Panic in http3.RoundTripper.Close

[ainar-g]

Discovered while testing #4456.

Panic occurs in RoundTripper.Close here:

if err := client.conn.CloseWithError(0, ""); err != nil 

My main hypothesis is that this occurs when there is a dialing error. See RoundTripper.getClient:

cl, ok := r.clients[hostname]
if !ok {
	if onlyCached {
		return nil, false, ErrNoCachedConn
	}
	cl = &roundTripperWithCount{
		dialing: make(chan struct{}),
	}
	go func() {
		defer close(cl.dialing)
		conn, rt, err := r.dial(ctx, hostname)
		if err != nil {
			cl.dialErr = err
			return
		}
		cl.conn = conn
		cl.rt = rt
	}()
	r.clients[hostname] = cl
}

Note how cl.conn is only set in the anonymous goroutine if r.dial is successful.

Which means that r.clients can contain clients with conn == nil, but methods Close, CloseIdleConnections, and getClient all lack nil checks and assume that a non-nil *roundTripperWithCount always has a non-nil conn.

[marten-seemann]

Thanks for reporting. Very valuable to get this resolved before shipping v0.43 :)