cleanup output-background subcommand handling

- fixes a double-free error when access() failed.

- refactor code to make memory managment (alloc/free) more straightforward
   - do not bring the temporary wordexp_t struct around
   - do not postpone errors handling

sway/commands/output/background.c

@@ -69,42 +69,49 @@ struct cmd_results *output_cmd_background(int argc, char **argv) {
 			return cmd_res;
 		}
 		free(src);
-		src = p.we_wordv[0];
+		src = strdup(p.we_wordv[0]);
+		wordfree(&p);
+		if (!src) {
+			wlr_log(L_ERROR, "Failed to duplicate string");
+			return cmd_results_new(CMD_FAILURE, "output",
+				"Unable to allocate resource");
+		}
+
 		if (config->reading && *src != '/') {
+			// src file is inside configuration dir
+
 			char *conf = strdup(config->current_config);
-			if (conf) {
-				char *conf_path = dirname(conf);
-				src = malloc(strlen(conf_path) + strlen(src) + 2);
-				if (!src) {
-					free(conf);
-					wordfree(&p);
-					wlr_log(L_ERROR,
-						"Unable to allocate resource: Not enough memory");
-					return cmd_results_new(CMD_FAILURE, "output",
+			if(!conf) {
+				wlr_log(L_ERROR, "Failed to duplicate string");
+				return cmd_results_new(CMD_FAILURE, "output",
 						"Unable to allocate resources");
-				}
-				sprintf(src, "%s/%s", conf_path, p.we_wordv[0]);
+			}
+
+			char *conf_path = dirname(conf);
+			char *rel_path = src;
+			src = malloc(strlen(conf_path) + strlen(src) + 2);
+			if (!src) {
+				free(rel_path);
 				free(conf);
-			} else {
-				wlr_log(L_ERROR, "Unable to allocate background source");
+				wlr_log(L_ERROR, "Unable to allocate memory");
+				return cmd_results_new(CMD_FAILURE, "output",
+						"Unable to allocate resources");
 			}
+
+			sprintf(src, "%s/%s", conf_path, rel_path);
+			free(rel_path);
+			free(conf);
 		}
 
 		if (access(src, F_OK) == -1) {
 			struct cmd_results *cmd_res = cmd_results_new(CMD_FAILURE, "output",
 				"Unable to access background file '%s': %s", src, strerror(errno));
 			free(src);
-			wordfree(&p);
 			return cmd_res;
 		}
 
-		output->background = strdup(src);
+		output->background = src;
 		output->background_option = strdup(mode);
-		if (src != p.we_wordv[0]) {
-			free(src);
-		}
-		wordfree(&p);
-
 		argc -= j + 1; argv += j + 1;
 	}