DatabasePlugin: add method GetStats()

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

src/db/ProxyDatabasePlugin.cxx

@@ -67,6 +67,10 @@ public:
 				     VisitString visit_string,
 				     GError **error_r) const override;
 
+	virtual bool GetStats(const DatabaseSelection &selection,
+			      DatabaseStats &stats,
+			      GError **error_r) const override;
+
 protected:
 	bool Configure(const struct config_param *param, GError **error_r);
 };
@@ -420,6 +424,27 @@ ProxyDatabase::VisitUniqueTags(const DatabaseSelection &selection,
 		result;
 }
 
+bool
+ProxyDatabase::GetStats(const DatabaseSelection &selection,
+			DatabaseStats &stats, GError **error_r) const
+{
+	// TODO: match
+	(void)selection;
+
+	struct mpd_stats *stats2 =
+		mpd_run_stats(connection);
+	if (stats2 == nullptr)
+		return CheckError(connection, error_r);
+
+	stats.song_count = mpd_stats_get_number_of_songs(stats2);
+	stats.total_duration = mpd_stats_get_db_play_time(stats2);
+	stats.artist_count = mpd_stats_get_number_of_artists(stats2);
+	stats.album_count = mpd_stats_get_number_of_albums(stats2);
+	mpd_stats_free(stats2);
+
+	return true;
+}
+
 const DatabasePlugin proxy_db_plugin = {
 	"proxy",
 	ProxyDatabase::Create,

src/db/SimpleDatabasePlugin.cxx

@@ -280,6 +280,13 @@ SimpleDatabase::VisitUniqueTags(const DatabaseSelection &selection,
 				 error_r);
 }
 
+bool
+SimpleDatabase::GetStats(const DatabaseSelection &selection,
+			 DatabaseStats &stats, GError **error_r) const
+{
+	return ::GetStats(*this, selection, stats, error_r);
+}
+
 bool
 SimpleDatabase::Save(GError **error_r)
 {

src/db/SimpleDatabasePlugin.hxx

@@ -58,6 +58,7 @@ public:
 
 	virtual bool Open(GError **error_r) override;
 	virtual void Close() override;
+
 	virtual struct song *GetSong(const char *uri_utf8,
 				     GError **error_r) const override;
 	virtual bool Visit(const DatabaseSelection &selection,
@@ -71,6 +72,10 @@ public:
 				     VisitString visit_string,
 				     GError **error_r) const override;
 
+	virtual bool GetStats(const DatabaseSelection &selection,
+			      DatabaseStats &stats,
+			      GError **error_r) const override;
+
 protected:
 	bool Configure(const struct config_param *param, GError **error_r);