command/storage: check if storage is already mounted

src/command/StorageCommands.cxx

@@ -203,6 +203,11 @@ handle_mount(Client &client, Request args, Response &r)
 		return CommandResult::ERROR;
 	}
 
+	if (composite.IsMounted(remote_uri)) {
+		r.Error(ACK_ERROR_ARG, "This storage is already mounted");
+		return CommandResult::ERROR;
+	}
+
 	auto &event_loop = instance.io_thread.GetEventLoop();
 	auto storage = CreateStorageURI(event_loop, remote_uri);
 	if (storage == nullptr) {

src/storage/CompositeStorage.hxx

@@ -121,6 +121,15 @@ public:
 		VisitMounts(uri, root, t);
 	}
 
+	/**
+	 * Is a storage with the given URI already mounted?
+	 */
+	gcc_pure gcc_nonnull_all
+	bool IsMounted(const char *storage_uri) const noexcept {
+		const std::lock_guard<Mutex> protect(mutex);
+		return IsMounted(root, storage_uri);
+	}
+
 	void Mount(const char *uri, std::unique_ptr<Storage> storage);
 	bool Unmount(const char *uri);
 
@@ -155,6 +164,22 @@ private:
 		}
 	}
 
+	gcc_pure gcc_nonnull_all
+	static bool IsMounted(const Directory &directory,
+			      const char *storage_uri) noexcept {
+		if (directory.storage) {
+			const auto uri = directory.storage->MapUTF8("");
+			if (uri == storage_uri)
+				return true;
+		}
+
+		for (const auto &i : directory.children)
+			if (IsMounted(i.second, storage_uri))
+				return true;
+
+		return false;
+	}
+
 	/**
 	 * Follow the given URI path, and find the outermost directory
 	 * which is a #Storage mount point.  If there are no mounts,

src/storage/StorageState.cxx

@@ -112,6 +112,11 @@ storage_state_restore(const char *line, TextFile &file, Instance &instance)
 		return true;
 	}
 
+	if (composite_storage.IsMounted(url.c_str())) {
+		LogError(storage_domain, "This storage is already mounted");
+		return true;
+	}
+
 	auto &event_loop = instance.io_thread.GetEventLoop();
 	auto storage = CreateStorageURI(event_loop, url.c_str());
 	if (storage == nullptr) {