p2p: fix discovery shutdown (#8725)

[battlmonstr]

Problem:
Some goroutines are blocked on shutdown:

1. table close <-tab.closed // because table loop pending
2. table loop <-refreshDone // because lookup shutdown blocks doRefresh
3. lookup shutdown <-it.replyCh // because it.queryfunc (findnode - ensureBond) is blocked, and not returning errClosed (if it returns and pushes to it.replyCh, then shutdown() will unblock)
4. findnode - ensureBond <-rm.errc // because the related replyMatcher was added after loop() exited, so there's nothing to push errClosed and unlock it

If addReplyMatcher channel is buffered, it is possible that UDPv4.pending() adds a new reply matcher after closeCtx.Done().
Such reply matcher's errc result channel will never be updated, because the UDPv4.loop() has exited at this point. Subsequent discovery operations will deadlock.

Solution:
Revert to an unbuffered channel.

[mh0lt]

This channel needs a buffer, or certainly it did because some call sequences deadlock otherwise. Is it causing shutdown to hang ?

I think that we need to add an additional check rather than just not buffering the channel. I'll take a look at the logging it seems to me that we should add a status check.

Looking at the code think this fix is what is needed:

func (t *UDPv4) pending(id enode.ID, ip net.IP, port int, ptype byte, callback replyMatchFunc) *replyMatcher {
	ch := make(chan error, 1)
	p := &replyMatcher{from: id, ip: ip, port: port, ptype: ptype, callback: callback, errc: ch}

	if t.closeCtx.Err() != nil {  <-- we need to check if the contexts is previously closed before starting to wait
		ch <- errClosed
	} else {
		select {
		case t.addReplyMatcher <- p:
			// loop will handle it
		case <-t.closeCtx.Done():
			ch <- errClosed
		}
	}

	return p
}