db/DatabasePlaylist: pass Database reference around

Reduce global variable usage, move to frontend code.

src/db/DatabasePlaylist.cxx

@@ -21,7 +21,6 @@
 #include "DatabasePlaylist.hxx"
 #include "Selection.hxx"
 #include "PlaylistFile.hxx"
-#include "DatabaseGlue.hxx"
 #include "DatabasePlugin.hxx"
 #include "DetachedSong.hxx"
 #include "Mapper.hxx"
@@ -37,17 +36,14 @@ AddSong(const char *playlist_path_utf8,
 }
 
 bool
-search_add_to_playlist(const char *uri, const char *playlist_path_utf8,
+search_add_to_playlist(const Database &db,
+		       const char *uri, const char *playlist_path_utf8,
 		       const SongFilter *filter,
 		       Error &error)
 {
-	const Database *db = GetDatabase(error);
-	if (db == nullptr)
-		return false;
-
 	const DatabaseSelection selection(uri, true, filter);
 
 	using namespace std::placeholders;
 	const auto f = std::bind(AddSong, playlist_path_utf8, _1, _2);
-	return db->Visit(selection, f, error);
+	return db.Visit(selection, f, error);
 }

src/db/DatabasePlaylist.hxx

@@ -22,12 +22,14 @@
 
 #include "Compiler.h"
 
+class Database;
 class SongFilter;
 class Error;
 
-gcc_nonnull(1,2)
+gcc_nonnull(2,3)
 bool
-search_add_to_playlist(const char *uri, const char *path_utf8,
+search_add_to_playlist(const Database &db,
+		       const char *uri, const char *path_utf8,
 		       const SongFilter *filter,
 		       Error &error);
 

src/db/DatabaseSong.cxx

@@ -19,23 +19,18 @@
 
 #include "config.h"
 #include "DatabaseSong.hxx"
-#include "DatabaseGlue.hxx"
 #include "DatabasePlugin.hxx"
 #include "DetachedSong.hxx"
 #include "Mapper.hxx"
 
 DetachedSong *
-DatabaseDetachSong(const char *uri, Error &error)
+DatabaseDetachSong(const Database &db, const char *uri, Error &error)
 {
-	const Database *db = GetDatabase(error);
-	if (db == nullptr)
-		return nullptr;
-
-	const LightSong *tmp = db->GetSong(uri, error);
+	const LightSong *tmp = db.GetSong(uri, error);
 	if (tmp == nullptr)
 		return nullptr;
 
 	DetachedSong *song = new DetachedSong(map_song_detach(*tmp));
-	db->ReturnSong(tmp);
+	db.ReturnSong(tmp);
 	return song;
 }

src/db/DatabaseSong.hxx

@@ -22,6 +22,7 @@
 
 #include "Compiler.h"
 
+class Database;
 class DetachedSong;
 class Error;
 
@@ -33,6 +34,6 @@ class Error;
  */
 gcc_malloc gcc_nonnull_all
 DetachedSong *
-DatabaseDetachSong(const char *uri, Error &error);
+DatabaseDetachSong(const Database &db, const char *uri, Error &error);
 
 #endif