fix: kubectl port fwd should return on canceled context (#6700)

Debugging the flaky `TestDevGracefulCancel/multi-config-microservices`
integration test showed that the kubectl port forwarder was stuck with
goroutines waiting on a channels(one per resource).

Search for `goroutine 235` and `goroutine 234` in this Kokoro log:
https://source.cloud.google.com/results/invocations/a9749ab5-8762-4319-a2be-f67c7440f7a2/targets/skaffold%2Fpresubmit/log

This change means that the forwarder also listens for context canceled.

**Related**: #6424, #6643, #6662, #6685

pkg/skaffold/kubernetes/portforward/kubectl_forwarder.go

@@ -81,7 +81,20 @@ func (k *KubectlForwarder) Start(out io.Writer) {
 func (k *KubectlForwarder) Forward(parentCtx context.Context, pfe *portForwardEntry) error {
 	errChan := make(chan error, 1)
 	go k.forward(parentCtx, pfe, errChan)
-	return <-errChan
+	l := log.Entry(parentCtx)
+	resourceName := ""
+	if pfe != nil {
+		resourceName = pfe.resource.Name
+	}
+	l.Tracef("KubectlForwarder.Forward(%s): waiting on errChan", resourceName)
+	select {
+	case <-parentCtx.Done():
+		l.Tracef("KubectlForwarder.Forward(%s): parentCtx canceled, returning nil error", resourceName)
+		return nil
+	case err := <-errChan:
+		l.Tracef("KubectlForwarder.Forward(%s): got error on errChan, returning: %+v", resourceName, err)
+		return err
+	}
 }
 
 func (k *KubectlForwarder) forward(ctx context.Context, pfe *portForwardEntry, errChan chan error) {

pkg/skaffold/kubernetes/portforward/kubectl_forwarder_test.go

@@ -481,3 +481,25 @@ func TestStartAndForward(t *testing.T) {
 		})
 	}
 }
+
+func TestForwardReturnsNilOnContextCancelled(t *testing.T) {
+	k := NewKubectlForwarder(&kubectl.CLI{})
+	k.Start(ioutil.Discard)
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{}, 1)
+	go func() {
+		pfe := newPortForwardEntry(0, latestV1.PortForwardResource{}, "", "", "", "", 8080, false)
+		err := k.Forward(ctx, pfe)
+		if err != nil {
+			t.Errorf("expected nil error, got %+v", err)
+		}
+		close(done)
+	}()
+	cancel()
+	select {
+	case <-done:
+		// expected
+	case <-time.After(3 * time.Second):
+		t.Fatalf("forwarder did not return on context cancel")
+	}
+}