request for camera '%s'.",
 				   state->cam_->id().c_str()),
 				  ("libcamera::Camera::createRequest() failed"));
 		gst_task_stop(self->task);
 		return;
+
+	case -ENOBUFS:
+	default:
+		break;
 	}
 
-	/* Process one completed request, if available. */
+	/*
+	 * Process one completed request, if available, and record if further
+	 * requests are ready for processing.
+	 */
 	ret = state->processRequest();
 	switch (ret) {
+	case 0:
+		/* Another completed request is available, resume the task. */
+		doResume = true;
+		break;
+
 	case -EPIPE:
 		gst_task_stop(self->task);
 		return;
 
-	case -ENODATA:
+	case -ENOBUFS:
-		gst_task_pause(self->task);
+	default:
-		return;
+		break;
 	}
 
-	/*
+	/* Resume the task for another iteration if needed. */
-	 * Here we need to decide if we want to pause. This needs to
+	if (doResume)
-	 * happen in lock step with the callback thread which may want
+		gst_task_resume(self->task);
-	 * to resume the task and might push pending buffers.
-	 */
-	bool do_pause;
-
-	{
-		MutexLocker locker(state->lock_);
-		do_pause = state->completedRequests_.empty();
-	}
-
-	if (do_pause)
-		gst_task_pause(self->task);
 }
 
 static void