CompositeStorage: add method GetMount()

src/storage/CompositeStorage.cxx

@@ -210,6 +210,19 @@ CompositeStorage::~CompositeStorage()
 {
 }
 
+Storage *
+CompositeStorage::GetMount(const char *uri)
+{
+	const ScopeLock protect(mutex);
+
+	auto result = FindStorage(uri);
+	if (*result.uri != 0)
+		/* not a mount point */
+		return nullptr;
+
+	return result.directory->storage;
+}
+
 void
 CompositeStorage::Mount(const char *uri, Storage *storage)
 {

src/storage/CompositeStorage.hxx

@@ -95,6 +95,17 @@ public:
 	CompositeStorage();
 	virtual ~CompositeStorage();
 
+	/**
+	 * Get the #Storage at the specified mount point.  Returns
+	 * nullptr if the given URI is not a mount point.
+	 *
+	 * The returned pointer is unprotected.  No other thread is
+	 * allowed to unmount the given mount point while the return
+	 * value is being used.
+	 */
+	gcc_pure gcc_nonnull_all
+	Storage *GetMount(const char *uri);
+
 	/**
 	 * Call the given function for each mounted storage, including
 	 * the root storage.  Passes mount point URI and the a const