p2p/discover: fix expired matcher iteration in discover reply loop

[Sahil-4555]

This update resolves the matcher cleanup loop in the discover reply handling code. When an expired matcher is removed from the list, calling Remove(el) invalidates its links, therefore invoking el.Next() subsequently may cause the loop to terminate prematurely. To avoid this, the next element is preserved before removal, allowing iteration to proceed appropriately across the entire list. This ensures that all expired matchers in the same tick are processed and that contTimeouts are not undercounted when multiple matchers expire simultaneously.

[Copilot]

Pull request overview

Fixes iteration over pending discv4 reply matchers when elements are removed during traversal, preventing premature termination of cleanup/match loops in the UDPv4 discover reply handling.

Changes:

• Replaces direct container/list traversal in UDPv4.loop() with a removal-safe iterator helper.
• Adds a generic iterList helper (based on iter.Seq2) that preserves the next element before yielding, so current elements can be removed safely during iteration.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
p2p/discover/v4_udp.go	Uses removal-safe iteration for scheduling, reply matching, and timeout cleanup loops.
p2p/discover/common.go	Adds iterList helper and required imports (container/list, iter).

Comments suppressed due to low confidence (1)

p2p/discover/v4_udp.go:510

• This change fixes a subtle iteration/removal bug in the timeout cleanup loop. Please add a regression test (e.g., multiple matchers expiring at once) to ensure all expired matchers are processed/removed in a single timeout tick and to prevent reintroducing the premature-termination issue.

			// Notify and remove callbacks whose deadline is in the past.
			for p, el := range iterList[*replyMatcher](plist) {
				if now.After(p.deadline) || now.Equal(p.deadline) {
					p.errc <- errTimeout
					plist.Remove(el)
					contTimeouts++

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.