p2p/discover: fix timeout loop early exit when removing expired matchers (#34743)

Save `el.Next()` before calling `plist.Remove(el)` so iteration
continues correctly. Previously the loop exited after removing the first
expired matcher because `Remove` invalidates the element's links.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

p2p/discover/common.go

@@ -17,9 +17,11 @@
 package discover
 
 import (
+	"container/list"
 	"crypto/ecdsa"
 	crand "crypto/rand"
 	"encoding/binary"
+	"iter"
 	"math/rand"
 	"net"
 	"net/netip"
@@ -143,3 +145,16 @@ func (r *reseedingRandom) Shuffle(n int, swap func(i, j int)) {
 	defer r.mu.Unlock()
 	r.cur.Shuffle(n, swap)
 }
+
+// iterList iterates over the elements of the given list.
+func iterList[T any](l *list.List) iter.Seq2[T, *list.Element] {
+	return func(yield func(T, *list.Element) bool) {
+		for el := l.Front(); el != nil; {
+			next := el.Next()
+			if !yield(el.Value.(T), el) {
+				return
+			}
+			el = next
+		}
+	}
+}

p2p/discover/v4_udp.go

@@ -446,9 +446,8 @@ func (t *UDPv4) loop() {
 		}
 		// Start the timer so it fires when the next pending reply has expired.
 		now := time.Now()
-		for el := plist.Front(); el != nil; el = el.Next() {
-			nextTimeout = el.Value.(*replyMatcher)
-			if dist := nextTimeout.deadline.Sub(now); dist < 2*respTimeout {
+		for p, el := range iterList[*replyMatcher](plist) {
+			if dist := p.deadline.Sub(now); dist < 2*respTimeout {
 				timeout.Reset(dist)
 				return
 			}
@@ -478,8 +477,7 @@ func (t *UDPv4) loop() {
 
 		case r := <-t.gotreply:
 			var matched bool // whether any replyMatcher considered the reply acceptable.
-			for el := plist.Front(); el != nil; el = el.Next() {
-				p := el.Value.(*replyMatcher)
+			for p, el := range iterList[*replyMatcher](plist) {
 				if p.from == r.from && p.ptype == r.data.Kind() && p.ip == r.ip {
 					ok, requestDone := p.callback(r.data)
 					matched = matched || ok
@@ -499,8 +497,7 @@ func (t *UDPv4) loop() {
 			nextTimeout = nil
 
 			// Notify and remove callbacks whose deadline is in the past.
-			for el := plist.Front(); el != nil; el = el.Next() {
-				p := el.Value.(*replyMatcher)
+			for p, el := range iterList[*replyMatcher](plist) {
 				if now.After(p.deadline) || now.Equal(p.deadline) {
 					p.errc <- errTimeout
 					plist.Remove(el)