[gtk/wip/chergert/fix-x11-selection-deadlock] x11: fix deadlock when finalizing output stream

[Christian Hergert <chergert src gnome org>]

commit ccf44b694b91d131e32aa84f9f62985e4b1adcc8
Author: Christian Hergert <chergert redhat com>
Date:   Fri Sep 11 14:38:24 2020 -0700

    x11: fix deadlock when finalizing output stream
    
    If we have a very large selection (many MB of textbuffer text) we can
    possibly deadlock here trying to flush as the serializer is disposed
    and finalized on the main thread.
    
    This avoids it similarly to other places by checking that we are allowed
    to flush at this point.
    
    Fixes: #3153

 gdk/x11/gdkselectionoutputstream-x11.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/gdk/x11/gdkselectionoutputstream-x11.c b/gdk/x11/gdkselectionoutputstream-x11.c
index b9aad4c3f3..6c10a15f2c 100644
--- a/gdk/x11/gdkselectionoutputstream-x11.c
+++ b/gdk/x11/gdkselectionoutputstream-x11.c
@@ -439,7 +439,11 @@ gdk_x11_selection_output_stream_flush (GOutputStream  *output_stream,
   g_main_context_invoke (NULL, gdk_x11_selection_output_stream_invoke_flush, stream);
 
   g_mutex_lock (&priv->mutex);
-  if (gdk_x11_selection_output_stream_needs_flush_unlocked (stream))
+  /* We have to check gdk_x11_selection_output_stream_can_flush() because if
+   * we cannot, nothing will necessarily complete our g_cond_wait().
+   */
+  if (!priv->delete_pending &&
+      gdk_x11_selection_output_stream_needs_flush_unlocked (stream))
     g_cond_wait (&priv->cond, &priv->mutex);
   g_mutex_unlock (&priv->mutex);