playlist/Song: support backslash in relative URIs

Closes https://github.com/MusicPlayerDaemon/MPD/issues/607

NEWS

@@ -1,4 +1,6 @@
 ver 0.21.12 (not yet released)
+* Windows
+  - support backslash in relative URIs loaded from playlists
 
 ver 0.21.11 (2019/07/03)
 * input

src/playlist/PlaylistSong.cxx

@@ -66,6 +66,22 @@ playlist_check_translate_song(DetachedSong &song, const char *base_uri,
 		base_uri = nullptr;
 
 	const char *uri = song.GetURI();
+
+#ifdef _WIN32
+	if (!PathTraitsUTF8::IsAbsolute(uri) && strchr(uri, '\\') != nullptr) {
+		/* Windows uses the backslash as path separator, but
+		   the MPD protocol uses the (forward) slash by
+		   definition; to allow backslashes in relative URIs
+		   loaded from playlist files, this step converts all
+		   backslashes to (forward) slashes */
+
+		std::string new_uri(uri);
+		std::replace(new_uri.begin(), new_uri.end(), '\\', '/');
+		song.SetURI(std::move(new_uri));
+		uri = song.GetURI();
+	}
+#endif
+
 	if (base_uri != nullptr && !uri_has_scheme(uri) &&
 	    !PathTraitsUTF8::IsAbsolute(uri))
 		song.SetURI(PathTraitsUTF8::Build(base_uri, uri));

test/test_translate_song.cxx

@@ -270,3 +270,23 @@ TEST_F(TranslateSongTest, Relative)
 						  insecure_loader));
 	EXPECT_EQ(se, ToString(song4));
 }
+
+TEST_F(TranslateSongTest, Backslash)
+{
+	const SongLoader loader(reinterpret_cast<const Database *>(1),
+				storage);
+
+	DetachedSong song1("foo\\bar.ogg", MakeTag2b());
+#ifdef _WIN32
+	/* on Windows, all backslashes are converted to slashes in
+	   relative paths from playlists */
+	auto se = ToString(DetachedSong(uri2, MakeTag2c()));
+	EXPECT_TRUE(playlist_check_translate_song(song1, nullptr,
+						  loader));
+	EXPECT_EQ(se, ToString(song1));
+#else
+	/* backslash only supported on Windows */
+	EXPECT_FALSE(playlist_check_translate_song(song1, nullptr,
+						   loader));
+#endif
+}