Fix potential p2p shutdown hangup (#10839)

Copy PR #10626 into `main`

Co-authored-by: Mark Holt <135143369+mh0lt@users.noreply.github.com>

p2p/discover/v4_udp.go

@@ -606,14 +606,15 @@ func (t *UDPv4) loop() {
 			return
 
 		case p := <-t.addReplyMatcher:
-			func() {
-				mutex.Lock()
-				defer mutex.Unlock()
-				p.deadline = time.Now().Add(t.replyTimeout)
-				listUpdate <- plist.PushBack(p)
-			}()
+			mutex.Lock()
+			p.deadline = time.Now().Add(t.replyTimeout)
+			back := plist.PushBack(p)
+			mutex.Unlock()
+			listUpdate <- back
 
 		case r := <-t.gotreply:
+			var removals []*list.Element
+
 			func() {
 				mutex.Lock()
 				defer mutex.Unlock()
@@ -629,7 +630,7 @@ func (t *UDPv4) loop() {
 						if requestDone {
 							p.errc <- nil
 							plist.Remove(el)
-							listUpdate <- el
+							removals = append(removals, el)
 						}
 						// Reset the continuous timeout counter (time drift detection)
 						contTimeouts = 0
@@ -638,6 +639,10 @@ func (t *UDPv4) loop() {
 				r.matched <- matched
 			}()
 
+			for _, el := range removals {
+				listUpdate <- el
+			}
+
 		case key := <-t.gotkey:
 			go func() {
 				if key, err := v4wire.DecodePubkey(crypto.S256(), key); err == nil {