From 46e4f0b5c1d269e29d26a273016b18afbd13bbc4 Mon Sep 17 00:00:00 2001
From: Felix Lange <fjl@twurst.com>
Date: Fri, 29 Aug 2025 14:23:45 +0200
Subject: [PATCH] p2p/discover: add waitForNodes

---
 p2p/discover/lookup.go | 17 +++--------------
 p2p/discover/table.go  | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 14 deletions(-)

diff --git a/p2p/discover/lookup.go b/p2p/discover/lookup.go
index 09808b71e0..db1bbe32f9 100644
--- a/p2p/discover/lookup.go
+++ b/p2p/discover/lookup.go
@@ -19,7 +19,6 @@ package discover
 import (
 	"context"
 	"errors"
-	"time"
 
 	"github.com/ethereum/go-ethereum/p2p/enode"
 )
@@ -106,11 +105,10 @@ func (it *lookup) startQueries() bool {
 	// The first query returns nodes from the local table.
 	if it.queries == -1 {
 		closest := it.tab.findnodeByID(it.result.target, bucketSize, false)
-		// 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.
+		// Avoid finishing the lookup too quickly if table is empty.
+		// Wait for the table to fill.
 		if len(closest.entries) == 0 {
-			it.slowdown()
+			it.tab.waitForNodes(1)
 		}
 		it.queries = 1
 		it.replyCh <- closest.entries
@@ -130,15 +128,6 @@ 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 *enode.Node, reply chan<- []*enode.Node) {
 	r, err := it.queryfunc(n)
 	if !errors.Is(err, errClosed) { // avoid recording failures on shutdown.
diff --git a/p2p/discover/table.go b/p2p/discover/table.go
index b6c35aaaa9..63aa655256 100644
--- a/p2p/discover/table.go
+++ b/p2p/discover/table.go
@@ -32,6 +32,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/common/mclock"
+	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/metrics"
 	"github.com/ethereum/go-ethereum/p2p/enode"
@@ -84,6 +85,7 @@ type Table struct {
 	closeReq        chan struct{}
 	closed          chan struct{}
 
+	nodeFeed        event.FeedOf[*enode.Node]
 	nodeAddedHook   func(*bucket, *tableNode)
 	nodeRemovedHook func(*bucket, *tableNode)
 }
@@ -567,6 +569,8 @@ func (tab *Table) nodeAdded(b *bucket, n *tableNode) {
 	}
 	n.addedToBucket = time.Now()
 	tab.revalidation.nodeAdded(tab, n)
+
+	tab.nodeFeed.Send(n.Node)
 	if tab.nodeAddedHook != nil {
 		tab.nodeAddedHook(b, n)
 	}
@@ -702,3 +706,36 @@ func (tab *Table) deleteNode(n *enode.Node) {
 	b := tab.bucket(n.ID())
 	tab.deleteInBucket(b, n.ID())
 }
+
+// waitForNodes blocks until the table contains at least n nodes.
+func (tab *Table) waitForNodes(n int) bool {
+	getlength := func() (count int) {
+		for _, b := range &tab.buckets {
+			count += len(b.entries)
+		}
+		return count
+	}
+
+	var ch chan *enode.Node
+	for {
+		tab.mutex.Lock()
+		if getlength() >= n {
+			tab.mutex.Unlock()
+			return true
+		}
+		if ch == nil {
+			// Init subscription.
+			ch = make(chan *enode.Node)
+			sub := tab.nodeFeed.Subscribe(ch)
+			defer sub.Unsubscribe()
+		}
+		tab.mutex.Unlock()
+
+		// Wait for a node add event.
+		select {
+		case <-ch:
+		case <-tab.closeReq:
+			return false
+		}
+	}
+}