p2p/discover: slow down lookups on empty table (#20389)

* p2p/discover: slow down lookups on empty table

* p2p/discover: wake from slowdown sleep when table is closed

p2p/discover/lookup.go

@@ -18,6 +18,7 @@ package discover
 
 import (
 	"context"
+	"time"
 
 	"github.com/ethereum/go-ethereum/p2p/enode"
 )
@@ -106,6 +107,12 @@ func (it *lookup) startQueries() bool {
 		it.tab.mutex.Lock()
 		closest := it.tab.closest(it.result.target, bucketSize, false)
 		it.tab.mutex.Unlock()
+		// Avoid finishing the lookup too quickly if table is empty. It'd be better to wait
+		// for the table to fill in this case, but there is no good mechanism for that
+		// yet.
+		if len(closest.entries) == 0 {
+			it.slowdown()
+		}
 		it.queries = 1
 		it.replyCh <- closest.entries
 		return true
@@ -124,6 +131,15 @@ func (it *lookup) startQueries() bool {
 	return it.queries > 0
 }
 
+func (it *lookup) slowdown() {
+	sleep := time.NewTimer(1 * time.Second)
+	defer sleep.Stop()
+	select {
+	case <-sleep.C:
+	case <-it.tab.closeReq:
+	}
+}
+
 func (it *lookup) query(n *node, reply chan<- []*node) {
 	fails := it.tab.db.FindFails(n.ID(), n.IP())
 	r, err := it.queryfunc(n)