`panic: invalid uri: ` when prefetching a track

[aykevl]

In some circumstances (not yet sure when this happens but it seems to have something to do with playing whole albums), next.Uri is an empty string here:

go-librespot/cmd/daemon/controls.go

Line 25 in b95e6ac

nextId := librespot.SpotifyIdFromUri(next.Uri)

This leads to a crash:

go-librespot/ids.go

Lines 110 to 114 in b95e6ac

	func SpotifyIdFromUri(uri string) SpotifyId {
	id, err := SpotifyIdFromUriSafe(uri)
	if err != nil {
	panic(err)
	}

The next object in this case looks like this:

&connectstate.ContextTrack{state:impl.MessageState{NoUnkeyedLiterals:pragma.NoUnkeyedLiterals{}, DoNotCompare:pragma.DoNotCompare{}, DoNotCopy:pragma.DoNotCopy{}, atomicMessageInfo:(*impl.MessageInfo)(nil)}, sizeCache:0, unknownFields:[]uint8(nil), Uri:"", Uid:"a1e0611f0a14df708f14", Gid:[]uint8{0x7, 0x7d, 0x48, 0x4f, 0x2c, 0x99, 0x4d, 0x24, 0x93, 0xcf, 0x57, 0x1, 0xfd, 0x71, 0xd7, 0x31}, Metadata:map[string]string{"context_uri":"spotify:album:0tiOfqFu4d3UJ2r1LsoG3Z", "entity_uri":"spotify:album:0tiOfqFu4d3UJ2r1LsoG3Z"}}

If you look into it, there's an album ID ("entity_uri":"spotify:album:0tiOfqFu4d3UJ2r1LsoG3Z"), and something that looks like a track ID (Uid:"a1e0611f0a14df708f14"). So it looks like there's enough information to prefetch this track, but I'm not entirely sure how to implement it.
(Interestingly this only happens while prefetching, perhaps the code that pulls the track ID when actually playing a track takes care of this case? In which case it would make sense to generalize that code and use it in both places).

[gregnebu]

+1 I faced the same yesterday, and came to the same ccl. Unfortunately I'm not familiar enough with the code base to indeed suggest a good fix

[aykevl]

I have a fix at #98, but have a hard time verifying whether it works as expected (since it happens so rarely). If you know how to reproduce the issue, you can put some logging in the else case here to see whether it is triggered and whether it works correctly:

	if next.Uri != "" {
		nextId = librespot.SpotifyIdFromUri(next.Uri)
	} else {
		nextId = librespot.SpotifyIdFromGid(librespot.SpotifyIdTypeTrack, next.Gid)
	}

[aykevl]

#110 avoids the panic. It still won't prefetch, but that only means the next track will have a slight delay before it starts. That's much better than panicking.

I think it works well enough for now.