db/upnp: move lazy Open() call to new class LazyDatabase

Generic approach for the workaround.

Makefile.am

@@ -397,6 +397,7 @@ libfs_a_SOURCES = \
 libdb_plugins_a_SOURCES = \
 	src/DatabaseRegistry.cxx src/DatabaseRegistry.hxx \
 	src/DatabaseHelpers.cxx src/DatabaseHelpers.hxx \
+	src/db/LazyDatabase.cxx src/db/LazyDatabase.hxx \
 	src/db/SimpleDatabasePlugin.cxx src/db/SimpleDatabasePlugin.hxx
 
 if HAVE_LIBMPDCLIENT

src/db/LazyDatabase.cxx

@@ -0,0 +1,103 @@
+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "LazyDatabase.hxx"
+
+#include <assert.h>
+
+LazyDatabase::~LazyDatabase()
+{
+	assert(!open);
+
+	delete db;
+}
+
+bool
+LazyDatabase::EnsureOpen(Error &error) const
+{
+	if (open)
+		return true;
+
+	if (!db->Open(error))
+		return false;
+
+	open = true;
+	return true;
+}
+
+void
+LazyDatabase::Close()
+{
+	if (open) {
+		open = false;
+		db->Close();
+	}
+}
+
+Song *
+LazyDatabase::GetSong(const char *uri, Error &error) const
+{
+	return EnsureOpen(error)
+		? db->GetSong(uri, error)
+		: nullptr;
+}
+
+void
+LazyDatabase::ReturnSong(Song *song) const
+{
+	assert(open);
+
+	db->ReturnSong(song);
+}
+
+bool
+LazyDatabase::Visit(const DatabaseSelection &selection,
+		    VisitDirectory visit_directory,
+		    VisitSong visit_song,
+		    VisitPlaylist visit_playlist,
+		    Error &error) const
+{
+	return EnsureOpen(error) &&
+		db->Visit(selection, visit_directory, visit_song,
+			  visit_playlist, error);
+}
+
+bool
+LazyDatabase::VisitUniqueTags(const DatabaseSelection &selection,
+			      TagType tag_type,
+			      VisitString visit_string,
+			      Error &error) const
+{
+	return EnsureOpen(error) &&
+		db->VisitUniqueTags(selection, tag_type, visit_string, error);
+}
+
+bool
+LazyDatabase::GetStats(const DatabaseSelection &selection,
+		       DatabaseStats &stats, Error &error) const
+{
+	return EnsureOpen(error) && db->GetStats(selection, stats, error);
+}
+
+time_t
+LazyDatabase::GetUpdateStamp() const
+{
+	return open ? db->GetUpdateStamp() : 0;
+}

src/db/LazyDatabase.hxx

@@ -0,0 +1,69 @@
+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_LAZY_DATABASE_PLUGIN_HXX
+#define MPD_LAZY_DATABASE_PLUGIN_HXX
+
+#include "DatabasePlugin.hxx"
+
+/**
+ * A wrapper for a #Database object that gets opened on the first
+ * database access.  This works around daemonization problems with
+ * some plugins.
+ */
+class LazyDatabase final : public Database {
+	Database *const db;
+
+	mutable bool open;
+
+public:
+	gcc_nonnull_all
+	LazyDatabase(Database *_db)
+		:db(_db), open(false) {}
+
+	virtual ~LazyDatabase();
+
+	virtual void Close() override;
+
+	virtual Song *GetSong(const char *uri_utf8,
+			      Error &error) const override;
+	virtual void ReturnSong(Song *song) const;
+
+	virtual bool Visit(const DatabaseSelection &selection,
+			   VisitDirectory visit_directory,
+			   VisitSong visit_song,
+			   VisitPlaylist visit_playlist,
+			   Error &error) const override;
+
+	virtual bool VisitUniqueTags(const DatabaseSelection &selection,
+				     TagType tag_type,
+				     VisitString visit_string,
+				     Error &error) const override;
+
+	virtual bool GetStats(const DatabaseSelection &selection,
+			      DatabaseStats &stats,
+			      Error &error) const override;
+
+	virtual time_t GetUpdateStamp() const override;
+
+private:
+	bool EnsureOpen(Error &error) const;
+};
+
+#endif

src/db/UpnpDatabasePlugin.cxx

@@ -24,6 +24,7 @@
 #include "upnp/Discovery.hxx"
 #include "upnp/ContentDirectoryService.hxx"
 #include "upnp/Directory.hxx"
+#include "LazyDatabase.hxx"
 #include "DatabasePlugin.hxx"
 #include "DatabaseSelection.hxx"
 #include "DatabaseError.hxx"
@@ -99,7 +100,6 @@ protected:
 	bool Configure(const config_param &param, Error &error);
 
 private:
-	bool reallyOpen(Error &error);
 	bool VisitServer(ContentDirectoryService* server,
 			 const std::vector<std::string> &vpath,
 			 const DatabaseSelection &selection,
@@ -188,11 +188,15 @@ UpnpDatabase::Create(gcc_unused EventLoop &loop,
 		     const config_param &param, Error &error)
 {
 	UpnpDatabase *db = new UpnpDatabase();
-	if (db && !db->Configure(param, error)) {
+	if (!db->Configure(param, error)) {
 		delete db;
-		db = nullptr;
+		return nullptr;
 	}
-	return db;
+
+	/* libupnp loses its ability to receive multicast messages
+	   apparently due to daemonization; using the LazyDatabase
+	   wrapper works around this problem */
+	return new LazyDatabase(db);
 }
 
 bool
@@ -201,23 +205,8 @@ UpnpDatabase::Configure(const config_param &, Error &)
 	return true;
 }
 
-// Open is called before mpd daemonizes, and daemonizing messes with
-// the UpNP lib (it loses its ability to receive mcast messages
-// apparently).
-// So we delay actual opening until the first client call.
-//
-// Unfortunately the servers may take a few seconds to answer the
-// first search on the network, so we would either have to sleep for a
-// relatively long time (maybe 5S), or accept that the directory may
-// not be complete on the first client call. There is no simple
-// solution to this issue, we would need a call from mpd after daemonizing.
 bool
-UpnpDatabase::Open(Error &)
-{
-	return true;
-}
-
-bool UpnpDatabase::reallyOpen(Error &error)
+UpnpDatabase::Open(Error &error)
 {
 	if (m_root)
 		return true;
@@ -320,9 +309,6 @@ upnpItemToSong(const UPnPDirObject &dirent, const char *uri)
 Song *
 UpnpDatabase::GetSong(const char *uri, Error &error) const
 {
-	if (!const_cast<UpnpDatabase *>(this)->reallyOpen(error))
-		return nullptr;
-
 	if (!m_superdir || !m_superdir->ok()) {
 		error.Set(upnp_domain,
 			  "UpnpDatabase::GetSong() superdir is sick");
@@ -813,9 +799,6 @@ UpnpDatabase::Visit(const DatabaseSelection &selection,
 		    VisitPlaylist visit_playlist,
 		    Error &error) const
 {
-	if (!const_cast<UpnpDatabase *>(this)->reallyOpen(error))
-		return false;
-
 	std::vector<ContentDirectoryService> servers;
 	if (!m_superdir || !m_superdir->ok() ||
 	    !m_superdir->getDirServices(servers)) {
@@ -873,9 +856,6 @@ UpnpDatabase::VisitUniqueTags(const DatabaseSelection &selection,
 			      VisitString visit_string,
 			      Error &error) const
 {
-	if (!const_cast<UpnpDatabase *>(this)->reallyOpen(error))
-		return false;
-
 	if (!visit_string)
 		return true;