seat: Create inert objects for missing capabilities

We should throw a protocol error if the relevant capability has never
existed when get_(pointer|keyboard|touch) is called. Otherwise, it
should succeed, even if the capability is not currently present.

This follows the spec, and avoids possible races with the client when
capabilities are lost.

Closes: https://github.com/swaywm/wlroots/issues/2227

include/wlr/types/wlr_seat.h

@@ -214,6 +214,7 @@ struct wlr_seat {
 
 	char *name;
 	uint32_t capabilities;
+	uint32_t accumulated_capabilities;
 	struct timespec last_event;
 
 	struct wlr_data_source *selection_source;

types/seat/wlr_seat.c

@@ -19,9 +19,9 @@ static void seat_handle_get_pointer(struct wl_client *client,
 		struct wl_resource *seat_resource, uint32_t id) {
 	struct wlr_seat_client *seat_client =
 		wlr_seat_client_from_resource(seat_resource);
-	if (!(seat_client->seat->capabilities & WL_SEAT_CAPABILITY_POINTER)) {
-		wlr_log(WLR_ERROR, "Client sent get_pointer on seat without the "
-			"pointer capability");
+	if (!(seat_client->seat->accumulated_capabilities & WL_SEAT_CAPABILITY_POINTER)) {
+		wl_resource_post_error(seat_resource, 0,
+				"wl_seat.get_pointer called when no pointer capability has existed");
 		return;
 	}
 
@@ -33,9 +33,9 @@ static void seat_handle_get_keyboard(struct wl_client *client,
 		struct wl_resource *seat_resource, uint32_t id) {
 	struct wlr_seat_client *seat_client =
 		wlr_seat_client_from_resource(seat_resource);
-	if (!(seat_client->seat->capabilities & WL_SEAT_CAPABILITY_KEYBOARD)) {
-		wlr_log(WLR_ERROR, "Client sent get_keyboard on seat without the "
-			"keyboard capability");
+	if (!(seat_client->seat->accumulated_capabilities & WL_SEAT_CAPABILITY_KEYBOARD)) {
+		wl_resource_post_error(seat_resource, 0,
+				"wl_seat.get_keyboard called when no keyboard capability has existed");
 		return;
 	}
 
@@ -47,9 +47,9 @@ static void seat_handle_get_touch(struct wl_client *client,
 		struct wl_resource *seat_resource, uint32_t id) {
 	struct wlr_seat_client *seat_client =
 		wlr_seat_client_from_resource(seat_resource);
-	if (!(seat_client->seat->capabilities & WL_SEAT_CAPABILITY_TOUCH)) {
-		wlr_log(WLR_ERROR, "Client sent get_touch on seat without the "
-			"touch capability");
+	if (!(seat_client->seat->accumulated_capabilities & WL_SEAT_CAPABILITY_TOUCH)) {
+		wl_resource_post_error(seat_resource, 0,
+				"wl_seat.get_touch called when no touch capability has existed");
 		return;
 	}
 
@@ -311,6 +311,7 @@ struct wlr_seat_client *wlr_seat_client_for_wl_client(struct wlr_seat *wlr_seat,
 void wlr_seat_set_capabilities(struct wlr_seat *wlr_seat,
 		uint32_t capabilities) {
 	wlr_seat->capabilities = capabilities;
+	wlr_seat->accumulated_capabilities |= capabilities;
 
 	struct wlr_seat_client *client;
 	wl_list_for_each(client, &wlr_seat->clients, link) {

types/seat/wlr_seat_keyboard.c

@@ -410,6 +410,11 @@ void seat_client_create_keyboard(struct wlr_seat_client *seat_client,
 		keyboard_handle_resource_destroy);
 	wl_list_insert(&seat_client->keyboards, wl_resource_get_link(resource));
 
+	if ((seat_client->seat->capabilities & WL_SEAT_CAPABILITY_KEYBOARD) == 0) {
+		wl_resource_set_user_data(resource, NULL);
+		return;
+	}
+
 	struct wlr_keyboard *keyboard = seat_client->seat->keyboard_state.keyboard;
 	if (keyboard == NULL) {
 		return;

types/seat/wlr_seat_pointer.c

@@ -410,6 +410,11 @@ void seat_client_create_pointer(struct wlr_seat_client *seat_client,
 		&pointer_handle_resource_destroy);
 	wl_list_insert(&seat_client->pointers, wl_resource_get_link(resource));
 
+	if ((seat_client->seat->capabilities & WL_SEAT_CAPABILITY_POINTER) == 0) {
+		wl_resource_set_user_data(resource, NULL);
+		return;
+	}
+
 	struct wlr_seat_client *focused_client =
 		seat_client->seat->pointer_state.focused_client;
 	struct wlr_surface *focused_surface =

types/seat/wlr_seat_touch.c

@@ -366,6 +366,10 @@ void seat_client_create_touch(struct wlr_seat_client *seat_client,
 	wl_resource_set_implementation(resource, &touch_impl, seat_client,
 		&touch_handle_resource_destroy);
 	wl_list_insert(&seat_client->touches, wl_resource_get_link(resource));
+
+	if ((seat_client->seat->capabilities & WL_SEAT_CAPABILITY_TOUCH) == 0) {
+		wl_resource_set_user_data(resource, NULL);
+	}
 }
 
 void seat_client_destroy_touch(struct wl_resource *resource) {