Ensure we push atmost once to the nackCh

test/xds/xds_server_certificate_providers_test.go

@@ -24,6 +24,7 @@ import (
 	"fmt"
 	"net"
 	"strconv"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -110,6 +111,7 @@ func (s) TestServerSideXDS_WithNoCertificateProvidersInBootstrap_Failure(t *test
 	// Spin up an xDS management server that pushes on a channel when it
 	// receives a NACK for an LDS response.
 	nackCh := make(chan struct{}, 1)
+	nackSeen := atomic.Bool{}
 	mgmtServer := e2e.StartManagementServer(t, e2e.ManagementServerOptions{
 		OnStreamRequest: func(_ int64, req *v3discoverypb.DiscoveryRequest) error {
 			if req.GetTypeUrl() != "type.googleapis.com/envoy.config.listener.v3.Listener" {
@@ -118,6 +120,12 @@ func (s) TestServerSideXDS_WithNoCertificateProvidersInBootstrap_Failure(t *test
 			if req.GetErrorDetail() == nil {
 				return nil
 			}
+			// Pushing to the channel more than once will block the xds
+			// server until a reader clears the channel. So we avoid pushing more
+			// than once.
+			if !nackSeen.CompareAndSwap(false, true) {
+				return nil
+			}
 			select {
 			case nackCh <- struct{}{}:
 			case <-ctx.Done():
@@ -237,6 +245,7 @@ func (s) TestServerSideXDS_WithValidAndInvalidSecurityConfiguration(t *testing.T
 	// Spin up an xDS management server that pushes on a channel when it
 	// receives a NACK for an LDS response.
 	nackCh := make(chan struct{}, 1)
+	nackSeen := atomic.Bool{}
 	managementServer := e2e.StartManagementServer(t, e2e.ManagementServerOptions{
 		OnStreamRequest: func(_ int64, req *v3discoverypb.DiscoveryRequest) error {
 			if req.GetTypeUrl() != "type.googleapis.com/envoy.config.listener.v3.Listener" {
@@ -245,6 +254,12 @@ func (s) TestServerSideXDS_WithValidAndInvalidSecurityConfiguration(t *testing.T
 			if req.GetErrorDetail() == nil {
 				return nil
 			}
+			// Pushing to the channel more than once will block the xds
+			// server until a reader clears the channel. So we avoid pushing more
+			// than once.
+			if !nackSeen.CompareAndSwap(false, true) {
+				return nil
+			}
 			select {
 			case nackCh <- struct{}{}:
 			case <-ctx.Done():