directory: simplify list update handling logic

Now the "update" command can be issued multiple times regardless
of whether the client is in list mode or not.

We serialize the update tasks to prevent updates from trampling
over each other and will spawn another update task
once the current one is finished updating and reaped.

Right now we cap the queue size to 32 which is probably enough (I
bet most people usually run update with no argument anyways);
but we can make it grow/shrink dynamically if needed.  There'll
still be a hard-coded limit to prevent DoS attacks, though.

src/directory.c

@@ -53,13 +53,18 @@ enum update_progress {
 	UPDATE_PROGRESS_DONE = 2
 } progress;
 
+/* make this dynamic?, or maybe this is big enough... */
+static char *update_paths[32];
+static size_t update_paths_nr;
+
 static Directory *music_root;
 
 static time_t directory_dbModTime;
 
 static pthread_t update_thr;
 
-static int directory_updateJobId;
+static const int update_task_id_max = 1 << 15;
+static int update_task_id;
 
 static enum update_return
 addToDirectory(Directory * directory, const char *name);
@@ -99,74 +104,84 @@ static char *getDbFile(void)
 
 int isUpdatingDB(void)
 {
-	return (progress != UPDATE_PROGRESS_IDLE) ? directory_updateJobId : 0;
+	return (progress != UPDATE_PROGRESS_IDLE) ? update_task_id : 0;
+}
+
+static void * update_task(void *_path)
+{
+	enum update_return ret = UPDATE_RETURN_NOUPDATE;
+
+	if (_path) {
+		ret = updatePath((char *)_path);
+		free(_path);
+	} else {
+		ret = updateDirectory(music_root);
+	}
+
+	if (ret == UPDATE_RETURN_UPDATED && writeDirectoryDB() < 0)
+		ret = UPDATE_RETURN_ERROR;
+	progress = UPDATE_PROGRESS_DONE;
+	wakeup_main_task();
+	return (void *)ret;
+}
+
+static void spawn_update_task(char *path)
+{
+	pthread_attr_t attr;
+
+	assert(pthread_equal(pthread_self(), main_task));
+
+	progress = UPDATE_PROGRESS_RUNNING;
+	pthread_attr_init(&attr);
+	if (pthread_create(&update_thr, &attr, update_task, path))
+		FATAL("Failed to spawn update task: %s\n", strerror(errno));
+	if (++update_task_id > update_task_id_max)
+		update_task_id = 1;
+	DEBUG("spawned thread for update job id %i\n", update_task_id);
 }
 
 void reap_update_task(void)
 {
 	enum update_return ret;
 
+	assert(pthread_equal(pthread_self(), main_task));
+
 	if (progress != UPDATE_PROGRESS_DONE)
 		return;
 	if (pthread_join(update_thr, (void **)&ret))
 		FATAL("error joining update thread: %s\n", strerror(errno));
 	if (ret == UPDATE_RETURN_UPDATED)
 		playlistVersionChange();
-	progress = UPDATE_PROGRESS_IDLE;
-}
 
-static void * update_task(void *arg)
-{
-	List *path_list = (List *)arg;
-	enum update_return ret = UPDATE_RETURN_NOUPDATE;
-
-	if (path_list) {
-		ListNode *node = path_list->firstNode;
-
-		while (node) {
-			switch (updatePath(node->key)) {
-			case UPDATE_RETURN_ERROR:
-				ret = UPDATE_RETURN_ERROR;
-				goto out;
-			case UPDATE_RETURN_NOUPDATE:
-				break;
-			case UPDATE_RETURN_UPDATED:
-				ret = UPDATE_RETURN_UPDATED;
-			}
-			node = node->nextNode;
-		}
-		freeList(path_list);
+	if (update_paths_nr) {
+		char *path = update_paths[0];
+		memmove(&update_paths[0], &update_paths[1],
+		        --update_paths_nr * sizeof(char *));
+		spawn_update_task(path);
 	} else {
-		ret = updateDirectory(music_root);
+		progress = UPDATE_PROGRESS_IDLE;
 	}
-
-	if (ret == UPDATE_RETURN_UPDATED && writeDirectoryDB() < 0)
-		ret = UPDATE_RETURN_ERROR;
-out:
-	progress = UPDATE_PROGRESS_DONE;
-	wakeup_main_task();
-	return (void *)ret;
 }
 
-int updateInit(List * path_list)
+int directory_update_init(char *path)
 {
-	pthread_attr_t attr;
+	assert(pthread_equal(pthread_self(), main_task));
 
-	if (progress != UPDATE_PROGRESS_IDLE)
-		return -1;
+	if (progress != UPDATE_PROGRESS_IDLE) {
+		int next_task_id;
 
-	progress = UPDATE_PROGRESS_RUNNING;
+		if (!path)
+			return -1;
+		if (update_paths_nr == ARRAY_SIZE(update_paths))
+			return -1;
+		assert(update_paths_nr < ARRAY_SIZE(update_paths));
+		update_paths[update_paths_nr++] = path;
+		next_task_id = update_task_id + update_paths_nr;
 
-	pthread_attr_init(&attr);
-	if (pthread_create(&update_thr, &attr, update_task, path_list))
-		FATAL("Failed to spawn update task: %s\n", strerror(errno));
-	directory_updateJobId++;
-	if (directory_updateJobId > 1 << 15)
-		directory_updateJobId = 1;
-	DEBUG("updateInit: spawned update thread for update job id %i\n",
-	      (int)directory_updateJobId);
-
-	return (int)directory_updateJobId;
+		return next_task_id > update_task_id_max ?  1 : next_task_id;
+	}
+	spawn_update_task(path);
+	return update_task_id;
 }
 
 static void directory_set_stat(Directory * dir, const struct stat *st)