Always push changed mt events when syncing

If the start and end of a touch are dropped, the slot, according to the
kernel, may have a different state. We should inform the client of these
changes even if the slot is not currently active.

For most axes this doesn't matter too much as we expect them to change
during an active touch anyway so we don't expect the kernel's caching to
be a problem. However where the ABS_MT_TOOL_TYPE changed during a sync
we need to inform the client of the new tool type so that future
touchese won't be erroneously treated as e.g. palms.

For a full reproducer see the test case but it comes down to:
- touch down with MT_TOOL_PALM, make sure libevdev reads the state
- change that slot to MT_TOOL_FINGER, trigger a sync
- ensure that libevdev pushes out that tool type change even if the
  slot is not currently active

Co-authored-by: Peter Hutterer <peter.hutterer@who-t.net>
Part-of: <https://gitlab.freedesktop.org/libevdev/libevdev/-/merge_requests/124>

libevdev/libevdev.c

@@ -805,25 +805,36 @@ push_mt_sync_events(struct libevdev *dev,
 	int rc;
 
 	for (int slot = 0; slot < dev->num_slots;  slot++) {
-		/* stopped touches were already terminated in
+		bool have_slot_event = false;
-		 * terminate_slots */
-		if (changes[slot].state == TOUCH_STOPPED ||
-		    !bit_is_set(changes[slot].axes, ABS_MT_SLOT))
-			continue;
 
-		queue_push_event(dev, EV_ABS, ABS_MT_SLOT, slot);
+		if (!bit_is_set(changes[slot].axes, ABS_MT_SLOT))
-		last_reported_slot = slot;
+			continue;
 
 		for (int axis = ABS_MT_MIN; axis <= ABS_MT_MAX; axis++) {
 			if (axis == ABS_MT_SLOT ||
 			    !libevdev_has_event_code(dev, EV_ABS, axis))
 				continue;
 
-			if (bit_is_set(changes[slot].axes, axis))
+			if (bit_is_set(changes[slot].axes, axis)) {
+                                /* We already sent the tracking id -1 in
+                                 * terminate_slots so don't do that again. There
+                                 * may be other axes like ABS_MT_TOOL_TYPE that
+                                 * need to be synced despite no touch being active */
+                                if (axis == ABS_MT_TRACKING_ID &&
+                                    *slot_value(dev, slot, axis) == -1)
+					continue;
+
+				if (!have_slot_event) {
+					queue_push_event(dev, EV_ABS, ABS_MT_SLOT, slot);
+					last_reported_slot = slot;
+					have_slot_event = true;
+				}
+
 				queue_push_event(dev, EV_ABS, axis,
 						 *slot_value(dev, slot, axis));
 			}
 		}
+	}
 
 	/* add one last slot event to make sure the client is on the same
 	   slot as the kernel */

test/test-libevdev-events.c

@@ -1008,6 +1008,206 @@ START_TEST(test_syn_delta_tracking_ids_btntool)
 }
 END_TEST
 
+START_TEST(test_syn_delta_mt_tool_type)
+{
+	struct uinput_device* uidev;
+	struct libevdev *dev;
+	int rc;
+	struct input_event ev;
+	int i;
+	const int num_slots = 15;
+	int slot = -1;
+	unsigned long terminated[NLONGS(num_slots)];
+	struct input_absinfo abs[7] = {
+		{ .value = ABS_X, .maximum = 1000 },
+		{ .value = ABS_Y, .maximum = 1000 },
+		{ .value = ABS_MT_POSITION_X, .maximum = 1000 },
+		{ .value = ABS_MT_POSITION_Y, .maximum = 1000 },
+		{ .value = ABS_MT_TOOL_TYPE, .maximum = MT_TOOL_PALM },
+		{ .value = ABS_MT_SLOT, .maximum = num_slots },
+		{ .value = ABS_MT_TRACKING_ID, .minimum = -1, .maximum = 0xff },
+	};
+
+	test_create_abs_device(&uidev, &dev,
+			       ARRAY_LENGTH(abs), abs,
+			       EV_SYN, SYN_REPORT,
+			       -1);
+
+	for (i = num_slots; i >= 0; i--) {
+		int tool_type = MT_TOOL_FINGER;
+		switch (i) {
+			case 0:
+			case 1:
+			case 2:
+			case 3:
+				tool_type = MT_TOOL_FINGER;
+				break;
+			case 4:
+			case 5:
+			case 6:
+			case 7:
+				tool_type = MT_TOOL_PALM;
+				break;
+		}
+		uinput_device_event_multiple(uidev,
+					     EV_ABS, ABS_MT_SLOT, i,
+					     EV_ABS, ABS_MT_TRACKING_ID, i,
+					     EV_ABS, ABS_X, 100 + i,
+					     EV_ABS, ABS_Y, 500 + i,
+					     EV_ABS, ABS_MT_POSITION_X, 100 + i,
+					     EV_ABS, ABS_MT_POSITION_Y, 500 + i,
+					     EV_ABS, ABS_MT_TOOL_TYPE, tool_type,
+					     EV_SYN, SYN_REPORT, 0,
+					     -1, -1);
+		do {
+			rc = libevdev_next_event(dev, LIBEVDEV_READ_FLAG_NORMAL, &ev);
+			ck_assert_int_ne(rc, LIBEVDEV_READ_STATUS_SYNC);
+		} while (rc >= 0);
+	}
+
+	/* we have a bunch of touches now, and libevdev knows it. Change all
+	 * touches */
+	for (i = num_slots; i >= 0; i--) {
+		uinput_device_event(uidev, EV_ABS, ABS_MT_SLOT, i);
+		switch (i) {
+			/* Slot 0 is a finger and stays a finger */
+			case 0:
+			/* Slot 4 is a palm and stays a palm */
+			case 4:
+				uinput_device_event_multiple(uidev,
+							     EV_ABS, ABS_X, 200 + i,
+							     EV_ABS, ABS_Y, 700 + i,
+							     EV_ABS, ABS_MT_POSITION_X, 200 + i,
+							     EV_ABS, ABS_MT_POSITION_Y, 700 + i,
+							     -1, -1);
+				break;
+			/* Slot 1 is a finger and changes active touch to palm */
+			case 1:
+				uinput_device_event(uidev, EV_ABS, ABS_MT_TOOL_TYPE, MT_TOOL_PALM);
+				break;
+			/* Slot 2 is a finger and terminates */
+			case 2:
+			/* Slot 6 is a palm and terminates */
+			case 6:
+				uinput_device_event(uidev, EV_ABS, ABS_MT_TRACKING_ID, -1);
+				break;
+			/* Slot 3 is a finger and restarts as finger */
+			case 3:
+			/* Slot 5 is a palm and restarts as finger */
+			case 5:
+				uinput_device_event_multiple(uidev,
+							     EV_ABS, ABS_MT_TRACKING_ID, num_slots + i,
+							     EV_ABS, ABS_X, 200 + i,
+							     EV_ABS, ABS_Y, 700 + i,
+							     EV_ABS, ABS_MT_POSITION_X, 200 + i,
+							     EV_ABS, ABS_MT_POSITION_Y, 700 + i,
+							     EV_ABS, ABS_MT_TOOL_TYPE, MT_TOOL_FINGER,
+							     -1, -1);
+				break;
+			/* Slot 7 is a palm and restarts and terminates again as finger */
+			case 7:
+				uinput_device_event(uidev, EV_ABS, ABS_MT_TRACKING_ID, -1);
+				uinput_device_event(uidev, EV_SYN, SYN_REPORT, 0);
+				uinput_device_event_multiple(uidev,
+							     EV_ABS, ABS_MT_TRACKING_ID, num_slots + i,
+							     EV_ABS, ABS_X, 200 + i,
+							     EV_ABS, ABS_Y, 700 + i,
+							     EV_ABS, ABS_MT_POSITION_X, 200 + i,
+							     EV_ABS, ABS_MT_POSITION_Y, 700 + i,
+							     EV_ABS, ABS_MT_TOOL_TYPE, MT_TOOL_FINGER,
+							     -1, -1);
+				uinput_device_event(uidev, EV_ABS, ABS_MT_TRACKING_ID, -1);
+				uinput_device_event(uidev, EV_SYN, SYN_REPORT, 0);
+				break;
+		}
+		uinput_device_event(uidev, EV_SYN, SYN_REPORT, 0);
+	}
+
+	/* Force sync */
+	rc = libevdev_next_event(dev, LIBEVDEV_READ_FLAG_FORCE_SYNC, &ev);
+	ck_assert_int_eq(rc, LIBEVDEV_READ_STATUS_SYNC);
+
+	/* now check for the right tracking IDs */
+	memset(terminated, 0, sizeof(terminated));
+	slot = -1;
+	while ((rc = libevdev_next_event(dev, LIBEVDEV_READ_FLAG_SYNC, &ev)) != -EAGAIN) {
+		if (libevdev_event_is_code(&ev, EV_SYN, SYN_REPORT))
+			continue;
+
+		if (libevdev_event_is_code(&ev, EV_ABS, ABS_MT_SLOT)) {
+			slot = ev.value;
+			continue;
+		}
+
+		if (libevdev_event_is_code(&ev, EV_ABS, ABS_X) ||
+		    libevdev_event_is_code(&ev, EV_ABS, ABS_Y))
+			continue;
+
+		ck_assert_int_ne(slot, -1);
+
+		if (libevdev_event_is_code(&ev, EV_ABS, ABS_MT_TRACKING_ID)) {
+			switch (slot) {
+				case 0:
+				case 1:
+				case 4:
+					ck_abort_msg("No ABS_MT_TRACKING_ID expected for this slot");
+					break;
+				case 2:
+				case 6:
+				case 7:
+					ck_assert_int_eq(ev.value, -1);
+					break;
+				case 3:
+				case 5:
+					if (!bit_is_set(terminated, slot)) {
+						ck_assert_int_eq(ev.value, -1);
+						set_bit(terminated, slot);
+					} else {
+						ck_assert_int_eq(ev.value, num_slots + slot);
+					}
+					break;
+			}
+
+			continue;
+		}
+
+		if (libevdev_event_is_code(&ev, EV_ABS, ABS_MT_TOOL_TYPE)) {
+			switch (slot) {
+				case 0:
+				case 2:
+				case 3:
+				case 4:
+				case 6:
+					ck_abort_msg("No ABS_MT_TOOL_TYPE expected for this slot");
+					break;
+				case 1:
+					ck_assert_int_eq(ev.value, MT_TOOL_PALM);
+					break;
+				case 5:
+				case 7:
+					ck_assert_int_eq(ev.value, MT_TOOL_FINGER);
+					break;
+			}
+			continue;
+		}
+
+		switch(ev.code) {
+			case ABS_MT_POSITION_X:
+				ck_assert_int_eq(ev.value, 200 + slot);
+				break;
+			case ABS_MT_POSITION_Y:
+				ck_assert_int_eq(ev.value, 700 + slot);
+				break;
+			default:
+				ck_abort();
+		}
+	}
+
+	uinput_device_free(uidev);
+	libevdev_free(dev);
+}
+END_TEST
+
 START_TEST(test_syn_delta_late_sync)
 {
 	struct uinput_device* uidev;
@@ -2060,6 +2260,7 @@ TEST_SUITE_ROOT_PRIVILEGES(libevdev_events)
 	add_test(s, test_syn_delta_late_sync);
 	add_test(s, test_syn_delta_tracking_ids);
 	add_test(s, test_syn_delta_tracking_ids_btntool);
+	add_test(s, test_syn_delta_mt_tool_type);
 
 	add_test(s, test_skipped_sync);
 	add_test(s, test_incomplete_sync);