Keep canary alive when primary promotion fails

When the canary analysis succeeds, Flagger copies the canary pod spec
to the primary and waits for the primary rollout to finish. If the
primary fails to become ready, the non-retriable readiness error
triggered the standard analysis rollback, which routes all traffic to
the primary and scales the canary to zero.

During promotion the primary already runs the new (failing) spec while
the canary is the only healthy copy of the new revision still serving
traffic. Rolling back therefore sends all traffic to the broken primary
and deletes the working canary, taking the application down.

Halt the promotion instead: when the primary is not ready and the canary
is in the Promoting or Finalising phase, mark the rollout as failed and
alert, but keep the canary running and leave routing untouched until the
primary recovers or a corrected revision is applied.

Fixes #1898

Signed-off-by: Pedram Pourmohammad <eragon.pedy@gmail.com>

pkg/controller/scheduler.go

@@ -297,7 +297,14 @@ func (c *Controller) advanceCanary(name string, namespace string) {
 		if err != nil {
 			c.recordEventWarningf(cd, "%v", err)
 			if !retriable {
-				c.rollback(cd, canaryController, meshRouter, scalerReconciler)
+				// during promotion the canary is the only healthy copy, halt
+				// instead of rolling back traffic to the unhealthy primary
+				if cd.Status.Phase == flaggerv1.CanaryPhasePromoting ||
+					cd.Status.Phase == flaggerv1.CanaryPhaseFinalising {
+					c.handleFailedPromotion(cd, canaryController, err)
+				} else {
+					c.rollback(cd, canaryController, meshRouter, scalerReconciler)
+				}
 			}
 			return
 		}
@@ -989,6 +996,29 @@ func (c *Controller) rollback(canary *flaggerv1.Canary, canaryController canary.
 	c.runPostRolloutHooks(canary, flaggerv1.CanaryPhaseFailed)
 }
 
+// handleFailedPromotion marks the rollout as failed when the primary is unhealthy
+// during promotion, without scaling the canary down or routing to the primary.
+func (c *Controller) handleFailedPromotion(canary *flaggerv1.Canary, canaryController canary.Controller, err error) {
+	c.recordEventWarningf(canary, "Promotion of %s.%s failed, primary not ready: %v",
+		canary.Spec.TargetRef.Name, canary.Namespace, err)
+	c.alert(canary, fmt.Sprintf("Promotion failed, primary not ready: %v", err),
+		false, flaggerv1.SeverityError)
+
+	if err := canaryController.SetStatusPhase(canary, flaggerv1.CanaryPhaseFailed); err != nil {
+		c.logger.With("canary", fmt.Sprintf("%s.%s", canary.Name, canary.Namespace)).Errorf("%v", err)
+		return
+	}
+
+	c.recorder.SetStatus(canary, flaggerv1.CanaryPhaseFailed)
+	c.recorder.IncFailures(metrics.CanaryMetricLabels{
+		Name:               canary.Spec.TargetRef.Name,
+		Namespace:          canary.Namespace,
+		DeploymentStrategy: canary.DeploymentStrategy(),
+		AnalysisStatus:     metrics.AnalysisStatusCompleted,
+	})
+	c.runPostRolloutHooks(canary, flaggerv1.CanaryPhaseFailed)
+}
+
 func (c *Controller) setPhaseInitialized(cd *flaggerv1.Canary, canaryController canary.Controller) error {
 	if cd.Status.Phase == "" || cd.Status.Phase == flaggerv1.CanaryPhaseInitializing {
 		cd.Status.Phase = flaggerv1.CanaryPhaseInitialized

pkg/controller/scheduler_deployment_fixture_test.go

@@ -82,6 +82,39 @@ func (f fixture) makeReady(t *testing.T, name string) {
 	require.NoError(t, err)
 }
 
+// makePrimaryNotReady puts the primary into a stuck rollout (progress deadline exceeded).
+func (f fixture) makePrimaryNotReady(t *testing.T) {
+	primaryName := fmt.Sprintf("%s-primary", f.canary.Spec.TargetRef.Name)
+	p, err := f.kubeClient.AppsV1().
+		Deployments("default").
+		Get(context.TODO(), primaryName, metav1.GetOptions{})
+	require.NoError(t, err)
+
+	p.Status = appsv1.DeploymentStatus{
+		ObservedGeneration: p.Generation,
+		Replicas:           1,
+		UpdatedReplicas:    1,
+		ReadyReplicas:      0,
+		AvailableReplicas:  0,
+		Conditions: []appsv1.DeploymentCondition{
+			{
+				Type:   appsv1.DeploymentProgressing,
+				Status: corev1.ConditionFalse,
+				Reason: "ProgressDeadlineExceeded",
+			},
+			{
+				Type:           appsv1.DeploymentAvailable,
+				Status:         corev1.ConditionFalse,
+				Reason:         "MinimumReplicasUnavailable",
+				LastUpdateTime: metav1.Now(),
+			},
+		},
+	}
+
+	_, err = f.kubeClient.AppsV1().Deployments("default").Update(context.TODO(), p, metav1.UpdateOptions{})
+	require.NoError(t, err)
+}
+
 func newDeploymentFixture(c *flaggerv1.Canary) fixture {
 	if c == nil {
 		c = newDeploymentTestCanary()

pkg/controller/scheduler_deployment_test.go

@@ -116,6 +116,67 @@ func TestScheduler_DeploymentRollback(t *testing.T) {
 	assert.Equal(t, flaggerv1.CanaryPhaseFailed, c.Status.Phase)
 }
 
+// when the primary fails to become ready during promotion, the healthy canary
+// must be kept instead of rolled back to the broken primary (#1898)
+func TestScheduler_DeploymentPromotionPrimaryNotReady(t *testing.T) {
+	mocks := newDeploymentFixture(nil)
+
+	// initializing
+	mocks.ctrl.advanceCanary("podinfo", "default")
+	mocks.makePrimaryReady(t)
+
+	// initialized
+	mocks.ctrl.advanceCanary("podinfo", "default")
+
+	// update
+	dep2 := newDeploymentTestDeploymentV2()
+	_, err := mocks.kubeClient.AppsV1().Deployments("default").Update(context.TODO(), dep2, metav1.UpdateOptions{})
+	require.NoError(t, err)
+
+	// detect changes -> progressing, canary scaled up
+	mocks.ctrl.advanceCanary("podinfo", "default")
+	mocks.makeCanaryReady(t)
+	require.NoError(t, assertPhase(mocks.flaggerClient, "podinfo", flaggerv1.CanaryPhaseProgressing))
+
+	canaryDep, err := mocks.kubeClient.AppsV1().Deployments("default").Get(context.TODO(), "podinfo", metav1.GetOptions{})
+	require.NoError(t, err)
+	require.NotNil(t, canaryDep.Spec.Replicas)
+	canaryReplicas := *canaryDep.Spec.Replicas
+	require.Greater(t, canaryReplicas, int32(0))
+
+	// simulate: analysis succeeded, spec promoted to primary, now finishing promotion
+	cd, err := mocks.flaggerClient.FlaggerV1beta1().Canaries("default").Get(context.TODO(), "podinfo", metav1.GetOptions{})
+	require.NoError(t, err)
+	err = mocks.deployer.SetStatusPhase(cd, flaggerv1.CanaryPhasePromoting)
+	require.NoError(t, err)
+
+	// known routing state at promotion time (split between primary and canary)
+	require.NoError(t, mocks.router.SetRoutes(mocks.canary, 50, 50, false))
+
+	// the promoted primary fails to roll out
+	mocks.makePrimaryNotReady(t)
+
+	// advance: Flagger observes the primary is stuck
+	mocks.ctrl.advanceCanary("podinfo", "default")
+
+	// the canary must NOT be scaled to zero - it is the only healthy copy serving traffic
+	canaryDep, err = mocks.kubeClient.AppsV1().Deployments("default").Get(context.TODO(), "podinfo", metav1.GetOptions{})
+	require.NoError(t, err)
+	require.NotNil(t, canaryDep.Spec.Replicas)
+	assert.Equal(t, canaryReplicas, *canaryDep.Spec.Replicas, "canary must not be scaled to zero when promotion fails")
+
+	// traffic must NOT be shifted entirely onto the broken primary
+	primaryWeight, canaryWeight, _, err := mocks.router.GetRoutes(mocks.canary)
+	require.NoError(t, err)
+	assert.NotEqual(t, 100, primaryWeight, "traffic must not be routed entirely to the unhealthy primary")
+	assert.Greater(t, canaryWeight, 0, "canary must keep serving traffic when promotion fails")
+
+	// the rollout is reported as failed so it stops advancing and alerts
+	c, err := mocks.flaggerClient.FlaggerV1beta1().Canaries("default").Get(context.TODO(), "podinfo", metav1.GetOptions{})
+	require.NoError(t, err)
+	assert.Equal(t, flaggerv1.CanaryPhaseFailed, c.Status.Phase)
+}
+
 func TestScheduler_DeploymentSkipAnalysis(t *testing.T) {
 	mocks := newDeploymentFixture(nil)
 	// initializing