db/DatabaseLock: add class ScopeDatabaseUnlock

src/db/DatabaseLock.hxx

@@ -107,4 +107,18 @@ public:
 	}
 };
 
+/**
+ * Unlock the database while in the current scope.
+ */
+class ScopeDatabaseUnlock {
+public:
+	ScopeDatabaseUnlock() {
+		db_unlock();
+	}
+
+	~ScopeDatabaseUnlock() {
+		db_lock();
+	}
+};
+
 #endif

src/db/plugins/simple/Directory.cxx

@@ -232,14 +232,12 @@ Directory::Walk(bool recursive, const SongFilter *filter,
 		/* TODO: eliminate this unlock/lock; it is necessary
 		   because the child's SimpleDatabasePlugin::Visit()
 		   call will lock it again */
-		db_unlock();
-		bool result = WalkMount(GetPath(), *mounted_database,
-					recursive, filter,
-					visit_directory, visit_song,
-					visit_playlist,
-					error);
-		db_lock();
-		return result;
+		const ScopeDatabaseUnlock unlock;
+		return WalkMount(GetPath(), *mounted_database,
+				 recursive, filter,
+				 visit_directory, visit_song,
+				 visit_playlist,
+				 error);
 	}
 
 	if (visit_song) {

src/db/update/Editor.cxx

@@ -36,15 +36,14 @@ DatabaseEditor::DeleteSong(Directory &dir, Song *del)
 	/* first, prevent traversers in main task from getting this */
 	dir.RemoveSong(del);
 
-	db_unlock(); /* temporary unlock, because update_remove_song() blocks */
+	/* temporary unlock, because update_remove_song() blocks */
+	const ScopeDatabaseUnlock unlock;
 
 	/* now take it out of the playlist (in the main_task) */
 	remove.Remove(del);
 
 	/* finally, all possible references gone, free it */
 	del->Free();
-
-	db_lock();
 }
 
 void