DatabasePlugin: make Visit() const

src/DatabasePlugin.hxx

@@ -59,7 +59,7 @@ public:
 	 * directory (UTF-8)
 	 */
 	virtual struct song *GetSong(const char *uri_utf8,
-				     GError **error_r) = 0;
+				     GError **error_r) const = 0;
 
 	/**
 	 * Visit the selected entities.
@@ -68,7 +68,7 @@ public:
 			   VisitDirectory visit_directory,
 			   VisitSong visit_song,
 			   VisitPlaylist visit_playlist,
-			   GError **error_r) = 0;
+			   GError **error_r) const = 0;
 };
 
 struct DatabasePlugin {

src/db/SimpleDatabasePlugin.cxx

@@ -208,7 +208,7 @@ SimpleDatabase::Close()
 }
 
 struct song *
-SimpleDatabase::GetSong(const char *uri, GError **error_r)
+SimpleDatabase::GetSong(const char *uri, GError **error_r) const
 {
 	assert(root != NULL);
 
@@ -241,7 +241,7 @@ SimpleDatabase::Visit(const struct db_selection *selection,
 		      VisitDirectory visit_directory,
 		      VisitSong visit_song,
 		      VisitPlaylist visit_playlist,
-		      GError **error_r)
+		      GError **error_r) const
 {
 	const struct directory *directory = LookupDirectory(selection->uri);
 	if (directory == NULL) {

src/db/SimpleDatabasePlugin.hxx

@@ -59,12 +59,12 @@ public:
 	virtual bool Open(GError **error_r) override;
 	virtual void Close() override;
 	virtual struct song *GetSong(const char *uri_utf8,
-				     GError **error_r) override;
+				     GError **error_r) const override;
 	virtual bool Visit(const struct db_selection *selection,
 			   VisitDirectory visit_directory,
 			   VisitSong visit_song,
 			   VisitPlaylist visit_playlist,
-			   GError **error_r) override;
+			   GError **error_r) const override;
 
 protected:
 	bool Configure(const struct config_param *param, GError **error_r);