db/simple: add option to hide CUE target songs

This reduces duplicates in the music database by hiding the original
song file when it is referenced by a CUE sheet.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1275

NEWS

@@ -9,6 +9,7 @@ ver 0.23 (not yet released)
   - proxy: require MPD 0.20 or later
   - proxy: require libmpdclient 2.11 or later
   - proxy: split search into chunks to avoid exceeding the output buffer
+  - simple: add option to hide CUE target songs
   - upnp: support libnpupnp instead of libupnp
 * archive
   - zzip, iso9660: ignore file names which are invalid UTF-8

doc/plugins.rst

@@ -23,6 +23,13 @@ The default plugin. Stores a copy of the database in memory. A file is used for
      - The path of the cache directory for additional storages mounted at runtime. This setting is necessary for the **mount** protocol command.
    * - **compress yes|no**
      - Compress the database file using gzip? Enabled by default (if built with zlib).
+   * - **hide_playlist_targets yes|no**
+     - Hide songs which are referenced by playlists?  Thas is,
+       playlist files which are represented in the database as virtual
+       directories (playlist plugin setting ``as_directory``).  This
+       option is enabled by default and avoids duplicate songs; one
+       copy for the original file, and another copy in the virtual
+       directory of a CUE file referring to it.
 
 proxy
 -----

src/db/plugins/simple/Directory.cxx

@@ -109,21 +109,21 @@ Directory::FindChild(std::string_view name) const noexcept
 	return nullptr;
 }
 
-bool
-Directory::TargetExists(std::string_view _target) const noexcept
+Song *
+Directory::LookupTargetSong(std::string_view _target) noexcept
 {
 	StringView target{_target};
 
 	if (target.SkipPrefix("../")) {
 		if (parent == nullptr)
-			return false;
+			return nullptr;
 
-		return parent->TargetExists(target);
+		return parent->LookupTargetSong(target);
 	}
 
 	/* sorry for the const_cast ... */
-	const auto lr = const_cast<Directory *>(this)->LookupDirectory(target);
-	return lr.directory->FindSong(lr.rest) != nullptr;
+	const auto lr = LookupDirectory(target);
+	return lr.directory->FindSong(lr.rest);
 }
 
 void
@@ -229,6 +229,7 @@ Directory::Sort() noexcept
 
 void
 Directory::Walk(bool recursive, const SongFilter *filter,
+		bool hide_playlist_targets,
 		const VisitDirectory& visit_directory, const VisitSong& visit_song,
 		const VisitPlaylist& visit_playlist) const
 {
@@ -247,7 +248,10 @@ Directory::Walk(bool recursive, const SongFilter *filter,
 	}
 
 	if (visit_song) {
-		for (auto &song : songs){
+		for (auto &song : songs) {
+			if (hide_playlist_targets && song.in_playlist)
+				continue;
+
 			const auto song2 = song.Export();
 			if (filter == nullptr || filter->Match(song2))
 				visit_song(song2);
@@ -265,6 +269,7 @@ Directory::Walk(bool recursive, const SongFilter *filter,
 
 		if (recursive)
 			child.Walk(recursive, filter,
+				   hide_playlist_targets,
 				   visit_directory, visit_song,
 				   visit_playlist);
 	}

src/db/plugins/simple/Directory.hxx

@@ -214,7 +214,7 @@ public:
 	LookupResult LookupDirectory(std::string_view uri) noexcept;
 
 	[[gnu::pure]]
-	bool TargetExists(std::string_view target) const noexcept;
+	Song *LookupTargetSong(std::string_view target) noexcept;
 
 	[[gnu::pure]]
 	bool IsEmpty() const noexcept {
@@ -303,6 +303,7 @@ public:
 	 * Caller must lock #db_mutex.
 	 */
 	void Walk(bool recursive, const SongFilter *match,
+		  bool hide_playlist_targets,
 		  const VisitDirectory& visit_directory, const VisitSong& visit_song,
 		  const VisitPlaylist& visit_playlist) const;
 

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -60,6 +60,7 @@ inline SimpleDatabase::SimpleDatabase(const ConfigBlock &block)
 #ifdef ENABLE_ZLIB
 	 compress(block.GetBlockValue("compress", true)),
 #endif
+	 hide_playlist_targets(block.GetBlockValue("hide_playlist_targets", true)),
 	 cache_path(block.GetPath("cache_directory"))
 {
 	if (path.IsNull())
@@ -306,6 +307,7 @@ SimpleDatabase::Visit(const DatabaseSelection &selection,
 			visit_directory(r.directory->Export());
 
 		r.directory->Walk(selection.recursive, selection.filter,
+				  hide_playlist_targets,
 				  visit_directory, visit_song,
 				  visit_playlist);
 		helper.Commit();

src/db/plugins/simple/SimpleDatabasePlugin.hxx

@@ -44,6 +44,8 @@ class SimpleDatabase : public Database {
 	bool compress;
 #endif
 
+	bool hide_playlist_targets;
+
 	/**
 	 * The path where cache files for Mount() are located.
 	 */

src/db/plugins/simple/Song.hxx

@@ -101,6 +101,12 @@ struct Song {
 	 */
 	AudioFormat audio_format = AudioFormat::Undefined();
 
+	/**
+	 * Is this song referenced by at least one playlist file that
+	 * is part of the database?
+	 */
+	bool in_playlist = false;
+
 	template<typename F>
 	Song(F &&_filename, Directory &_parent) noexcept
 		:parent(_parent), filename(std::forward<F>(_filename)) {}

src/db/update/Playlist.cxx

@@ -141,10 +141,18 @@ UpdateWalk::PurgeDanglingFromPlaylists(Directory &directory) noexcept
 
 	directory.ForEachSongSafe([&](Song &song){
 		if (!song.target.empty() &&
-		    !PathTraitsUTF8::IsAbsoluteOrHasScheme(song.target.c_str()) &&
-		    !directory.TargetExists(song.target)) {
-			editor.DeleteSong(directory, &song);
-			modified = true;
+		    !PathTraitsUTF8::IsAbsoluteOrHasScheme(song.target.c_str())) {
+			Song *target = directory.LookupTargetSong(song.target.c_str());
+			if (target == nullptr) {
+				/* the target does not exist: remove
+				   the virtual song */
+				editor.DeleteSong(directory, &song);
+				modified = true;
+			} else {
+				/* the target exists: mark it (for
+				   option "hide_playlist_targets") */
+				target->in_playlist = true;
+			}
 		}
 	});
 }

src/db/update/Walk.hxx

@@ -88,6 +88,9 @@ private:
 	/**
 	 * Remove all virtual songs inside playlists whose "target"
 	 * field points to a non-existing song file.
+	 *
+	 * It also looks up all target songs and sets their
+	 * "in_playlist" field.
 	 */
 	void PurgeDanglingFromPlaylists(Directory &directory) noexcept;