backends/native: Surround device state queries/updates with RW lock

Wrap all keyboard state updates, and all pointer/stylus/touch cursor
position with a write lock, and ::query_state() (The only entrypoint
to this state from other threads) with a read lock.

The principle is that query_state may be called from different threads
(UI so far, but maybe KMS too in the future), while the input thread
may (or may not) be updating it. This state is fetched "atomically"
(eg. x/y will be consistently old or new, if the input thread were
updating it at the same time).

There's other places deep in backends/native that read this state,
they all will run in the input thread, so they count as "other readers"
to the other thread. Those changes are already mutually exclusive with
updates, so they don't explicitly need the RW lock.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>

3 files changed, 50 insertions, 6 deletions

diff --git a/src/backends/native/meta-input-device-native.c b/src/backends/native/meta-input-device-native.c
index ff1faedeb..240392b86 100644
--- a/src/backends/native/meta-input-device-native.c
+++ b/src/backends/native/meta-input-device-native.c
@@ -435,6 +435,8 @@ update_internal_xkb_state (MetaInputDeviceNative *device,
   xkb_mod_mask_t group_mods;
   struct xkb_state *xkb_state;
 
+  g_rw_lock_writer_lock (&seat_impl->state_lock);
+
   xkb_state = meta_seat_impl_get_xkb_state (seat_impl);
   depressed_mods = xkb_state_serialize_mods (xkb_state, XKB_STATE_MODS_DEPRESSED);
   latched_mods = xkb_state_serialize_mods (xkb_state, XKB_STATE_MODS_LATCHED);
@@ -457,6 +459,8 @@ update_internal_xkb_state (MetaInputDeviceNative *device,
                          locked_mods,
                          0, 0, group_mods);
   notify_stickykeys_mask (device);
+
+  g_rw_lock_writer_unlock (&seat_impl->state_lock);
 }
 
 static void
diff --git a/src/backends/native/meta-seat-impl.c b/src/backends/native/meta-seat-impl.c
index 19092c01d..323222312 100644
--- a/src/backends/native/meta-seat-impl.c
+++ b/src/backends/native/meta-seat-impl.c
@@ -435,6 +435,8 @@ new_absolute_motion_event (MetaSeatImpl       *seat_impl,
   clutter_event_set_device (event, seat_impl->core_pointer);
   clutter_event_set_source_device (event, input_device);
 
+  g_rw_lock_writer_lock (&seat_impl->state_lock);
+
   if (clutter_input_device_get_device_type (input_device) == CLUTTER_TABLET_DEVICE)
     {
       MetaInputDeviceNative *device_native =
@@ -458,6 +460,8 @@ new_absolute_motion_event (MetaSeatImpl       *seat_impl,
       seat_impl->pointer_y = y;
     }
 
+  g_rw_lock_writer_unlock (&seat_impl->state_lock);
+
   return event;
 }
 
@@ -2031,10 +2035,14 @@ process_device_event (MetaSeatImpl          *seat_impl,
         y = libinput_event_touch_get_y_transformed (touch_event,
                                                     stage_height);
 
+        g_rw_lock_writer_lock (&seat_impl->state_lock);
+
         touch_state = meta_seat_impl_acquire_touch_state (seat_impl, seat_slot);
         touch_state->coords.x = x;
         touch_state->coords.y = y;
 
+        g_rw_lock_writer_unlock (&seat_impl->state_lock);
+
         meta_seat_impl_notify_touch_event (seat_impl, device,
                                            CLUTTER_TOUCH_BEGIN,
                                            time_us,
@@ -2066,7 +2074,10 @@ process_device_event (MetaSeatImpl          *seat_impl,
                                            touch_state->seat_slot,
                                            touch_state->coords.x,
                                            touch_state->coords.y);
+
+        g_rw_lock_writer_lock (&seat_impl->state_lock);
         meta_seat_impl_release_touch_state (seat_impl, seat_slot);
+        g_rw_lock_writer_unlock (&seat_impl->state_lock);
         break;
       }
 
@@ -2093,13 +2104,18 @@ process_device_event (MetaSeatImpl          *seat_impl,
         y = libinput_event_touch_get_y_transformed (touch_event,
                                                     stage_height);
 
+        g_rw_lock_writer_lock (&seat_impl->state_lock);
         touch_state = meta_seat_impl_lookup_touch_state (seat_impl, seat_slot);
+        if (touch_state)
+          {
+            touch_state->coords.x = x;
+            touch_state->coords.y = y;
+          }
+        g_rw_lock_writer_unlock (&seat_impl->state_lock);
+
         if (!touch_state)
           break;
 
-        touch_state->coords.x = x;
-        touch_state->coords.y = y;
-
         meta_seat_impl_notify_touch_event (seat_impl, device,
                                            CLUTTER_TOUCH_UPDATE,
                                            time_us,
@@ -2611,6 +2627,8 @@ meta_seat_impl_finalize (GObject *object)
 
   g_free (seat_impl->seat_id);
 
+  g_rw_lock_clear (&seat_impl->state_lock);
+
   G_OBJECT_CLASS (meta_seat_impl_parent_class)->finalize (object);
 }
 
@@ -2656,6 +2674,9 @@ meta_seat_impl_query_state (MetaSeatImpl         *seat_impl,
                             ClutterModifierType  *modifiers)
 {
   MetaInputDeviceNative *device_native = META_INPUT_DEVICE_NATIVE (device);
+  gboolean retval = FALSE;
+
+  g_rw_lock_reader_lock (&seat_impl->state_lock);
 
   if (sequence)
     {
@@ -2665,7 +2686,7 @@ meta_seat_impl_query_state (MetaSeatImpl         *seat_impl,
       slot = meta_event_native_sequence_get_slot (sequence);
       touch_state = meta_seat_impl_lookup_touch_state (seat_impl, slot);
       if (!touch_state)
-        return FALSE;
+        goto out;
 
       if (coords)
         {
@@ -2676,7 +2697,7 @@ meta_seat_impl_query_state (MetaSeatImpl         *seat_impl,
       if (modifiers)
         *modifiers = meta_xkb_translate_modifiers (seat_impl->xkb, 0);
 
-      return TRUE;
+      retval = TRUE;
     }
   else
     {
@@ -2692,8 +2713,12 @@ meta_seat_impl_query_state (MetaSeatImpl         *seat_impl,
                                                      seat_impl->button_state);
         }
 
-      return TRUE;
+      retval = TRUE;
     }
+
+ out:
+  g_rw_lock_reader_unlock (&seat_impl->state_lock);
+  return retval;
 }
 
 static void
@@ -2764,6 +2789,8 @@ meta_seat_impl_class_init (MetaSeatImplClass *klass)
 static void
 meta_seat_impl_init (MetaSeatImpl *seat_impl)
 {
+  g_rw_lock_init (&seat_impl->state_lock);
+
   seat_impl->repeat = TRUE;
   seat_impl->repeat_delay = 250;     /* ms */
   seat_impl->repeat_interval = 33;   /* ms */
@@ -2803,6 +2830,8 @@ meta_seat_impl_update_xkb_state (MetaSeatImpl *seat_impl)
   xkb_mod_mask_t locked_mods;
   struct xkb_keymap *xkb_keymap;
 
+  g_rw_lock_writer_lock (&seat_impl->state_lock);
+
   xkb_keymap = meta_keymap_native_get_keyboard_map (seat_impl->keymap);
 
   latched_mods = xkb_state_serialize_mods (seat_impl->xkb,
@@ -2826,6 +2855,8 @@ meta_seat_impl_update_xkb_state (MetaSeatImpl *seat_impl)
     xkb_keymap_led_get_index (xkb_keymap, XKB_LED_NAME_SCROLL);
 
   meta_seat_impl_sync_leds (seat_impl);
+
+  g_rw_lock_writer_unlock (&seat_impl->state_lock);
 }
 
 /**
@@ -2941,6 +2972,8 @@ meta_seat_impl_set_keyboard_layout_index (MetaSeatImpl       *seat_impl,
 
   g_return_if_fail (META_IS_SEAT_IMPL (seat_impl));
 
+  g_rw_lock_writer_lock (&seat_impl->state_lock);
+
   state = seat_impl->xkb;
 
   depressed_mods = xkb_state_serialize_mods (state, XKB_STATE_MODS_DEPRESSED);
@@ -2950,6 +2983,8 @@ meta_seat_impl_set_keyboard_layout_index (MetaSeatImpl       *seat_impl,
   xkb_state_update_mask (state, depressed_mods, latched_mods, locked_mods, 0, 0, idx);
 
   seat_impl->layout_idx = idx;
+
+  g_rw_lock_writer_unlock (&seat_impl->state_lock);
 }
 
 /**
@@ -2982,6 +3017,8 @@ meta_seat_impl_set_keyboard_numlock (MetaSeatImpl *seat_impl,
 
   g_return_if_fail (META_IS_SEAT_IMPL (seat_impl));
 
+  g_rw_lock_writer_lock (&seat_impl->state_lock);
+
   keymap = seat_impl->keymap;
   xkb_keymap = meta_keymap_native_get_keyboard_map (keymap);
 
@@ -3009,6 +3046,8 @@ meta_seat_impl_set_keyboard_numlock (MetaSeatImpl *seat_impl,
                          group_mods);
 
   meta_seat_impl_sync_leds (seat_impl);
+
+  g_rw_lock_writer_unlock (&seat_impl->state_lock);
 }
 
 /**
diff --git a/src/backends/native/meta-seat-impl.h b/src/backends/native/meta-seat-impl.h
index d04b1ae7f..027fd624b 100644
--- a/src/backends/native/meta-seat-impl.h
+++ b/src/backends/native/meta-seat-impl.h
@@ -58,6 +58,7 @@ struct _MetaSeatImpl
   char *seat_id;
   MetaEventSource *event_source;
   struct libinput *libinput;
+  GRWLock state_lock;
 
   GSList *devices;