Add disable path overlap validation flag

  ## What this PR does / why we need it:

In kubernetes#5651 there was a
request to throw an error when there are two ingresses defining the same
host and path, which was implemented as part of the validation webhook.

  Despite of this there are clear rules on the ingress controller that
describes what the controller would do in [such situation (the oldest
rule wins)](https://github.com/kubernetes/ingress-nginx/blob/main/docs/how-it-works.md?plain=1#L27)

  Some users are relying on this validation behaviour to prevent
misconfigurations, but there are use cases where allowing it, maybe
temporarily, is helpful.

Change-Id: I9d4124d1c36876b06d63100cd10988eaf2f41db9

internal/ingress/controller/controller.go

@@ -111,10 +111,11 @@ type Configuration struct {
 
 	IngressClassConfiguration *ingressclass.IngressClassConfiguration
 
-	ValidationWebhook         string
-	ValidationWebhookCertPath string
-	ValidationWebhookKeyPath  string
-	DisableFullValidationTest bool
+	ValidationWebhook            string
+	ValidationWebhookCertPath    string
+	ValidationWebhookKeyPath     string
+	DisableFullValidationTest    bool
+	DisablePathOverlapValidation bool
 
 	GlobalExternalAuth  *ngx_config.GlobalExternalAuth
 	MaxmindEditionFiles *[]string
@@ -319,11 +320,14 @@ func (n *NGINXController) CheckIngress(ing *networking.Ingress) error {
 	startTest := time.Now().UnixNano() / 1000000
 	_, servers, pcfg := n.getConfiguration(ings)
 
-	err := checkOverlap(ing, allIngresses, servers)
-	if err != nil {
-		n.metricCollector.IncCheckErrorCount(ing.ObjectMeta.Namespace, ing.Name)
-		return err
+	if !n.cfg.DisablePathOverlapValidation {
+		err := checkOverlap(ing, allIngresses, servers)
+		if err != nil {
+			n.metricCollector.IncCheckErrorCount(ing.ObjectMeta.Namespace, ing.Name)
+			return err
+		}
 	}
+
 	testedSize := len(ings)
 	if n.cfg.DisableFullValidationTest {
 		_, _, pcfg = n.getConfiguration(ings[len(ings)-1:])

internal/ingress/controller/controller_test.go

@@ -339,6 +339,83 @@ func TestCheckIngress(t *testing.T) {
 				t.Errorf("with a new ingress without error, no error should be returned")
 			}
 		})
+
+		t.Run("When there is a duplicated ingress with same host and path it should error", func(t *testing.T) {
+			ing := &networking.Ingress{
+				ObjectMeta: metav1.ObjectMeta{
+					Name:      "test-ingress",
+					Namespace: "test-namespace",
+					Annotations: map[string]string{
+						"kubernetes.io/ingress.class": "nginx",
+					},
+				},
+				Spec: networking.IngressSpec{
+					Rules: []networking.IngressRule{
+						{
+							Host: "example.com",
+							IngressRuleValue: networking.IngressRuleValue{
+								HTTP: &networking.HTTPIngressRuleValue{
+									Paths: []networking.HTTPIngressPath{
+										{
+											Path:     "/",
+											PathType: &pathTypePrefix,
+											Backend: networking.IngressBackend{
+												Service: &networking.IngressServiceBackend{
+													Name: "http-svc",
+													Port: networking.ServiceBackendPort{
+														Number: 80,
+														Name:   "http",
+													},
+												},
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			}
+
+			nginx.store = &fakeIngressStore{
+				ingresses: []*ingress.Ingress{
+					{
+						Ingress:           *ing,
+						ParsedAnnotations: &annotations.Ingress{},
+					},
+				},
+			}
+			duplicatedIngress := ing.DeepCopy()
+			duplicatedIngress.ObjectMeta.Name = "duplicated-ingress"
+
+			nginx.cfg.DisablePathOverlapValidation = false
+			nginx.command = testNginxTestCommand{
+				t:        t,
+				err:      nil,
+				expected: "_,example.com",
+			}
+
+			err = nginx.CheckIngress(duplicatedIngress)
+			if err == nil {
+				t.Errorf("expected errors but none occurred")
+			}
+			t.Run("when disablePathOverlap is enabled should not throw any error", func(t *testing.T) {
+				duplicatedIngress := ing.DeepCopy()
+				duplicatedIngress.ObjectMeta.Name = "duplicated-ingress"
+
+				nginx.cfg.DisablePathOverlapValidation = true
+				nginx.command = testNginxTestCommand{
+					t:        t,
+					err:      nil,
+					expected: "_,example.com",
+				}
+
+				err = nginx.CheckIngress(duplicatedIngress)
+				if err != nil {
+					t.Errorf("expected no errors but one %+v occurred", err)
+				}
+			})
+		})
 	})
 
 	t.Run("When the ingress is marked as deleted", func(t *testing.T) {

pkg/flags/flags.go

@@ -194,6 +194,11 @@ Takes the form "<host>:port". If not provided, no admission controller is starte
 			`The path of the validating webhook certificate PEM.`)
 		validationWebhookKey = flags.String("validating-webhook-key", "",
 			`The path of the validating webhook key PEM.`)
+		disablePathOverlapValidation = flags.Bool("disable-path-overlap-validation", false,
+			`Disable path overlap validation. Validation webhook blocks creating ingresses with the same hostname and path in the same ingressClass.
+        These flags turn off this validation as it helps split ingresses or move them between namespaces as it relies on the existing model rules:
+        'If the same path for the same host is defined in more than one Ingress, the oldest rule wins'`)
+
 		disableFullValidationTest = flags.Bool("disable-full-test", false,
 			`Disable full test of all merged ingresses at the admission stage and tests the template of the ingress being created or updated  (full test of all ingresses is enabled by default).`)
 
@@ -315,37 +320,38 @@ https://blog.maxmind.com/2019/12/18/significant-changes-to-accessing-and-using-g
 	ngx_config.EnableSSLChainCompletion = *enableSSLChainCompletion
 
 	config := &controller.Configuration{
-		APIServerHost:               *apiserverHost,
-		KubeConfigFile:              *kubeConfigFile,
-		UpdateStatus:                *updateStatus,
-		ElectionID:                  *electionID,
-		EnableProfiling:             *profiling,
-		EnableMetrics:               *enableMetrics,
-		MetricsPerHost:              *metricsPerHost,
-		MetricsBuckets:              histogramBuckets,
-		ReportStatusClasses:         *reportStatusClasses,
-		MonitorMaxBatchSize:         *monitorMaxBatchSize,
-		DisableServiceExternalName:  *disableServiceExternalName,
-		EnableSSLPassthrough:        *enableSSLPassthrough,
-		ResyncPeriod:                *resyncPeriod,
-		DefaultService:              *defaultSvc,
-		Namespace:                   *watchNamespace,
-		WatchNamespaceSelector:      namespaceSelector,
-		ConfigMapName:               *configMap,
-		TCPConfigMapName:            *tcpConfigMapName,
-		UDPConfigMapName:            *udpConfigMapName,
-		DisableFullValidationTest:   *disableFullValidationTest,
-		DefaultSSLCertificate:       *defSSLCertificate,
-		DeepInspector:               *deepInspector,
-		PublishService:              *publishSvc,
-		PublishStatusAddress:        *publishStatusAddress,
-		UpdateStatusOnShutdown:      *updateStatusOnShutdown,
-		ShutdownGracePeriod:         *shutdownGracePeriod,
-		PostShutdownGracePeriod:     *postShutdownGracePeriod,
-		UseNodeInternalIP:           *useNodeInternalIP,
-		SyncRateLimit:               *syncRateLimit,
-		HealthCheckHost:             *healthzHost,
-		DynamicConfigurationRetries: *dynamicConfigurationRetries,
+		APIServerHost:                *apiserverHost,
+		KubeConfigFile:               *kubeConfigFile,
+		UpdateStatus:                 *updateStatus,
+		ElectionID:                   *electionID,
+		EnableProfiling:              *profiling,
+		EnableMetrics:                *enableMetrics,
+		MetricsPerHost:               *metricsPerHost,
+		MetricsBuckets:               histogramBuckets,
+		ReportStatusClasses:          *reportStatusClasses,
+		MonitorMaxBatchSize:          *monitorMaxBatchSize,
+		DisableServiceExternalName:   *disableServiceExternalName,
+		EnableSSLPassthrough:         *enableSSLPassthrough,
+		ResyncPeriod:                 *resyncPeriod,
+		DefaultService:               *defaultSvc,
+		Namespace:                    *watchNamespace,
+		WatchNamespaceSelector:       namespaceSelector,
+		ConfigMapName:                *configMap,
+		TCPConfigMapName:             *tcpConfigMapName,
+		UDPConfigMapName:             *udpConfigMapName,
+		DisableFullValidationTest:    *disableFullValidationTest,
+		DisablePathOverlapValidation: *disablePathOverlapValidation,
+		DefaultSSLCertificate:        *defSSLCertificate,
+		DeepInspector:                *deepInspector,
+		PublishService:               *publishSvc,
+		PublishStatusAddress:         *publishStatusAddress,
+		UpdateStatusOnShutdown:       *updateStatusOnShutdown,
+		ShutdownGracePeriod:          *shutdownGracePeriod,
+		PostShutdownGracePeriod:      *postShutdownGracePeriod,
+		UseNodeInternalIP:            *useNodeInternalIP,
+		SyncRateLimit:                *syncRateLimit,
+		HealthCheckHost:              *healthzHost,
+		DynamicConfigurationRetries:  *dynamicConfigurationRetries,
 		ListenPorts: &ngx_config.ListenPorts{
 			Default:  *defServerPort,
 			Health:   *healthzPort,