db/Song: translate relative "real_uri" paths

src/db/DatabaseSong.cxx

@@ -23,6 +23,8 @@
 #include "song/LightSong.hxx"
 #include "storage/StorageInterface.hxx"
 #include "util/ScopeExit.hxx"
+#include "util/UriExtract.hxx"
+#include "util/UriRelative.hxx"
 
 #include <assert.h>
 
@@ -32,9 +34,19 @@ DatabaseDetachSong(const Storage *storage, const LightSong &song) noexcept
 	DetachedSong detached(song);
 	assert(detached.IsInDatabase());
 
-	if (!detached.HasRealURI() && storage != nullptr) {
+	if (storage != nullptr) {
+		if (!detached.HasRealURI()) {
 			const auto uri = song.GetURI();
 			detached.SetRealURI(storage->MapUTF8(uri.c_str()));
+		} else if (uri_is_relative_path(detached.GetRealURI())) {
+			/* if the "RealURI" is relative, translate it
+			   using the song's "URI" attribute, because
+			   it's assumed to be relative to it */
+			const auto real_uri =
+				uri_apply_relative(detached.GetRealURI(),
+						   song.GetURI());
+			detached.SetRealURI(storage->MapUTF8(real_uri.c_str()));
+		}
 	}
 
 	return detached;