gst: libcamerasrc: Prevent src task deadlock on exhausted buffer pool

Allow GstLibcameraPool to notify the source when a new buffer has become
available in a previously exhausted buffer pool. This can be used to
resume a src task that got paused because it couldn't acquire a buffer.

Without this change the src task will never resume from pause once the
pool gets exhausted.

To trigger the deadlock (it doesn't happen every time), run:

  gst-launch-1.0 libcamerasrc ! queue ! glimagesink

Signed-off-by: Jakub Adam <jakub.adam@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/gstreamer/gstlibcamerapool.cpp

@@ -14,6 +14,13 @@
 
 using namespace libcamera;
 
+enum {
+	SIGNAL_BUFFER_NOTIFY,
+	N_SIGNALS
+};
+
+static guint signals[N_SIGNALS];
+
 struct _GstLibcameraPool {
 	GstBufferPool parent;
 
@@ -55,7 +62,12 @@ static void
 gst_libcamera_pool_release_buffer(GstBufferPool *pool, GstBuffer *buffer)
 {
 	GstLibcameraPool *self = GST_LIBCAMERA_POOL(pool);
+	bool do_notify = gst_atomic_queue_length(self->queue) == 0;
+
 	gst_atomic_queue_push(self->queue, buffer);
+
+	if (do_notify)
+		g_signal_emit(self, signals[SIGNAL_BUFFER_NOTIFY], 0);
 }
 
 static void
@@ -90,6 +102,11 @@ gst_libcamera_pool_class_init(GstLibcameraPoolClass *klass)
 	pool_class->acquire_buffer = gst_libcamera_pool_acquire_buffer;
 	pool_class->reset_buffer = gst_libcamera_pool_reset_buffer;
 	pool_class->release_buffer = gst_libcamera_pool_release_buffer;
+
+	signals[SIGNAL_BUFFER_NOTIFY] = g_signal_new("buffer-notify",
+						     G_OBJECT_CLASS_TYPE(klass), G_SIGNAL_RUN_LAST,
+						     0, nullptr, nullptr, nullptr,
+						     G_TYPE_NONE, 0);
 }
 
 GstLibcameraPool *

src/gstreamer/gstlibcamerasrc.cpp

@@ -439,6 +439,9 @@ gst_libcamera_src_task_enter(GstTask *task, GThread *thread, gpointer user_data)
 		const StreamConfiguration &stream_cfg = state->config_->at(i);
 		GstLibcameraPool *pool = gst_libcamera_pool_new(self->allocator,
 								stream_cfg.stream());
+		g_signal_connect_swapped(pool, "buffer-notify",
+					 G_CALLBACK(gst_libcamera_resume_task), task);
+
 		gst_libcamera_pad_set_pool(srcpad, pool);
 		gst_flow_combiner_add_pad(self->flow_combiner, srcpad);
 	}