db/simple: fix dangling LightSong::tag reference in moved ExportedSong

After commit 1afa33c3c7, an old bug was revealed:
SimpleDatabase::GetSong() constructs an ExportedSong instance by
moving the return value of Song::Export(), which causes the
LightSong::tag field to be dangling on the moved-from
ExportedSong::tag_buffer.  This broke tags from CUE sheets.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1070

NEWS

@@ -1,4 +1,6 @@
 ver 0.22.5 (not yet released)
+* database
+  - simple: fix missing CUE sheet metadata in "addid" command
 * tags
   - id: translate TPE3 to Conductor, not Performer
 * archive

src/db/plugins/simple/ExportedSong.hxx

@@ -37,6 +37,15 @@ public:
 	ExportedSong(const char *_uri, Tag &&_tag) noexcept
 		:LightSong(_uri, tag_buffer),
 		 tag_buffer(std::move(_tag)) {}
+
+	/* this custom move constructor is necessary so LightSong::tag
+	   points to this instance's #Tag field instead of leaving a
+	   dangling reference to the source object's #Tag field */
+	ExportedSong(ExportedSong &&src) noexcept
+		:LightSong(src, tag_buffer),
+		 tag_buffer(std::move(src.tag_buffer)) {}
+
+	ExportedSong &operator=(ExportedSong &&) = delete;
 };
 
 #endif

src/song/LightSong.hxx

@@ -88,6 +88,19 @@ struct LightSong {
 	LightSong(const char *_uri, const Tag &_tag) noexcept
 		:uri(_uri), tag(_tag) {}
 
+	/**
+	 * A copy constructor which copies all fields, but only sets
+	 * the tag to a caller-provided reference.  This is used by
+	 * the #ExportedSong move constructor.
+	 */
+	LightSong(const LightSong &src, const Tag &_tag) noexcept
+		:directory(src.directory), uri(src.uri),
+		 real_uri(src.real_uri),
+		 tag(_tag),
+		 mtime(src.mtime),
+		 start_time(src.start_time), end_time(src.end_time),
+		 audio_format(src.audio_format) {}
+
 	gcc_pure
 	std::string GetURI() const noexcept {
 		if (directory == nullptr)