output: introduce wlr_output_test

backend/drm/drm.c

@@ -334,6 +334,10 @@ static bool drm_connector_attach_render(struct wlr_output *output,
 	return make_drm_surface_current(&conn->crtc->primary->surf, buffer_age);
 }
 
+static bool drm_connector_test(struct wlr_output *output) {
+	return true;
+}
+
 static bool drm_connector_commit_buffer(struct wlr_output *output) {
 	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
 	struct wlr_drm_backend *drm = get_drm_backend_from_backend(output->backend);
@@ -455,6 +459,10 @@ static bool drm_connector_set_custom_mode(struct wlr_output *output,
 static bool drm_connector_commit(struct wlr_output *output) {
 	struct wlr_drm_backend *drm = get_drm_backend_from_backend(output->backend);
 
+	if (!drm_connector_test(output)) {
+		return false;
+	}
+
 	if (!drm->session->active) {
 		return false;
 	}
@@ -501,7 +509,7 @@ static bool drm_connector_commit(struct wlr_output *output) {
 
 static void fill_empty_gamma_table(size_t size,
 		uint16_t *r, uint16_t *g, uint16_t *b) {
-	assert(0xFFFF < UINT64_MAX / (size - 1));	
+	assert(0xFFFF < UINT64_MAX / (size - 1));
 	for (uint32_t i = 0; i < size; ++i) {
 		uint16_t val = (uint64_t)0xffff * i / (size - 1);
 		r[i] = g[i] = b[i] = val;
@@ -1068,6 +1076,7 @@ static const struct wlr_output_impl output_impl = {
 	.move_cursor = drm_connector_move_cursor,
 	.destroy = drm_connector_destroy,
 	.attach_render = drm_connector_attach_render,
+	.test = drm_connector_test,
 	.commit = drm_connector_commit,
 	.set_gamma = set_drm_connector_gamma,
 	.get_gamma_size = drm_connector_get_gamma_size,

backend/headless/output.c

@@ -57,7 +57,7 @@ static bool output_attach_render(struct wlr_output *wlr_output,
 		buffer_age);
 }
 
-static bool output_commit(struct wlr_output *wlr_output) {
+static bool output_test(struct wlr_output *wlr_output) {
 	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_ENABLED) {
 		wlr_log(WLR_DEBUG, "Cannot disable a headless output");
 		return false;
@@ -65,6 +65,17 @@ static bool output_commit(struct wlr_output *wlr_output) {
 
 	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_MODE) {
 		assert(wlr_output->pending.mode_type == WLR_OUTPUT_STATE_MODE_CUSTOM);
+	}
+
+	return true;
+}
+
+static bool output_commit(struct wlr_output *wlr_output) {
+	if (!output_test(wlr_output)) {
+		return false;
+	}
+
+	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_MODE) {
 		if (!output_set_custom_mode(wlr_output,
 				wlr_output->pending.custom_mode.width,
 				wlr_output->pending.custom_mode.height,

backend/wayland/output.c

@@ -197,17 +197,28 @@ static bool output_attach_buffer(struct wlr_output *wlr_output,
 	return true;
 }
 
+static bool output_test(struct wlr_output *wlr_output) {
+	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_ENABLED) {
+		wlr_log(WLR_DEBUG, "Cannot disable a Wayland output");
+		return false;
+	}
+
+	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_MODE) {
+		assert(wlr_output->pending.mode_type == WLR_OUTPUT_STATE_MODE_CUSTOM);
+	}
+
+	return true;
+}
+
 static bool output_commit(struct wlr_output *wlr_output) {
 	struct wlr_wl_output *output =
 		get_wl_output_from_output(wlr_output);
 
-	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_ENABLED) {
+	if (!output_test(wlr_output)) {
-		wlr_log(WLR_DEBUG, "Cannot disable a Wayland output");
 		return false;
 	}
 
 	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_MODE) {
-		assert(wlr_output->pending.mode_type == WLR_OUTPUT_STATE_MODE_CUSTOM);
 		if (!output_set_custom_mode(wlr_output,
 				wlr_output->pending.custom_mode.width,
 				wlr_output->pending.custom_mode.height,
@@ -419,6 +430,7 @@ static const struct wlr_output_impl output_impl = {
 	.destroy = output_destroy,
 	.attach_render = output_attach_render,
 	.attach_buffer  = output_attach_buffer,
+	.test = output_test,
 	.commit = output_commit,
 	.set_cursor = output_set_cursor,
 	.move_cursor = output_move_cursor,

backend/x11/output.c

@@ -99,17 +99,28 @@ static bool output_attach_render(struct wlr_output *wlr_output,
 	return wlr_egl_make_current(&x11->egl, output->surf, buffer_age);
 }
 
+static bool output_test(struct wlr_output *wlr_output) {
+	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_ENABLED) {
+		wlr_log(WLR_DEBUG, "Cannot disable an X11 output");
+		return false;
+	}
+
+	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_MODE) {
+		assert(wlr_output->pending.mode_type == WLR_OUTPUT_STATE_MODE_CUSTOM);
+	}
+
+	return true;
+}
+
 static bool output_commit(struct wlr_output *wlr_output) {
 	struct wlr_x11_output *output = get_x11_output_from_output(wlr_output);
 	struct wlr_x11_backend *x11 = output->x11;
 
-	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_ENABLED) {
+	if (!output_test(wlr_output)) {
-		wlr_log(WLR_DEBUG, "Cannot disable an X11 output");
 		return false;
 	}
 
 	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_MODE) {
-		assert(wlr_output->pending.mode_type == WLR_OUTPUT_STATE_MODE_CUSTOM);
 		if (!output_set_custom_mode(wlr_output,
 				wlr_output->pending.custom_mode.width,
 				wlr_output->pending.custom_mode.height,
@@ -152,6 +163,7 @@ static bool output_commit(struct wlr_output *wlr_output) {
 static const struct wlr_output_impl output_impl = {
 	.destroy = output_destroy,
 	.attach_render = output_attach_render,
+	.test = output_test,
 	.commit = output_commit,
 };
 

include/wlr/interfaces/wlr_output.h

@@ -21,6 +21,7 @@ struct wlr_output_impl {
 	bool (*move_cursor)(struct wlr_output *output, int x, int y);
 	void (*destroy)(struct wlr_output *output);
 	bool (*attach_render)(struct wlr_output *output, int *buffer_age);
+	bool (*test)(struct wlr_output *output);
 	bool (*commit)(struct wlr_output *output);
 	bool (*set_gamma)(struct wlr_output *output, size_t size,
 		const uint16_t *r, const uint16_t *g, const uint16_t *b);

include/wlr/types/wlr_output.h

@@ -337,6 +337,14 @@ bool wlr_output_preferred_read_format(struct wlr_output *output,
  */
 void wlr_output_set_damage(struct wlr_output *output,
 	pixman_region32_t *damage);
+/**
+ * Test whether the pending output state would be accepted by the backend. If
+ * this function returns true, `wlr_output_commit` can only fail due to a
+ * runtime error.
+ *
+ * This function doesn't mutate the pending state.
+ */
+bool wlr_output_test(struct wlr_output *output);
 /**
  * Commit the pending output state. If `wlr_output_attach_render` has been
  * called, the pending frame will be submitted for display and a `frame` event

types/wlr_output.c

@@ -473,16 +473,11 @@ static void output_state_clear(struct wlr_output_state *state) {
 	state->committed = 0;
 }
 
-bool wlr_output_commit(struct wlr_output *output) {
+static bool output_basic_test(struct wlr_output *output) {
-	if (output->pending.committed & WLR_OUTPUT_STATE_BUFFER) {
+	if ((output->pending.committed & WLR_OUTPUT_STATE_BUFFER) &&
-		if (output->frame_pending) {
+			output->frame_pending) {
-			wlr_log(WLR_ERROR, "Tried to commit a buffer while a frame is pending");
+		wlr_log(WLR_DEBUG, "Tried to commit a buffer while a frame is pending");
-			goto error;
+		return false;
-		}
-		if (output->idle_frame != NULL) {
-			wl_event_source_remove(output->idle_frame);
-			output->idle_frame = NULL;
-		}
 	}
 
 	bool enabled = output->enabled;
@@ -491,16 +486,38 @@ bool wlr_output_commit(struct wlr_output *output) {
 	}
 
 	if (!enabled && output->pending.committed & WLR_OUTPUT_STATE_BUFFER) {
-		wlr_log(WLR_ERROR, "Tried to commit a buffer on a disabled output");
+		wlr_log(WLR_DEBUG, "Tried to commit a buffer on a disabled output");
-		goto error;
+		return false;
 	}
 	if (!enabled && output->pending.committed & WLR_OUTPUT_STATE_MODE) {
-		wlr_log(WLR_ERROR, "Tried to modeset a disabled output");
+		wlr_log(WLR_DEBUG, "Tried to modeset a disabled output");
-		goto error;
+		return false;
 	}
 	if (!enabled && output->pending.committed & WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED) {
-		wlr_log(WLR_ERROR, "Tried to enable adaptive sync on a disabled output");
+		wlr_log(WLR_DEBUG, "Tried to enable adaptive sync on a disabled output");
-		goto error;
+		return false;
+	}
+
+	return true;
+}
+
+bool wlr_output_test(struct wlr_output *output) {
+	if (!output_basic_test(output)) {
+		return false;
+	}
+	return output->impl->test(output);
+}
+
+bool wlr_output_commit(struct wlr_output *output) {
+	if (!output_basic_test(output)) {
+		wlr_log(WLR_ERROR, "Basic output test failed");
+		return false;
+	}
+
+	if ((output->pending.committed & WLR_OUTPUT_STATE_BUFFER) &&
+			output->idle_frame != NULL) {
+		wl_event_source_remove(output->idle_frame);
+		output->idle_frame = NULL;
 	}
 
 	struct timespec now;
@@ -565,10 +582,6 @@ bool wlr_output_commit(struct wlr_output *output) {
 
 	output_state_clear(&output->pending);
 	return true;
-
-error:
-	output_state_clear(&output->pending);
-	return false;
 }
 
 void wlr_output_rollback(struct wlr_output *output) {