fix: avoid scheduling an instance refresh if the context is done (#491)

Fixes #493

internal/alloydb/instance.go

@@ -189,7 +189,11 @@ func (i *Instance) OpenConns() *uint64 {
 // Close closes the instance; it stops the refresh cycle and prevents it from
 // making additional calls to the AlloyDB Admin API.
 func (i *Instance) Close() error {
+	i.resultGuard.Lock()
+	defer i.resultGuard.Unlock()
 	i.cancel()
+	i.cur.cancel()
+	i.next.cancel()
 	return nil
 }
 
@@ -230,6 +234,8 @@ func (i *Instance) result(ctx context.Context) (*refreshOperation, error) {
 		err = res.err
 	case <-ctx.Done():
 		err = ctx.Err()
+	case <-i.ctx.Done():
+		err = i.ctx.Err()
 	}
 	if err != nil {
 		return nil, err
@@ -260,6 +266,13 @@ 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 err := i.ctx.Err(); err != nil {
+			r.err = err
+			close(r.ready)
+			return
+		}
+
 		ctx, cancel := context.WithTimeout(i.ctx, i.refreshTimeout)
 		defer cancel()
 
@@ -280,6 +293,7 @@ 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 +311,6 @@ 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)
 	})

internal/alloydb/instance_test.go

@@ -19,7 +19,6 @@ import (
 	"crypto/rand"
 	"crypto/rsa"
 	"errors"
-	"strings"
 	"testing"
 	"time"
 
@@ -216,7 +215,7 @@ func TestClose(t *testing.T) {
 	i.Close()
 
 	_, _, err = i.ConnectInfo(ctx)
-	if !strings.Contains(err.Error(), "context was canceled or expired") {
+	if !errors.Is(err, context.Canceled) {
 		t.Fatalf("failed to retrieve connect info: %v", err)
 	}
 }