p2p/discover: remove hot-spin in table refresh trigger (#32912)

This fixes a regression introduced in #32518. In that PR, we removed the
slowdown logic that would throttle lookups when the table runs empty.
Said logic was originally added in #20389.

Usually it's fine, but there exist pathological cases, such as hive
tests, where the node can only discover one other node, so it can only
ever query that node and won't get any results. In cases like these, we
need to throttle the creation of lookups to avoid crazy CPU usage.

Author: fjl

p2p/discover/lookup.go

@@ -153,6 +153,7 @@ type lookupIterator struct {
 	cancel        func()
 	lookup        *lookup
 	tabRefreshing <-chan struct{}
+	lastLookup    time.Time
 }
 
 type lookupFunc func(ctx context.Context) *lookup
@@ -185,6 +186,9 @@ func (it *lookupIterator) Next() bool {
 			return false
 		}
 		if it.lookup == nil {
+			// Ensure enough time has passed between lookup creations.
+			it.slowdown()
+
 			it.lookup = it.nextLookup(it.ctx)
 			if it.lookup.empty() {
 				// If the lookup is empty right after creation, it means the local table
@@ -235,6 +239,25 @@ func (it *lookupIterator) lookupFailed(tab *Table, timeout time.Duration) {
 	tab.waitForNodes(tout, 1)
 }
 
+// slowdown applies a delay between creating lookups. This exists to prevent hot-spinning
+// in some test environments where lookups don't yield any results.
+func (it *lookupIterator) slowdown() {
+	const minInterval = 1 * time.Second
+
+	now := time.Now()
+	diff := now.Sub(it.lastLookup)
+	it.lastLookup = now
+	if diff > minInterval {
+		return
+	}
+	wait := time.NewTimer(diff)
+	defer wait.Stop()
+	select {
+	case <-wait.C:
+	case <-it.ctx.Done():
+	}
+}
+
 // Close ends the iterator.
 func (it *lookupIterator) Close() {
 	it.cancel()