evdev: track KEY_SYSRQ frames and pass them even as repeat frames

Alt+Printscreen aka KEY_LEFTALT + KEY_SYSRQ is emulated by the kernel
and always posted with SYN_REPORT 1, see
drivers/tty/sysrq.c:sysrq_reinject_alt_sysrq()

The actual sequence when pressing Alt + Printscreen is to release Alt
first, then press it again:
- evdev:
    - [ 10, 674010,   1,  56,       0] # EV_KEY / KEY_LEFTALT               0
    - [ 10, 674010,   0,   0,       0] # ------------ SYN_REPORT (0) ---------- +2861ms
  - evdev:
    - [ 10, 674030,   1,  56,       1] # EV_KEY / KEY_LEFTALT               1
    - [ 10, 674030,   1,  99,       1] # EV_KEY / KEY_SYSRQ                 1
    - [ 10, 674030,   0,   0,       1] # ------------ SYN_REPORT (1) ---------- +0ms
  - evdev:
    - [ 10, 674031,   1,  99,       0] # EV_KEY / KEY_SYSRQ                 0
    - [ 10, 674031,   1,  56,       0] # EV_KEY / KEY_LEFTALT               0
    - [ 10, 674031,   0,   0,       1] # ------------ SYN_REPORT (1) ---------- +0ms

Handle that special case so we get our printscreen key to work as
expected anymore.

Fixes: 9a9466b6a9 ("evdev: discard any frame with EV_SYN SYN_REPORT 1")
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1273>

src/evdev.c

@@ -1043,6 +1043,7 @@ evdev_device_dispatch(void *data)
 	struct input_event ev;
 	int rc;
 	bool once = false;
+	bool had_sysrq = false;
 	_unref_(evdev_frame) *frame = evdev_frame_new(64);
 
 	/* If the compositor is repainting, this function is called only once
@@ -1083,17 +1084,27 @@ evdev_device_dispatch(void *data)
 					device,
 					"event frame overflow, discarding events.\n");
 			}
+			/* Alt+Printscreen is always a repeat frame, see
+			 * drivers/tty/sysrq.c:sysrq_reinject_alt_sysrq() in the
+			 * kernel
+			 */
+			if (ev.type == EV_KEY && ev.code == KEY_SYSRQ)
+				had_sysrq = true;
+
 			if (ev.type == EV_SYN && ev.code == SYN_REPORT) {
 				/* A SYN_REPORT 1 event is a kernel-inserted
 				 * auto-repeat. Nothing in libinput cares about kernel
 				 * repeats and the inserted frame causes issues with
 				 * timestamp deltas (see e.g. #1145)
+				 *
+				 * (well, except Alt+Printscreen (KEY_SYSRQ))
 				 */
-				if (ev.value != 1)
+				if (ev.value != 1 || had_sysrq)
 					evdev_device_dispatch_frame(libinput,
 								    device,
 								    frame);
 				evdev_frame_reset(frame);
+				had_sysrq = false;
 			}
 		} else if (rc == -ENODEV) {
 			evdev_device_remove(device);

test/test-keyboard.c

@@ -468,6 +468,61 @@ START_TEST(keyboard_no_scroll)
 }
 END_TEST
 
+START_TEST(keyboard_alt_printscreen)
+{
+	struct litest_device *dev = litest_current_device();
+	struct libinput *li = dev->libinput;
+
+	litest_drain_events(li);
+
+	/* repeat frame, ignored */
+	litest_event(dev, EV_KEY, KEY_A, 1);
+	litest_event(dev, EV_SYN, SYN_REPORT, 1);
+	litest_dispatch(li);
+	litest_assert_empty_queue(li);
+
+	/* not a repeat frame */
+	litest_event(dev, EV_KEY, KEY_LEFTALT, 1);
+	litest_event(dev, EV_SYN, SYN_REPORT, 0);
+	litest_dispatch(li);
+	litest_assert_key_event(li, KEY_LEFTALT, LIBINPUT_KEY_STATE_PRESSED);
+
+	/* normal repeat frame, ignored */
+	litest_event(dev, EV_KEY, KEY_LEFTALT, 2);
+	litest_event(dev, EV_SYN, SYN_REPORT, 1);
+	litest_dispatch(li);
+	litest_assert_empty_queue(li);
+
+	/* not repeat frame */
+	litest_event(dev, EV_KEY, KEY_LEFTALT, 0);
+	litest_event(dev, EV_SYN, SYN_REPORT, 0);
+	litest_dispatch(li);
+	litest_assert_key_event(li, KEY_LEFTALT, LIBINPUT_KEY_STATE_RELEASED);
+
+	/* special alt+printscreen repeat frame, *not* ignored */
+	litest_event(dev, EV_KEY, KEY_LEFTALT, 1);
+	litest_event(dev, EV_KEY, KEY_SYSRQ, 1);
+	litest_event(dev, EV_SYN, SYN_REPORT, 1);
+	litest_dispatch(li);
+
+	/* special alt+printscreen repeat frame, *not* ignored */
+	litest_event(dev, EV_KEY, KEY_LEFTALT, 0);
+	litest_event(dev, EV_KEY, KEY_SYSRQ, 0);
+	litest_event(dev, EV_SYN, SYN_REPORT, 1);
+	litest_dispatch(li);
+
+	/* Note: the kernel doesn't release the key combo until both keys are released
+	 * and the order is reshuffled so we have alt down, sysrq down, sysrq up, alt up
+	 */
+	litest_assert_key_event(li, KEY_LEFTALT, LIBINPUT_KEY_STATE_PRESSED);
+	litest_assert_key_event(li, KEY_SYSRQ, LIBINPUT_KEY_STATE_PRESSED);
+	litest_assert_key_event(li, KEY_SYSRQ, LIBINPUT_KEY_STATE_RELEASED);
+	litest_assert_key_event(li, KEY_LEFTALT, LIBINPUT_KEY_STATE_RELEASED);
+
+	litest_assert_empty_queue(li);
+}
+END_TEST
+
 TEST_COLLECTION(keyboard)
 {
 	/* clang-format off */
@@ -484,5 +539,7 @@ TEST_COLLECTION(keyboard)
 	litest_add(keyboard_leds, LITEST_ANY, LITEST_ANY);
 
 	litest_add(keyboard_no_scroll, LITEST_KEYS, LITEST_WHEEL);
+
+	litest_add_for_device(keyboard_alt_printscreen, LITEST_KEYBOARD);
 	/* clang-format on */
 }