db/Print: move sort/window emulation code to class DatabaseVisitorHelper

That way, each plugin can decide to implement it better.

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -23,6 +23,7 @@
 #include "db/DatabasePlugin.hxx"
 #include "db/DatabaseListener.hxx"
 #include "db/Selection.hxx"
+#include "db/VHelper.hxx"
 #include "db/DatabaseError.hxx"
 #include "db/PlaylistInfo.hxx"
 #include "db/LightDirectory.hxx"
@@ -804,6 +805,15 @@ try {
 	throw;
 }
 
+gcc_const
+static DatabaseSelection
+CheckSelection(DatabaseSelection selection) noexcept
+{
+	selection.uri.clear();
+	selection.filter = nullptr;
+	return selection;
+}
+
 void
 ProxyDatabase::Visit(const DatabaseSelection &selection,
 		     VisitDirectory visit_directory,
@@ -813,11 +823,14 @@ ProxyDatabase::Visit(const DatabaseSelection &selection,
 	// TODO: eliminate the const_cast
 	const_cast<ProxyDatabase *>(this)->EnsureConnected();
 
+	DatabaseVisitorHelper helper(CheckSelection(selection), visit_song);
+
 	if (!visit_directory && !visit_playlist && selection.recursive &&
 	    !selection.IsEmpty()) {
 		/* this optimized code path can only be used under
 		   certain conditions */
 		::SearchSongs(connection, selection, visit_song);
+		helper.Commit();
 		return;
 	}
 
@@ -825,6 +838,8 @@ ProxyDatabase::Visit(const DatabaseSelection &selection,
 	::Visit(connection, selection.uri.c_str(),
 		selection.recursive, selection.filter,
 		visit_directory, visit_song, visit_playlist);
+
+	helper.Commit();
 }
 
 void

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -26,6 +26,7 @@
 #include "db/Helpers.hxx"
 #include "db/Stats.hxx"
 #include "db/UniqueTags.hxx"
+#include "db/VHelper.hxx"
 #include "db/LightDirectory.hxx"
 #include "Directory.hxx"
 #include "Song.hxx"
@@ -265,6 +266,15 @@ SimpleDatabase::ReturnSong(gcc_unused const LightSong *song) const noexcept
 	}
 }
 
+gcc_const
+static DatabaseSelection
+CheckSelection(DatabaseSelection selection) noexcept
+{
+	selection.uri.clear();
+	selection.filter = nullptr;
+	return selection;
+}
+
 void
 SimpleDatabase::Visit(const DatabaseSelection &selection,
 		      VisitDirectory visit_directory,
@@ -286,6 +296,8 @@ SimpleDatabase::Visit(const DatabaseSelection &selection,
 		return;
 	}
 
+	DatabaseVisitorHelper helper(CheckSelection(selection), visit_song);
+
 	if (r.uri == nullptr) {
 		/* it's a directory */
 
@@ -295,6 +307,7 @@ SimpleDatabase::Visit(const DatabaseSelection &selection,
 		r.directory->Walk(selection.recursive, selection.filter,
 				  visit_directory, visit_song,
 				  visit_playlist);
+		helper.Commit();
 		return;
 	}
 
@@ -306,6 +319,7 @@ SimpleDatabase::Visit(const DatabaseSelection &selection,
 				if (selection.Match(song2))
 					visit_song(song2);
 
+				helper.Commit();
 				return;
 			}
 		}

src/db/plugins/upnp/UpnpDatabasePlugin.cxx

@@ -27,6 +27,7 @@
 #include "db/Interface.hxx"
 #include "db/DatabasePlugin.hxx"
 #include "db/Selection.hxx"
+#include "db/VHelper.hxx"
 #include "db/DatabaseError.hxx"
 #include "db/LightDirectory.hxx"
 #include "song/LightSong.hxx"
@@ -576,6 +577,15 @@ UpnpDatabase::VisitServer(const ContentDirectoryService &server,
 	}
 }
 
+gcc_const
+static DatabaseSelection
+CheckSelection(DatabaseSelection selection) noexcept
+{
+	selection.uri.clear();
+	selection.filter = nullptr;
+	return selection;
+}
+
 // Deal with the possibly multiple servers, call VisitServer if needed.
 void
 UpnpDatabase::Visit(const DatabaseSelection &selection,
@@ -583,6 +593,8 @@ UpnpDatabase::Visit(const DatabaseSelection &selection,
 		    VisitSong visit_song,
 		    VisitPlaylist visit_playlist) const
 {
+	DatabaseVisitorHelper helper(CheckSelection(selection), visit_song);
+
 	auto vpath = SplitString(selection.uri.c_str(), '/');
 	if (vpath.empty()) {
 		for (const auto &server : discovery->GetDirectories()) {
@@ -598,6 +610,7 @@ UpnpDatabase::Visit(const DatabaseSelection &selection,
 					    visit_playlist);
 		}
 
+		helper.Commit();
 		return;
 	}
 
@@ -608,6 +621,7 @@ UpnpDatabase::Visit(const DatabaseSelection &selection,
 	auto server = discovery->GetServer(servername.c_str());
 	VisitServer(server, std::move(vpath), selection,
 		    visit_directory, visit_song, visit_playlist);
+	helper.Commit();
 }
 
 void