player/Thread: never reuse decoder when switching radio streams

When switching to another song manually, the player checks if the
decoder is already decoding that song; if so, it will attempt to reuse
it by seeking it to the new position.  That however fails if the
decoder is not seekable (e.g. a radio stream) which leaves the user
unable to switch to that song with the bogus error message "Not
seekable".

NEWS

@@ -3,6 +3,8 @@ ver 0.20.19 (not yet released)
   - validate absolute seek time, reject negative values
 * input
   - mms: fix lockup bug and a crash bug
+* player
+  - fix spurious "Not seekable" error when switching radio streams
 * macOS: fix crash bug
 
 ver 0.20.18 (2018/02/24)

src/decoder/DecoderControl.hxx

@@ -308,9 +308,14 @@ struct DecoderControl {
 	bool IsCurrentSong(const DetachedSong &_song) const noexcept;
 
 	gcc_pure
-	bool LockIsCurrentSong(const DetachedSong &_song) const noexcept {
+	bool IsSeekableCurrentSong(const DetachedSong &_song) const noexcept {
+		return seekable && IsCurrentSong(_song);
+	}
+
+	gcc_pure
+	bool LockIsSeeakbleCurrentSong(const DetachedSong &_song) const noexcept {
 		const std::lock_guard<Mutex> protect(mutex);
-		return IsCurrentSong(_song);
+		return IsSeekableCurrentSong(_song);
 	}
 
 private:

src/player/Thread.cxx

@@ -584,7 +584,7 @@ Player::SeekDecoder()
 
 	const SongTime start_time = pc.next_song->GetStartTime();
 
-	if (!dc.LockIsCurrentSong(*pc.next_song)) {
+	if (!dc.LockIsSeeakbleCurrentSong(*pc.next_song)) {
 		/* the decoder is already decoding the "next" song -
 		   stop it and start the previous song again */