db/simple: prune CUE entries from database for non-existent songs

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1019

src/db/plugins/simple/Directory.cxx

@@ -109,6 +109,23 @@ Directory::FindChild(std::string_view name) const noexcept
 	return nullptr;
 }
 
+bool
+Directory::TargetExists(std::string_view _target) const noexcept
+{
+	StringView target{_target};
+
+	if (target.SkipPrefix("../")) {
+		if (parent == nullptr)
+			return false;
+
+		return parent->TargetExists(target);
+	}
+
+	/* sorry for the const_cast ... */
+	const auto lr = const_cast<Directory *>(this)->LookupDirectory(target);
+	return lr.directory->FindSong(lr.rest) != nullptr;
+}
+
 void
 Directory::PruneEmpty() noexcept
 {

src/db/plugins/simple/Directory.hxx

@@ -118,13 +118,17 @@ public:
 		return new Directory(std::string(), nullptr);
 	}
 
+	bool IsPlaylist() const noexcept {
+		return device == DEVICE_PLAYLIST;
+	}
+
 	/**
 	 * Is this really a regular file which is being treated like a
 	 * directory?
 	 */
 	bool IsReallyAFile() const noexcept {
 		return device == DEVICE_INARCHIVE ||
-			device == DEVICE_PLAYLIST ||
+			IsPlaylist() ||
 			device == DEVICE_CONTAINER;
 	}
 
@@ -210,6 +214,9 @@ public:
 	gcc_pure
 	LookupResult LookupDirectory(std::string_view uri) noexcept;
 
+	[[gnu::pure]]
+	bool TargetExists(std::string_view target) const noexcept;
+
 	gcc_pure
 	bool IsEmpty() const noexcept {
 		return children.empty() &&