stmtdiagnostics: fix incorrect usage of timeutil.Timer

The contract of `timeutil.Timer.Stop` is such that the timer can no
longer be used, and it was violated in `Registry.poll`. This is now
fixed by allocating a fresh timer after each `Stop` call.

Release note: None

pkg/sql/stmtdiagnostics/statement_diagnostics.go

@@ -157,25 +157,31 @@ func (r *Registry) Start(ctx context.Context, stopper *stop.Stopper) {
 
 func (r *Registry) poll(ctx context.Context) {
 	var (
-		timer timeutil.Timer
+		timer *timeutil.Timer
 		// We need to store timer.C reference separately because timer.Stop()
 		// (called when polling is disabled) puts timer into the pool and
 		// prohibits further usage of stored timer.C.
-		timerC              = timer.C
+		timerC              <-chan time.Time
 		lastPoll            time.Time
 		deadline            time.Time
 		pollIntervalChanged = make(chan struct{}, 1)
 		maybeResetTimer     = func() {
 			if interval := pollingInterval.Get(&r.st.SV); interval == 0 {
 				// Setting the interval to zero stops the polling.
-				timer.Stop()
+				if timer != nil {
+					timer.Stop()
+					timer = nil
+				}
 				// nil out the channel so that it'd block forever in the loop
 				// below (until the polling interval is changed).
 				timerC = nil
 			} else {
 				newDeadline := lastPoll.Add(interval)
 				if deadline.IsZero() || !deadline.Equal(newDeadline) {
 					deadline = newDeadline
+					if timer == nil {
+						timer = timeutil.NewTimer()
+					}
 					timer.Reset(timeutil.Until(deadline))
 					timerC = timer.C
 				}