Don't restart swaybg if the command hasn't changed

When outputs have changed but none of that has resulted in a change in
how swaybg should be run don't restart it. This avoids flickering the
background to grey as a result of bug #3693, and is generally good to
not churn through process termination and startup for a no-op.

include/sway/config.h

@@ -476,6 +476,7 @@ struct sway_config {
 
 	// swaybg
 	char *swaybg_command;
+	char **swaybg_full_command;
 	struct wl_client *swaybg_client;
 	struct wl_listener swaybg_client_destroy;
 
@@ -586,6 +587,7 @@ void config_add_swaynag_warning(char *fmt, ...);
 /**
  * Free config struct
  */
+void free_swaybg_full_command(char** cmd);
 void free_config(struct sway_config *config);
 
 void free_sway_variable(struct sway_variable *var);

sway/config.c

@@ -85,6 +85,19 @@ static void free_mode(struct sway_mode *mode) {
 	free(mode);
 }
 
+void free_swaybg_full_command(char **cmd) {
+	if (!cmd) {
+		return;
+	}
+
+	char **pos = cmd;
+	while (*pos) {
+		free(*pos);
+		pos++;
+	}
+	free(cmd);
+}
+
 void free_config(struct sway_config *config) {
 	if (!config) {
 		return;
@@ -160,6 +173,7 @@ void free_config(struct sway_config *config) {
 	free(config->floating_scroll_right_cmd);
 	free(config->font);
 	free(config->swaybg_command);
+	free_swaybg_full_command(config->swaybg_full_command);
 	free(config->swaynag_command);
 	free((char *)config->current_config_path);
 	free((char *)config->current_config);

sway/config/output.c

@@ -578,7 +578,35 @@ static void handle_swaybg_client_destroy(struct wl_listener *listener,
 	sway_config->swaybg_client = NULL;
 }
 
+static int compare_swaybg_cmd(char **cmd1, char **cmd2) {
+	while (true) {
+		if (*cmd1 == NULL && *cmd2 == NULL) {
+			return 0;
+		} else if (*cmd1 == NULL || *cmd2 == NULL) {
+			return 1;
+		} else if (strcmp(*cmd1, *cmd2) != 0) {
+			return 1;
+		} else {
+			cmd1++;
+			cmd2++;
+		}
+	}
+}
+
 static bool _spawn_swaybg(char **command) {
+	if (config->swaybg_full_command != NULL &&
+		compare_swaybg_cmd(command, config->swaybg_full_command) == 0) {
+		// We got the same command as before, ignore it
+		free_swaybg_full_command(command);
+		return true;
+	}
+
+	// Save the command for later comparisons
+	if (config->swaybg_full_command != NULL) {
+		free_swaybg_full_command(config->swaybg_full_command);
+	}
+	config->swaybg_full_command = command;
+
 	if (config->swaybg_client != NULL) {
 		wl_client_destroy(config->swaybg_client);
 	}
@@ -653,35 +681,34 @@ bool spawn_swaybg(void) {
 	}
 
 	size_t i = 0;
-	cmd[i++] = config->swaybg_command;
+	cmd[i++] = strdup(config->swaybg_command);
 
 	// Iterate all the outputs and write the command
-	list_t *output_configs = create_list();
 	struct sway_output *sway_output, *tmp;
 	wl_list_for_each_safe(sway_output, tmp, &root->all_outputs, link) {
 		struct output_config *oc = find_output_config(sway_output);
-		list_add(output_configs, oc);
 		if (!oc->background) {
 			continue;
 		}
 		if (strcmp(oc->background_option, "solid_color") == 0) {
-			cmd[i++] = "-o";
-			cmd[i++] = oc->name;
-			cmd[i++] = "-c";
-			cmd[i++] = oc->background;
+			cmd[i++] = strdup("-o");
+			cmd[i++] = strdup(oc->name);
+			cmd[i++] = strdup("-c");
+			cmd[i++] = strdup(oc->background);
 		} else {
-			cmd[i++] = "-o";
-			cmd[i++] = oc->name;
-			cmd[i++] = "-i";
-			cmd[i++] = oc->background;
-			cmd[i++] = "-m";
-			cmd[i++] = oc->background_option;
+			cmd[i++] = strdup("-o");
+			cmd[i++] = strdup(oc->name);
+			cmd[i++] = strdup("-i");
+			cmd[i++] = strdup(oc->background);
+			cmd[i++] = strdup("-m");
+			cmd[i++] = strdup(oc->background_option);
 			if (oc->background_fallback) {
-				cmd[i++] = "-c";
-				cmd[i++] = oc->background_fallback;
+				cmd[i++] = strdup("-c");
+				cmd[i++] = strdup(oc->background_fallback);
 			}
 		}
 		assert(i < length);
+		free_output_config(oc);
 	}
 
 	for (size_t k = 0; k < i; k++) {
@@ -690,11 +717,5 @@ bool spawn_swaybg(void) {
 
 	bool result = _spawn_swaybg(cmd);
 
-	free(cmd);
-	for (int k = 0; k < output_configs->length; k++) {
-		free_output_config(output_configs->items[k]);
-	}
-	list_free(output_configs);
-
 	return result;
 }