DatabasePlugin: add method ReturnSong()

Allow the plugin to allocate the GetSong() return value.

src/db/ProxyDatabasePlugin.cxx

@@ -56,6 +56,8 @@ public:
 	virtual void Close() override;
 	virtual struct song *GetSong(const char *uri_utf8,
 				     GError **error_r) const override;
+	virtual void ReturnSong(struct song *song) const;
+
 	virtual bool Visit(const DatabaseSelection &selection,
 			   VisitDirectory visit_directory,
 			   VisitSong visit_song,
@@ -191,6 +193,16 @@ ProxyDatabase::GetSong(const char *uri, GError **error_r) const
 	return nullptr;
 }
 
+void
+ProxyDatabase::ReturnSong(struct song *song) const
+{
+	assert(song != nullptr);
+	assert(song_in_database(song));
+	assert(song_is_detached(song));
+
+	song_free(song);
+}
+
 static bool
 Visit(struct mpd_connection *connection, const char *uri,
       bool recursive, VisitDirectory visit_directory, VisitSong visit_song,

src/db/SimpleDatabasePlugin.cxx

@@ -184,6 +184,10 @@ SimpleDatabase::Open(GError **error_r)
 	root = directory_new_root();
 	mtime = 0;
 
+#ifndef NDEBUG
+	borrowed_song_count = 0;
+#endif
+
 	GError *error = NULL;
 	if (!Load(&error)) {
 		directory_free(root);
@@ -204,6 +208,7 @@ void
 SimpleDatabase::Close()
 {
 	assert(root != NULL);
+	assert(borrowed_song_count == 0);
 
 	directory_free(root);
 }
@@ -219,10 +224,25 @@ SimpleDatabase::GetSong(const char *uri, GError **error_r) const
 	if (song == NULL)
 		g_set_error(error_r, db_quark(), DB_NOT_FOUND,
 			    "No such song: %s", uri);
+#ifndef NDEBUG
+	else
+		++const_cast<unsigned &>(borrowed_song_count);
+#endif
 
 	return song;
 }
 
+void
+SimpleDatabase::ReturnSong(gcc_unused struct song *song) const
+{
+	assert(song != nullptr);
+
+#ifndef NDEBUG
+	assert(borrowed_song_count > 0);
+	--const_cast<unsigned &>(borrowed_song_count);
+#endif
+}
+
 G_GNUC_PURE
 const struct directory *
 SimpleDatabase::LookupDirectory(const char *uri) const

src/db/SimpleDatabasePlugin.hxx

@@ -38,6 +38,10 @@ class SimpleDatabase : public Database {
 
 	time_t mtime;
 
+#ifndef NDEBUG
+	unsigned borrowed_song_count;
+#endif
+
 public:
 	gcc_pure
 	struct directory *GetRoot() {
@@ -61,6 +65,8 @@ public:
 
 	virtual struct song *GetSong(const char *uri_utf8,
 				     GError **error_r) const override;
+	virtual void ReturnSong(struct song *song) const;
+
 	virtual bool Visit(const DatabaseSelection &selection,
 			   VisitDirectory visit_directory,
 			   VisitSong visit_song,