[mutter/gnome-3-34] x11: Ensure flush() Flushes all output stream data

[Robert Mader <rmader src gnome org>]

commit 5f34a0f2452557005688348e97aa790b9095003e
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Apr 11 16:59:17 2020 +0200

    x11: Ensure flush() Flushes all output stream data
    
    This seemed to work under the assumption that a flush() call can
    only result in one INCR roundtrip. This is evidently not true, so
    we should hold things off until all pending data is actually flushed.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1198
    
    (cherry picked from commit 8a2b82897db66b3edbe985a370429b4f36b9918c)

 src/x11/meta-x11-selection-output-stream.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)
---
diff --git a/src/x11/meta-x11-selection-output-stream.c b/src/x11/meta-x11-selection-output-stream.c
index 9928da6b2..6095b5500 100644
--- a/src/x11/meta-x11-selection-output-stream.c
+++ b/src/x11/meta-x11-selection-output-stream.c
@@ -292,12 +292,17 @@ meta_x11_selection_output_stream_invoke_flush (gpointer data)
 {
   MetaX11SelectionOutputStream *stream =
     META_X11_SELECTION_OUTPUT_STREAM (data);
+  MetaX11SelectionOutputStreamPrivate *priv =
+    meta_x11_selection_output_stream_get_instance_private (stream);
 
   if (meta_x11_selection_output_stream_needs_flush (stream) &&
       meta_x11_selection_output_stream_can_flush (stream))
     meta_x11_selection_output_stream_perform_flush (stream);
 
-  return G_SOURCE_REMOVE;
+  if (priv->delete_pending || priv->data->len > 0)
+    return G_SOURCE_CONTINUE;
+  else
+    return G_SOURCE_REMOVE;
 }
 
 static gssize
@@ -477,10 +482,9 @@ meta_x11_selection_output_stream_flush_async (GOutputStream       *output_stream
         }
     }
 
+  g_assert (priv->pending_task == NULL);
+  priv->pending_task = task;
   meta_x11_selection_output_stream_perform_flush (stream);
-  g_task_return_boolean (task, TRUE);
-  g_object_unref (task);
-  return;
 }
 
 static gboolean