config/output: Store output configs sequentially

The current output configuration system merges configurations such that
only one configuration per match is stored. This is a simplification of
a previous design and has the benefit that a minimal number of output
configurations are retained, but to correctly merge and supersede
configurations it is required that an identifier can be resolved from a
connector, which leads to differences in how output configurations are
interpreted based on whether a display is currently connected or not.

Instead, append all new output configurations to the list. To mitigate
unbounded growth, supersede old configurations that are an exact match
by clearing the fiels that can no longer take effect. Once a particular
output configuration no longer contains any set fields, remove it from
the list.

Based on work by Pedro Côrte-Real <pedro@pedrocr.net> in
#5629 which targeted an older
iteration of the output configuration system.

Author: kennylevinsen

sway/config/output.c

@@ -82,6 +82,30 @@ struct output_config *new_output_config(const char *name) {
 	return oc;
 }
 
+static bool empty_output_config(struct output_config *oc) {
+	return oc->enabled == -1 &&
+		oc->width == -1 &&
+		oc->height == -1 &&
+		oc->x == INT_MAX &&
+		oc->y == INT_MAX &&
+		oc->scale == -1 &&
+		oc->scale_filter == SCALE_FILTER_DEFAULT &&
+		oc->subpixel == WL_OUTPUT_SUBPIXEL_UNKNOWN &&
+		oc->refresh_rate == -1 &&
+		oc->custom_mode == -1 &&
+		oc->drm_mode.type == (uint32_t)-1 &&
+		oc->transform == -1 &&
+		oc->max_render_time == -1 &&
+		oc->adaptive_sync == -1 &&
+		oc->render_bit_depth == RENDER_BIT_DEPTH_DEFAULT &&
+		oc->set_color_transform == false &&
+		oc->background == NULL &&
+		oc->background_option == NULL &&
+		oc->background_fallback == NULL &&
+		oc->power == -1 &&
+		oc->allow_tearing == -1;
+}
+
 // supersede_output_config clears all fields in dst that were set in src
 static void supersede_output_config(struct output_config *dst, struct output_config *src) {
 	if (src->enabled != -1) {
@@ -232,57 +256,37 @@ static void merge_output_config(struct output_config *dst, struct output_config
 }
 
 void store_output_config(struct output_config *oc) {
-	bool merged = false;
 	bool wildcard = strcmp(oc->name, "*") == 0;
-	struct sway_output *output = wildcard ? NULL : all_output_by_name_or_id(oc->name);
-
-	char id[128];
-	if (output) {
-		output_get_identifier(id, sizeof(id), output);
-	}
-
-	for (int i = 0; i < config->output_configs->length; i++) {
-		struct output_config *old = config->output_configs->items[i];
-
-		// If the old config matches the new config's name, regardless of
-		// whether it was name or identifier, merge on top of the existing
-		// config. If the new config is a wildcard, this also merges on top of
-		// old wildcard configs.
-		if (strcmp(old->name, oc->name) == 0) {
-			merge_output_config(old, oc);
-			merged = true;
-			continue;
-		}
-
-		// If the new config is a wildcard config we supersede all non-wildcard
-		// configs. Old wildcard configs have already been handled above.
-		if (wildcard) {
-			supersede_output_config(old, oc);
-			continue;
-		}
 
-		// If the new config matches an output's name, and the old config
-		// matches on that output's identifier, supersede it.
-		if (output && strcmp(old->name, id) == 0 &&
-				strcmp(oc->name, output->wlr_output->name) == 0) {
+	// Supersede any existing configurations that match by clearing settings
+	// that can no longer be reached anyway, which allows us to later garbage
+	// collect configurations that no longer have any value
+	struct output_config *old;
+	for (int idx = 0; idx < config->output_configs->length; idx++) {
+		old = config->output_configs->items[idx];
+		if (wildcard || strcmp(old->name, oc->name) == 0) {
 			supersede_output_config(old, oc);
 		}
 	}
 
-	sway_log(SWAY_DEBUG, "Config stored for output %s (enabled: %d) (%dx%d@%fHz "
-		"position %d,%d scale %f subpixel %s transform %d) (bg %s %s) (power %d) "
-		"(max render time: %d) (allow tearing: %d)",
-		oc->name, oc->enabled, oc->width, oc->height, oc->refresh_rate,
-		oc->x, oc->y, oc->scale, sway_wl_output_subpixel_to_string(oc->subpixel),
-		oc->transform, oc->background, oc->background_option, oc->power,
-		oc->max_render_time, oc->allow_tearing);
-
-	// If the configuration was not merged into an existing configuration, add
-	// it to the list. Otherwise we're done with it and can free it.
-	if (!merged) {
-		list_add(config->output_configs, oc);
-	} else {
+	// As a very minor optimization, if the last config is for the same name,
+	// we can merge with that instead of adding a new config, reducing the
+	// number of configs when sequentially changing different settings
+	if (old != NULL && strcmp(old->name, oc->name) == 0) {
+		merge_output_config(old, oc);
 		free_output_config(oc);
+	} else {
+		list_add(config->output_configs, oc);
+	}
+
+	// Remove any configurations that have had all their settings removed
+	for (int idx = 0; idx < config->output_configs->length; idx++) {
+		old = config->output_configs->items[idx];
+		if (empty_output_config(old)) {
+			list_del(config->output_configs, idx);
+			free_output_config(old);
+			idx--;
+		}
 	}
 }
 
@@ -588,14 +592,13 @@ static struct output_config *find_output_config_from_list(
 	char id[128];
 	output_get_identifier(id, sizeof(id), sway_output);
 
-	// We take a new config and merge on top, in order, the wildcard config,
-	// output config by name, and output config by identifier to form the final
-	// config. If there are multiple matches, they are merged in order.
-	struct output_config *oc = NULL;
-	const char *names[] = {"*", name, id, NULL};
-	for (const char **name = &names[0]; *name; name++) {
-		for (size_t idx = 0; idx < configs_len; idx++) {
-			oc = configs[idx];
+	// To create the output config for this output, we merge all configurations
+	// that match the output in the order they were stored, such that later
+	// configurations override earlier ones
+	for (size_t idx = 0; idx < configs_len; idx++) {
+		struct output_config *oc = configs[idx];
+		const char *names[] = {"*", name, id, NULL};
+		for (const char **name = &names[0]; *name; name++) {
 			if (strcmp(oc->name, *name) == 0) {
 				merge_output_config(result, oc);
 			}