x11: Limit touch replay pointer events to when replaying

When a touch sequence was rejected, the emulated pointer events would be
replayed with old timestamps. This caused issues with grabs as they
would be ignored due to being too old. This was mitigated by making sure
device event timestamps never travelled back in time by tampering with
any event that had a timestamp seemingly in the past.

This failed when the most recent timestamp that had been received were
much older than the timestamp of the new event. This could for example
happen when a session was left not interacted with for 40+ days or so;
when interacted with again, as any new timestamp would according to
XSERVER_TIME_IS_BEFORE() still be in the past compared to the "most
recent" one. The effect is that we'd always use the `latest_evtime` for
all new device events without ever updating it.

The end result of this was that passive grabs would become active when
interacted with, but would then newer be released, as the timestamps to
XIAllowEvents() would out of date, resulting in the desktop effectively
freezing, as the Shell would have an active pointer grab.

To avoid the situation where we get stuck with an old `latest_evtime`
timestamp, limit the tampering with device event timestamp to 1) only
pointer events, and 2) only during the replay sequence. The second part
is implemented by sending an asynchronous message via the X server after
rejecting a touch sequence, only potentially tampering with the device
event timestamps until the reply. This should avoid the stuck timestamp
as in those situations, we'll always have a relatively up to date
`latest_evtime` meaning XSERVER_TIME_IS_BEFORE() will not get confused.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/886

src/backends/x11/meta-backend-x11.c

@@ -67,6 +67,10 @@ struct _MetaBackendX11Private
   XSyncAlarm user_active_alarm;
   XSyncCounter counter;
 
+  int current_touch_replay_sync_serial;
+  int pending_touch_replay_sync_serial;
+  Atom touch_replay_sync_atom;
+
   int xinput_opcode;
   int xinput_event_base;
   int xinput_error_base;
@@ -175,6 +179,26 @@ meta_backend_x11_translate_device_event (MetaBackendX11 *x11,
   backend_x11_class->translate_device_event (x11, device_event);
 }
 
+static void
+maybe_translate_touch_replay_pointer_event (MetaBackendX11 *x11,
+                                            XIDeviceEvent  *device_event)
+{
+  MetaBackendX11Private *priv = meta_backend_x11_get_instance_private (x11);
+
+  if (!device_event->send_event &&
+      device_event->time != META_CURRENT_TIME &&
+      priv->current_touch_replay_sync_serial !=
+      priv->pending_touch_replay_sync_serial &&
+      XSERVER_TIME_IS_BEFORE (device_event->time, priv->latest_evtime))
+    {
+      /* Emulated pointer events received after XIRejectTouch is received
+       * on a passive touch grab will contain older timestamps, update those
+       * so we dont get InvalidTime at grabs.
+       */
+      device_event->time = priv->latest_evtime;
+    }
+}
+
 static void
 translate_device_event (MetaBackendX11 *x11,
                         XIDeviceEvent  *device_event)
@@ -184,19 +208,7 @@ translate_device_event (MetaBackendX11 *x11,
   meta_backend_x11_translate_device_event (x11, device_event);
 
   if (!device_event->send_event && device_event->time != META_CURRENT_TIME)
-    {
+    priv->latest_evtime = device_event->time;
-      if (XSERVER_TIME_IS_BEFORE (device_event->time, priv->latest_evtime))
-        {
-          /* Emulated pointer events received after XIRejectTouch is received
-           * on a passive touch grab will contain older timestamps, update those
-           * so we dont get InvalidTime at grabs.
-           */
-          device_event->time = priv->latest_evtime;
-        }
-
-      /* Update the internal latest evtime, for any possible later use */
-      priv->latest_evtime = device_event->time;
-    }
 }
 
 static void
@@ -261,6 +273,9 @@ maybe_spoof_event_as_stage_event (MetaBackendX11 *x11,
     case XI_Motion:
     case XI_ButtonPress:
     case XI_ButtonRelease:
+      maybe_translate_touch_replay_pointer_event (x11,
+                                                  (XIDeviceEvent *) input_event);
+      /* Intentional fall-through */
     case XI_KeyPress:
     case XI_KeyRelease:
     case XI_TouchBegin:
@@ -329,6 +344,17 @@ handle_host_xevent (MetaBackend *backend,
   gboolean bypass_clutter = FALSE;
   MetaDisplay *display;
 
+  switch (event->type)
+    {
+    case ClientMessage:
+      if (event->xclient.window == meta_backend_x11_get_xwindow (x11) &&
+          event->xclient.message_type == priv->touch_replay_sync_atom)
+        priv->current_touch_replay_sync_serial = event->xclient.data.l[0];
+      break;
+    default:
+      break;
+    }
+
   XGetEventData (priv->xdisplay, &event->xcookie);
 
   display = meta_get_display ();
@@ -533,6 +559,10 @@ meta_backend_x11_post_init (MetaBackend *backend)
   monitor_manager = meta_backend_get_monitor_manager (backend);
   g_signal_connect (monitor_manager, "monitors-changed-internal",
                     G_CALLBACK (on_monitors_changed), backend);
+
+  priv->touch_replay_sync_atom = XInternAtom (priv->xdisplay,
+                                              "_MUTTER_TOUCH_SEQUENCE_SYNC",
+                                              False);
 }
 
 static ClutterBackend *
@@ -610,6 +640,21 @@ meta_backend_x11_finish_touch_sequence (MetaBackend          *backend,
                       META_VIRTUAL_CORE_POINTER_ID,
                       meta_x11_event_sequence_get_touch_detail (sequence),
                       DefaultRootWindow (priv->xdisplay), event_mode);
+
+  if (state == META_SEQUENCE_REJECTED)
+    {
+      XClientMessageEvent ev;
+
+      ev = (XClientMessageEvent) {
+        .type = ClientMessage,
+        .window = meta_backend_x11_get_xwindow (x11),
+        .message_type = priv->touch_replay_sync_atom,
+        .format = 32,
+        .data.l[0] = ++priv->pending_touch_replay_sync_serial,
+      };
+      XSendEvent (priv->xdisplay, meta_backend_x11_get_xwindow (x11),
+                  False, 0, (XEvent *) &ev);
+    }
 }
 
 static void