database: generate GError when database is disabled

src/command.c

@@ -395,6 +395,12 @@ print_error(struct client *client, GError *error)
 		return print_playlist_result(client, result);
 	} else if (error->domain == db_quark()) {
 		switch ((enum db_error)error->code) {
+		case DB_DISABLED:
+			command_error(client, ACK_ERROR_NO_EXIST, "%s",
+				      error->message);
+			g_error_free(error);
+			return COMMAND_RETURN_ERROR;
+
 		case DB_NOT_FOUND:
 			g_error_free(error);
 			command_error(client, ACK_ERROR_NO_EXIST, "Not found");

src/database.c

@@ -27,6 +27,7 @@
 #include "directory.h"
 #include "stats.h"
 #include "conf.h"
+#include "glib_compat.h"
 
 #include <glib.h>
 
@@ -112,12 +113,14 @@ db_walk(const char *uri,
 	const struct db_visitor *visitor, void *ctx,
 	GError **error_r)
 {
-	struct directory *directory;
-
-	if (db == NULL)
-		return -1;
+	if (db == NULL) {
+		g_set_error_literal(error_r, db_quark(), DB_DISABLED,
+				    "No database");
+		return false;
+	}
 
-	if ((directory = db_get_directory(uri)) == NULL) {
+	struct directory *directory = db_get_directory(uri);
+	if (directory == NULL) {
 		struct song *song;
 		if (visitor->song != NULL &&
 		    (song = db_get_song(uri)) != NULL)

src/db_error.h

@@ -23,6 +23,12 @@
 #include <glib.h>
 
 enum db_error {
+	/**
+	 * The database is disabled, i.e. none is configured in this
+	 * MPD instance.
+	 */
+	DB_DISABLED,
+
 	DB_NOT_FOUND,
 };