StoragePlugin: add method MapToRelativeUTF8()

Replaces map_to_relative_path() from Mapper.cxx.

src/storage/StorageInterface.hxx

@@ -68,6 +68,14 @@ public:
 	gcc_pure
 	AllocatedPath MapChildFS(const char *uri_utf8,
 				 const char *child_utf8) const;
+
+	/**
+	 * Check if the given URI points inside this storage.  If yes,
+	 * then it returns a relative URI (pointing inside the given
+	 * string); if not, returns nullptr.
+	 */
+	gcc_pure
+	virtual const char *MapToRelativeUTF8(const char *uri_utf8) const = 0;
 };
 
 #endif

src/storage/plugins/LocalStorage.cxx

@@ -68,6 +68,8 @@ public:
 
 	virtual AllocatedPath MapFS(const char *uri_utf8) const override;
 
+	virtual const char *MapToRelativeUTF8(const char *uri_utf8) const override;
+
 private:
 	AllocatedPath MapFS(const char *uri_utf8, Error &error) const;
 };
@@ -130,6 +132,12 @@ LocalStorage::MapFS(const char *uri_utf8) const
 	return MapFS(uri_utf8, IgnoreError());
 }
 
+const char *
+LocalStorage::MapToRelativeUTF8(const char *uri_utf8) const
+{
+	return PathTraitsUTF8::Relative(base_utf8.c_str(), uri_utf8);
+}
+
 bool
 LocalStorage::GetInfo(const char *uri_utf8, bool follow, FileInfo &info,
 		      Error &error)

src/storage/plugins/SmbclientStorage.cxx

@@ -69,6 +69,8 @@ public:
 						      Error &error) override;
 
 	virtual std::string MapUTF8(const char *uri_utf8) const override;
+
+	virtual const char *MapToRelativeUTF8(const char *uri_utf8) const override;
 };
 
 std::string
@@ -82,6 +84,12 @@ SmbclientStorage::MapUTF8(const char *uri_utf8) const
 	return PathTraitsUTF8::Build(base.c_str(), uri_utf8);
 }
 
+const char *
+SmbclientStorage::MapToRelativeUTF8(const char *uri_utf8) const
+{
+	return PathTraitsUTF8::Relative(base.c_str(), uri_utf8);
+}
+
 static bool
 GetInfo(const char *path, FileInfo &info, Error &error)
 {