v4l2: Fix threading issues in orphaning mechanism

The pool orphaning function was colling internal _stop() virtual function
implementation. This is not thread safe, as a private lock inside the buffer
pool is supposed to be held. Fix this by keeping delayed _stop() and orphaning
the GstV4L2Allocator instead (REQBUFS(0)).

Then, protect the orphaned boolean with the object lock for the case a buffer
is being released after we have orphaned the buffer. That would otherwise
cause a QBUF to happen while the queue is no longer owned by the buffer pool.
This boolean is otherwise used and set from the streaming lock, or after
threads have been stopped (final cleanup).

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/648>

sys/v4l2/gstv4l2bufferpool.c

@@ -964,9 +964,6 @@ gst_v4l2_buffer_pool_stop (GstBufferPool * bpool)
   GstV4l2BufferPool *pool = GST_V4L2_BUFFER_POOL (bpool);
   gboolean ret;
 
-  if (pool->orphaned)
-    return gst_v4l2_buffer_pool_vallocator_stop (pool);
-
   GST_DEBUG_OBJECT (pool, "stopping pool");
 
   if (pool->group_released_handler > 0) {
@@ -981,7 +978,8 @@ gst_v4l2_buffer_pool_stop (GstBufferPool * bpool)
     pool->other_pool = NULL;
   }
 
-  gst_v4l2_buffer_pool_streamoff (pool);
+  if (!pool->orphaned)
+    gst_v4l2_buffer_pool_streamoff (pool);
 
   ret = GST_BUFFER_POOL_CLASS (parent_class)->stop (bpool);
 
@@ -997,6 +995,8 @@ gst_v4l2_buffer_pool_orphan (GstBufferPool ** bpool)
   GstV4l2BufferPool *pool = GST_V4L2_BUFFER_POOL (*bpool);
   gboolean ret;
 
+  g_return_val_if_fail (pool->orphaned == FALSE, FALSE);
+
   if (!GST_V4L2_ALLOCATOR_CAN_ORPHAN_BUFS (pool->vallocator))
     return FALSE;
 
@@ -1004,25 +1004,24 @@ gst_v4l2_buffer_pool_orphan (GstBufferPool ** bpool)
     return FALSE;
 
   GST_DEBUG_OBJECT (pool, "orphaning pool");
-
   gst_buffer_pool_set_active (*bpool, FALSE);
-  /*
-   * If the buffer pool has outstanding buffers, it will not be stopped
-   * by the base class when set inactive. Stop it manually and mark it
-   * as orphaned
-   */
-  ret = gst_v4l2_buffer_pool_stop (*bpool);
-  if (!ret)
-    ret = gst_v4l2_allocator_orphan (pool->vallocator);
 
-  if (!ret)
+  /* We lock to prevent racing with a return buffer in QBuf, and has a
-    goto orphan_failed;
+   * workaround of not being able to use the pool hidden activation lock. */
+  GST_OBJECT_LOCK (pool);
 
-  pool->orphaned = TRUE;
+  gst_v4l2_buffer_pool_streamoff (pool);
-  gst_object_unref (*bpool);
+  ret = gst_v4l2_allocator_orphan (pool->vallocator);
-  *bpool = NULL;
+  if (ret)
+    pool->orphaned = TRUE;
+
+  GST_OBJECT_UNLOCK (pool);
+
+  if (ret) {
+    gst_object_unref (*bpool);
+    *bpool = NULL;
+  }
 
-orphan_failed:
   return ret;
 }
 
@@ -1171,6 +1170,13 @@ gst_v4l2_buffer_pool_qbuf (GstV4l2BufferPool * pool, GstBuffer * buf,
   }
 
   GST_OBJECT_LOCK (pool);
+
+  /* If the pool was orphaned, don't try to queue any returned buffers.
+   * This is done with the objet lock in order to synchronize with
+   * orphaning. */
+  if (pool->orphaned)
+    goto was_orphaned;
+
   g_atomic_int_inc (&pool->num_queued);
   pool->buffers[index] = buf;
 
@@ -1188,6 +1194,13 @@ already_queued:
     GST_ERROR_OBJECT (pool, "the buffer %i was already queued", index);
     return GST_FLOW_ERROR;
   }
+was_orphaned:
+  {
+    GST_DEBUG_OBJECT (pool, "pool was orphaned, not queuing back buffer.");
+    GST_BUFFER_FLAG_SET (buf, GST_BUFFER_FLAG_TAG_MEMORY);
+    GST_OBJECT_UNLOCK (pool);
+    return GST_FLOW_FLUSHING;
+  }
 queue_failed:
   {
     GST_ERROR_OBJECT (pool, "could not queue a buffer %i", index);
@@ -1468,14 +1481,6 @@ gst_v4l2_buffer_pool_release_buffer (GstBufferPool * bpool, GstBuffer * buffer)
 
   GST_DEBUG_OBJECT (pool, "release buffer %p", buffer);
 
-  /* If the buffer's pool has been orphaned, dispose of it so that
-   * the pool resources can be freed */
-  if (pool->orphaned) {
-    GST_BUFFER_FLAG_SET (buffer, GST_BUFFER_FLAG_TAG_MEMORY);
-    pclass->release_buffer (bpool, buffer);
-    return;
-  }
-
   switch (obj->type) {
     case V4L2_BUF_TYPE_VIDEO_CAPTURE:
     case V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE: