DNS discovery spinloop

[holiman]

When running geth with a dns discovery setting where there set of nodes in the crawled public set is empty, the dns client goes into a spinloop, consuming 100% of a CPU core.

Testcase to repro this, where the last it.Next() spins :

diff --git a/p2p/dnsdisc/client_test.go b/p2p/dnsdisc/client_test.go
index 6a6705abf2..7be13176db 100644
--- a/p2p/dnsdisc/client_test.go
+++ b/p2p/dnsdisc/client_test.go
@@ -115,6 +115,21 @@ func TestIterator(t *testing.T) {
 	checkIterator(t, it, nodes)
 }
 
+// This test checks that randomIterator finds all entries.
+func TestEmptyIterator(t *testing.T) {
+	tree, url := makeTestTree("n", testNodes(nodesSeed1, 00), nil)
+	c := NewClient(Config{
+		Resolver:  mapResolver(tree.ToTXT("n")),
+		Logger:    testlog.Logger(t, log.LvlTrace),
+		RateLimit: 500,
+	})
+	it, err := c.NewIterator(url)
+	if err != nil {
+		t.Fatal(err)
+	}
+	it.Next()
+}
+
 // This test checks if closing randomIterator races.
 func TestIteratorClose(t *testing.T) {
 	nodes := testNodes(nodesSeed1, 500)

Can also be tested live using e.g. geth --goerli --syncmode=snap.