Avoid no-op output reconfiguring

When an output command doesn't actually change anything in the output
config the processing will still all be done and cause other changes.
Notably if you run a command like:

	output * dpms on

even if it does nothing it will still cause all the inputs to be
reconfigured and very noticeable amounts of input latency to occur.

To fix this save the current output config in the output data and on
configure, if the config is the same just return without doing anything
else.

Fixes swaywm#6280

Author: pedrocr

include/sway/config.h

@@ -638,6 +638,10 @@ struct output_config *new_output_config(const char *name);
 
 void merge_output_config(struct output_config *dst, struct output_config *src);
 
+struct output_config *duplicate_output_config(const struct output_config *src);
+
+bool compare_output_config(const struct output_config *dst, const struct output_config *src);
+
 bool apply_output_config(struct output_config *oc, struct sway_output *output);
 
 bool test_output_config(struct output_config *oc, struct sway_output *output);

include/sway/output.h

@@ -57,6 +57,8 @@ struct sway_output {
 	uint32_t refresh_nsec;
 	int max_render_time; // In milliseconds
 	struct wl_event_source *repaint_timer;
+
+	struct output_config *current_config;
 };
 
 struct sway_output *output_create(struct wlr_output *wlr_output);

sway/config/output.c

@@ -119,6 +119,67 @@ void merge_output_config(struct output_config *dst, struct output_config *src) {
 	}
 }
 
+struct output_config *duplicate_output_config(const struct output_config *src) {
+	struct output_config *oc = calloc(1, sizeof(struct output_config));
+	if (oc == NULL) {
+		return NULL;
+	}
+	oc->name = strdup(src->name);
+	if (oc->name == NULL) {
+		free(oc);
+		return NULL;
+	}
+	oc->enabled = src->enabled;
+	oc->width = src->width;
+	oc->height = src->height;
+	oc->refresh_rate = src->refresh_rate;
+	oc->custom_mode = src->custom_mode;
+	oc->x = src->x;
+	oc->y = src->y;
+	oc->scale = src->scale;
+	oc->scale_filter = src->scale_filter;
+	oc->transform = src->transform;
+	oc->subpixel = src->subpixel;
+	oc->max_render_time = src->max_render_time;
+	oc->adaptive_sync = src->adaptive_sync;
+	if (src->background) {
+		oc->background = strdup(src->background);
+	}
+	if (src->background_option) {
+		oc->background_option = strdup(src->background_option);
+	}
+	if (src->background_fallback) {
+		oc->background_fallback = strdup(src->background_fallback);
+	}
+	oc->dpms_state = src->dpms_state;
+	return oc;
+}
+
+bool compare_linked_strings(const char *first, const char *second) {
+	return (first == NULL && second == NULL) ||
+		(first != NULL && second != NULL && !strcmp(first, second));
+}
+
+bool compare_output_config(const struct output_config *dst, const struct output_config *src) {
+	return dst->enabled == src->enabled &&
+		dst->width == src->width &&
+		dst->height == src->height &&
+		dst->x == src->x &&
+		dst->y == src->y &&
+		dst->scale == src->scale &&
+		dst->scale_filter == src->scale_filter &&
+		dst->subpixel == src->subpixel &&
+		dst->refresh_rate == src->refresh_rate &&
+		dst->custom_mode == src->custom_mode &&
+		dst->transform == src->transform &&
+		dst->max_render_time == src->max_render_time &&
+		dst->adaptive_sync == src->adaptive_sync &&
+		compare_linked_strings(dst->background, src->background) &&
+		compare_linked_strings(dst->background_option, src->background_option) &&
+		compare_linked_strings(dst->background_fallback, src->background_fallback) &&
+		dst->dpms_state == src->dpms_state;
+}
+
 enum output_config_changes {
 	OUTPUT_CONFIG_ENABLED = 1 << 1,
 	OUTPUT_CONFIG_WIDTH = 1 << 2,
@@ -389,6 +450,16 @@ bool apply_output_config(struct output_config *oc, struct sway_output *output) {
 
 	struct wlr_output *wlr_output = output->wlr_output;
 
+	if (output->current_config && compare_output_config(oc, output->current_config)) {
+		// The output config is unchanged so bail out early
+		sway_log(SWAY_DEBUG, "Config for output %s is unchanged", wlr_output->name);
+		return true;
+	}
+	if (output->current_config) {
+		free_output_config(output->current_config);
+	}
+	output->current_config = duplicate_output_config(oc);
+
 	// Flag to prevent the output mode event handler from calling us
 	output->enabling = (!oc || oc->enabled);