wayland backend: fix use-after free on output destroy

==12021==ERROR: AddressSanitizer: heap-use-after-free on address 0x617000015698 at pc 0x7f1a9abe1c09 bp 0x7ffe9068f6b0 sp 0x7ffe9068f6a0
WRITE of size 4 at 0x617000015698 thread T0
    #0 0x7f1a9abe1c08 in pointer_handle_leave ../backend/wayland/wl_seat.c:40
    #1 0x7f1a96ae7d1d in ffi_call_unix64 (/lib64/libffi.so.6+0x5d1d)
    #2 0x7f1a96ae768e in ffi_call (/lib64/libffi.so.6+0x568e)
    #3 0x7f1a988e0d8a  (/lib64/libwayland-client.so.0+0x8d8a)
    #4 0x7f1a988dd927  (/lib64/libwayland-client.so.0+0x5927)
    #5 0x7f1a988debe3 in wl_display_dispatch_queue_pending (/lib64/libwayland-client.so.0+0x6be3)
    #6 0x7f1a9abdd6d6 in dispatch_events ../backend/wayland/backend.c:28
    #7 0x7f1a9a968c11 in wl_event_loop_dispatch (/lib64/libwayland-server.so.0+0x9c11)
    #8 0x7f1a9a967449 in wl_display_run (/lib64/libwayland-server.so.0+0x8449)
    #9 0x418dff in main ../rootston/main.c:81
    #10 0x7f1a99b5ef29 in __libc_start_main (/lib64/libc.so.6+0x20f29)
    #11 0x4057c9 in _start (/home/shared/wayland/wlroots/build/rootston/rootston+0x4057c9)

0x617000015698 is located 664 bytes inside of 696-byte region [0x617000015400,0x6170000156b8)
freed by thread T0 here:
    #0 0x7f1a9af754b8 in __interceptor_free (/lib64/libasan.so.4+0xde4b8)
    #1 0x7f1a9abe01ee in wlr_wl_output_destroy ../backend/wayland/output.c:194
    #2 0x7f1a9ac12918 in wlr_output_destroy ../types/wlr_output.c:299
    #3 0x7f1a9abe061b in xdg_toplevel_handle_close ../backend/wayland/output.c:255
    #4 0x7f1a96ae7d1d in ffi_call_unix64 (/lib64/libffi.so.6+0x5d1d)
    #5 0x7f1a96ae768e in ffi_call (/lib64/libffi.so.6+0x568e)
    #6 0x7f1a988e0d8a  (/lib64/libwayland-client.so.0+0x8d8a)
    #7 0x7f1a988dd927  (/lib64/libwayland-client.so.0+0x5927)
    #8 0x7f1a988debe3 in wl_display_dispatch_queue_pending (/lib64/libwayland-client.so.0+0x6be3)
    #9 0x7f1a9abdd6d6 in dispatch_events ../backend/wayland/backend.c:28
    #10 0x7f1a9a968c11 in wl_event_loop_dispatch (/lib64/libwayland-server.so.0+0x9c11)
    #11 0x7f1a9a967449 in wl_display_run (/lib64/libwayland-server.so.0+0x8449)
    #12 0x418dff in main ../rootston/main.c:81
    #13 0x7f1a99b5ef29 in __libc_start_main (/lib64/libc.so.6+0x20f29)
    #14 0x4057c9 in _start (/home/shared/wayland/wlroots/build/rootston/rootston+0x4057c9)

previously allocated by thread T0 here:
    #0 0x7f1a9af75a38 in __interceptor_calloc (/lib64/libasan.so.4+0xdea38)
    #1 0x7f1a9abe0703 in wlr_wl_output_create ../backend/wayland/output.c:272
    #2 0x7f1a9abdd8eb in wlr_wl_backend_start ../backend/wayland/backend.c:55
    #3 0x7f1a9abbeb49 in wlr_backend_start ../backend/backend.c:28
    #4 0x7f1a9abd8ce1 in multi_backend_start ../backend/multi/backend.c:24
    #5 0x7f1a9abbeb49 in wlr_backend_start ../backend/backend.c:28
    #6 0x418c32 in main ../rootston/main.c:58
    #7 0x7f1a99b5ef29 in __libc_start_main (/lib64/libc.so.6+0x20f29)
    #8 0x4057c9 in _start (/home/shared/wayland/wlroots/build/rootston/rootston+0x4057c9)

1 files changed, 16 insertions, 1 deletions

diff --git a/backend/wayland/wl_seat.c b/backend/wayland/wl_seat.c
index 841e693d..6ca59130 100644
--- a/backend/wayland/wl_seat.c
+++ b/backend/wayland/wl_seat.c
@@ -26,6 +26,11 @@ static void pointer_handle_enter(void *data, struct wl_pointer *wl_pointer,
 		// GNOME sends a pointer enter when the surface is being destroyed
 		return;
 	}
+	if (wlr_wl_pointer->current_output) {
+		wl_list_remove(&wlr_wl_pointer->output_destroy_listener.link);
+	}
+	wl_signal_add(&output->wlr_output.events.destroy,
+		&wlr_wl_pointer->output_destroy_listener);
 	wlr_wl_pointer->current_output = output;
 	output->enter_serial = serial;
 	wlr_wl_output_update_cursor(output);
@@ -49,7 +54,7 @@ static void pointer_handle_motion(void *data, struct wl_pointer *wl_pointer,
 	struct wlr_wl_pointer *wlr_wl_pointer =
 		(struct wlr_wl_pointer *)dev->pointer;
 	if (!wlr_wl_pointer->current_output) {
-		wlr_log(L_ERROR, "pointer motion event without current output");
+		wlr_log(L_DEBUG, "pointer motion event without current output");
 		return;
 	}
 
@@ -231,6 +236,14 @@ static struct wlr_input_device *allocate_device(struct wlr_wl_backend *backend,
 	return wlr_device;
 }
 
+static void wlr_wl_pointer_handle_output_destroy(struct wl_listener *listener,
+		void *data) {
+	struct wlr_wl_pointer *wlr_wl_pointer =
+		wl_container_of(listener, wlr_wl_pointer, output_destroy_listener);
+	wlr_wl_pointer->current_output = NULL;
+	wl_list_remove(&wlr_wl_pointer->output_destroy_listener.link);
+}
+
 static void seat_handle_capabilities(void *data, struct wl_seat *wl_seat,
 		enum wl_seat_capability caps) {
 	struct wlr_wl_backend *backend = data;
@@ -243,6 +256,8 @@ static void seat_handle_capabilities(void *data, struct wl_seat *wl_seat,
 			wlr_log(L_ERROR, "Unable to allocate wlr_wl_pointer");
 			return;
 		}
+		wlr_wl_pointer->output_destroy_listener.notify =
+			wlr_wl_pointer_handle_output_destroy;
 
 		struct wlr_input_device *wlr_device;
 		if (!(wlr_device = allocate_device(backend, WLR_INPUT_DEVICE_POINTER))) {