fix: avoid scheduling an instance refresh if the context is done

Fixes #493

internal/alloydb/instance.go

@@ -190,6 +190,7 @@ func (i *Instance) OpenConns() *uint64 {
 // making additional calls to the AlloyDB Admin API.
 func (i *Instance) Close() error {
 	i.cancel()
+    i.next.cancel()
 	return nil
 }
 
@@ -260,6 +261,11 @@ func (i *Instance) scheduleRefresh(d time.Duration) *refreshOperation {
 	r := &refreshOperation{}
 	r.ready = make(chan struct{})
 	r.timer = time.AfterFunc(d, func() {
+		// instance has been closed, don't schedule anything
+		if i.ctx.Err() != nil {
+			return
+		}
+
 		ctx, cancel := context.WithTimeout(i.ctx, i.refreshTimeout)
 		defer cancel()
 
@@ -280,6 +286,7 @@ func (i *Instance) scheduleRefresh(d time.Duration) *refreshOperation {
 		// result and schedule a new refresh
 		i.resultGuard.Lock()
 		defer i.resultGuard.Unlock()
+
 		// if failed, scheduled the next refresh immediately
 		if r.err != nil {
 			i.next = i.scheduleRefresh(0)
@@ -297,12 +304,6 @@ func (i *Instance) scheduleRefresh(d time.Duration) *refreshOperation {
 		// Update the current results, and schedule the next refresh in
 		// the future
 		i.cur = r
-		select {
-		case <-i.ctx.Done():
-			// instance has been closed, don't schedule anything
-			return
-		default:
-		}
 		t := refreshDuration(time.Now(), i.cur.result.expiry)
 		i.next = i.scheduleRefresh(t)
 	})