From 80bfd6dc5bc3738db595a4972893b149b1224800 Mon Sep 17 00:00:00 2001
From: Po Lu <luangruo@yahoo.com>
Date: Sun, 27 Nov 2022 19:17:38 +0800
Subject: [PATCH] Make frame synchronization more robust

* src/xterm.c (x_sync_wait_for_frame_drawn_event)
(x_sync_handle_frame_drawn): Only cancel frame synchronization
if hanging twice or more in a row.
* src/xterm.h (struct x_output, FRAME_X_DRAW_JUST_HUNG): New
flag.
---
 src/xterm.c | 46 +++++++++++++++++++++++++++++++++++++++-------
 src/xterm.h |  6 ++++++
 2 files changed, 45 insertions(+), 7 deletions(-)

diff --git a/src/xterm.c b/src/xterm.c
index ec605f5e914..7eaf59d54b1 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -6919,13 +6919,27 @@ x_sync_wait_for_frame_drawn_event (struct frame *f)
 		  x_sync_is_frame_drawn_event, (XPointer) f,
 		  make_timespec (1, 0)))
     {
-      /* TODO: display this warning in the echo area.  */
-      fprintf (stderr, "Warning: compositing manager spent more than 1 second "
-	       "drawing a frame.  Frame synchronization has been disabled\n");
-      FRAME_X_OUTPUT (f)->use_vsync_p = false;
+      /* The first time a draw hangs, treat it as a random fluctuation
+	 on the part of the compositor.  If the next draw continues to
+	 hang, disable frame synchronization.  */
+      if (FRAME_X_DRAW_JUST_HUNG (f))
+	{
+	  fprintf (stderr, "Warning: compositing manager spent more than 1 "
+		   "second drawing a frame.  Frame synchronization has "
+		   "been disabled\n");
+	  FRAME_X_OUTPUT (f)->use_vsync_p = false;
 
-      /* Also change the frame parameter to reflect the new state.  */
-      store_frame_param (f, Quse_frame_synchronization, Qnil);
+	  /* Also change the frame parameter to reflect the new
+	     state.  */
+	  store_frame_param (f, Quse_frame_synchronization, Qnil);
+	}
+      else
+	{
+	  fprintf (stderr, "Warning: compositing manager spent more than 1 "
+		   "second drawing a frame.  Frame synchronization will be "
+		   "disabled if this happens again\n");
+	  FRAME_X_DRAW_JUST_HUNG (f) = true;
+	}
     }
   else
     x_sync_note_frame_times (FRAME_DISPLAY_INFO (f), f, &event);
@@ -7128,8 +7142,26 @@ static void
 x_sync_handle_frame_drawn (struct x_display_info *dpyinfo,
 			   XEvent *message, struct frame *f)
 {
+  XSyncValue value, counter;
+
   if (FRAME_OUTER_WINDOW (f) == message->xclient.window)
-    FRAME_X_WAITING_FOR_DRAW (f) = false;
+    {
+      counter = FRAME_X_COUNTER_VALUE (f);
+
+      /* Check that the counter in the message is the same as the
+	 counter in the frame.  */
+      XSyncIntsToValue (&value,
+			message->xclient.data.l[0] & 0xffffffff,
+			message->xclient.data.l[1] & 0xffffffff);
+
+      if (XSyncValueEqual (value, counter))
+	FRAME_X_WAITING_FOR_DRAW (f) = false;
+
+      /* As long as a _NET_WM_FRAME_DRAWN message arrives, we know
+	 that the compositor is still sending events, so avoid timing
+	 out.  */
+      FRAME_X_DRAW_JUST_HUNG (f) = false;
+    }
 
   x_sync_note_frame_times (dpyinfo, f, message);
 }
diff --git a/src/xterm.h b/src/xterm.h
index c36920081d3..ee429e9c68d 100644
--- a/src/xterm.h
+++ b/src/xterm.h
@@ -1179,6 +1179,10 @@ struct x_output
      frame.  */
   bool_bf waiting_for_frame_p : 1;
 
+  /* Whether or not Emacs just skipped waiting for a frame due to a
+     timeout.  */
+  bool_bf draw_just_hung_p : 1;
+
 #if !defined USE_GTK && defined HAVE_CLOCK_GETTIME
   /* Whether or not Emacs should wait for the compositing manager to
      draw frames before starting a new frame.  */
@@ -1392,6 +1396,8 @@ extern void x_mark_frame_dirty (struct frame *f);
   FRAME_X_OUTPUT (f)->extended_frame_counter
 #define FRAME_X_WAITING_FOR_DRAW(f)		\
   FRAME_X_OUTPUT (f)->waiting_for_frame_p
+#define FRAME_X_DRAW_JUST_HUNG(f)		\
+  FRAME_X_OUTPUT (f)->draw_just_hung_p
 #define FRAME_X_COUNTER_VALUE(f)		\
   FRAME_X_OUTPUT (f)->current_extended_counter_value
 #endif
-- 
2.39.5