p2p/discover: schedule revalidation also when all nodes are excluded (#30239)

## Issue

If `nextTime` has passed, but all nodes are excluded, `get` would return
`nil` and `run` would therefore not invoke `schedule`. Then, we schedule
a timer for the past, as neither `nextTime` value has been updated. This
creates a busy loop, as the timer immediately returns.

## Fix

With this PR, revalidation will be also rescheduled when all nodes are
excluded.

---------

Co-authored-by: lightclient <lightclient@protonmail.com>

p2p/discover/table_reval.go

@@ -77,14 +77,18 @@ func (tr *tableRevalidation) nodeEndpointChanged(tab *Table, n *tableNode) {
 // It returns the next time it should be invoked, which is used in the Table main loop
 // to schedule a timer. However, run can be called at any time.
 func (tr *tableRevalidation) run(tab *Table, now mclock.AbsTime) (nextTime mclock.AbsTime) {
-	if n := tr.fast.get(now, &tab.rand, tr.activeReq); n != nil {
-		tr.startRequest(tab, n)
-		tr.fast.schedule(now, &tab.rand)
-	}
-	if n := tr.slow.get(now, &tab.rand, tr.activeReq); n != nil {
-		tr.startRequest(tab, n)
-		tr.slow.schedule(now, &tab.rand)
+	reval := func(list *revalidationList) {
+		if list.nextTime <= now {
+			if n := list.get(now, &tab.rand, tr.activeReq); n != nil {
+				tr.startRequest(tab, n)
+			}
+			// Update nextTime regardless if any requests were started because
+			// current value has passed.
+			list.schedule(now, &tab.rand)
+		}
 	}
+	reval(&tr.fast)
+	reval(&tr.slow)
 
 	return min(tr.fast.nextTime, tr.slow.nextTime)
 }
@@ -200,7 +204,7 @@ type revalidationList struct {
 
 // get returns a random node from the queue. Nodes in the 'exclude' map are not returned.
 func (list *revalidationList) get(now mclock.AbsTime, rand randomSource, exclude map[enode.ID]struct{}) *tableNode {
-	if now < list.nextTime || len(list.nodes) == 0 {
+	if len(list.nodes) == 0 {
 		return nil
 	}
 	for i := 0; i < len(list.nodes)*3; i++ {