derp/derphttp: NotePreferred is sent synchronously which causes failures with blocking IO

[kylecarbs]

The Recv function in derphttp_client.go is called after client creation which waits for NotePreferred to be sent if applicable.

tailscale/derp/derphttp/derphttp_client.go

Lines 448 to 453 in 2b892ad

	if c.preferred {
	if err := derpClient.NotePreferred(true); err != nil {
	go httpConn.Close()
	return nil, 0, err
	}
	}

This becomes problematic when using io.Pipe as a transport because the accept loop in derp_server.go only runs after flushing sendServerKey.

tailscale/derp/derp_server.go

Lines 646 to 651 in 2b892ad

	br := brw.Reader
	nc.SetDeadline(time.Now().Add(10 * time.Second))
	bw := &lazyBufioWriter{w: nc, lbw: brw.Writer}
	if err := s.sendServerKey(bw); err != nil {
	return fmt.Errorf("send server key: %v", err)
	}

It seems like the best course of action is to make NotePreferred run in a goroutine. The current implementation allows for this with a custom dialer, which is only allowed for non-regional clients.

I'll do this but first wanted assurance that there wouldn't be any odd issues I'm overlooking by making this change. If this isn't a problem and I'm mistaken, feel free to close!

[DentonGentry]

I think you're setting up time with one of the engineers here, this might be another topic to discuss.

[DentonGentry]

I guess we should also monitor google/gvisor#6113, putting it here in the bug so we'll see it during the next bug scrub.