This case occurs with streams closed with STOP_SENDING, then a fin bit being received too, then the data being read via Connection::stream_recv().

In high-level terms:

server opens stream - sends some data
client reads some data and writes data
server reads that data, sends STOP_SENDING, and writes some data, then sends the fin bit in a separate empty message of length 0
client sees stream as readable, then reads the data
client tries to write some data ... gets Err::Done from stream_send().

As far as I gathered from the documentation, Err::Done is supposed to be returned when no data could be written because of no capacity and Err::StreamStopped returned when the stream has been closed by the peer.

In this specific case though, the stream seems to already have been freed by quiche, thus StreamMap::get_or_create() returns Err::Done, which Connection::stream_send() simply forwards.

I would expect an Err::StreamStopped, or at least an Err::InvalidStreamState (signaling that a stream with that id could not be created, in that case, obviously, because the stream id is not matching what's allowed for the client/server).

Currently I'm working around it with an extra check of Connection::stream_writable() for Err::InvalidStreamState whenever Err::Done is reported, but that seems unnecessary/wrong to me, why would that be needed?

In case this is the expected behaviour though, I would like to ask for a note in the documentation to check stream_writable() explicitly if Err::Done is returned.