eth/downloader: drop beacon head updates if the syncer is restarting (#27397)

* eth/downloader: drop beacon head updates if the syncer is restarting

* eth/donwloader: v2 of the goroutine spike preventer

eth/downloader/skeleton.go

@@ -367,6 +367,13 @@ func (s *skeleton) sync(head *types.Header) (*types.Header, error) {
 		s.filler.resume()
 	}
 	defer func() {
+		// The filler needs to be suspended, but since it can block for a while
+		// when there are many blocks queued up for full-sync importing, run it
+		// on a separate goroutine and consume head messages that need instant
+		// replies.
+		done := make(chan struct{})
+		go func() {
+			defer close(done)
 			if filled := s.filler.suspend(); filled != nil {
 				// If something was filled, try to delete stale sync helpers. If
 				// unsuccessful, warn the user, but not much else we can do (it's
@@ -377,6 +384,17 @@ func (s *skeleton) sync(head *types.Header) (*types.Header, error) {
 				}
 			}
 		}()
+		// Wait for the suspend to finish, consuming head events in the meantime
+		// and dropping them on the floor.
+		for {
+			select {
+			case <-done:
+				return
+			case event := <-s.headEvents:
+				event.errc <- errors.New("beacon syncer reorging")
+			}
+		}
+	}()
 	// Create a set of unique channels for this sync cycle. We need these to be
 	// ephemeral so a data race doesn't accidentally deliver something stale on
 	// a persistent channel across syncs (yup, this happened)