Revert "envconfig: enable xDS client fallback by default (#7949)"

This reverts commit 46faf72.

internal/envconfig/envconfig.go

@@ -49,7 +49,7 @@ var (
 	// XDSFallbackSupport is the env variable that controls whether support for
 	// xDS fallback is turned on. If this is unset or is false, only the first
 	// xDS server in the list of server configs will be used.
-	XDSFallbackSupport = boolFromEnv("GRPC_EXPERIMENTAL_XDS_FALLBACK", true)
+	XDSFallbackSupport = boolFromEnv("GRPC_EXPERIMENTAL_XDS_FALLBACK", false)
 	// NewPickFirstEnabled is set if the new pickfirst leaf policy is to be used
 	// instead of the exiting pickfirst implementation. This can be enabled by
 	// setting the environment variable "GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST"

internal/xds/bootstrap/bootstrap_test.go

@@ -429,6 +429,10 @@ func (s) TestGetConfiguration_Success(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
+			origFallbackEnv := envconfig.XDSFallbackSupport
+			envconfig.XDSFallbackSupport = true
+			defer func() { envconfig.XDSFallbackSupport = origFallbackEnv }()
+
 			testGetConfigurationWithFileNameEnv(t, test.name, false, test.wantConfig)
 			testGetConfigurationWithFileContentEnv(t, test.name, false, test.wantConfig)
 		})
@@ -1198,9 +1202,9 @@ func (s) TestNode_ToProto(t *testing.T) {
 // Tests the case where the xDS fallback env var is set to false, and verifies
 // that only the first server from the list of server configurations is used.
 func (s) TestGetConfiguration_FallbackDisabled(t *testing.T) {
-	origFallbackEnv := envconfig.XDSFallbackSupport
-	envconfig.XDSFallbackSupport = false
-	defer func() { envconfig.XDSFallbackSupport = origFallbackEnv }()
+	// TODO(easwars): Default value of "GRPC_EXPERIMENTAL_XDS_FALLBACK"
+	// env var is currently false. When the default is changed to true,
+	// explicitly set it to false here.
 
 	cancel := setupBootstrapOverride(map[string]string{
 		"multipleXDSServers": `

xds/internal/xdsclient/tests/fallback_test.go

@@ -30,6 +30,7 @@ import (
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/credentials/insecure"
 	"google.golang.org/grpc/internal"
+	"google.golang.org/grpc/internal/envconfig"
 	"google.golang.org/grpc/internal/stubserver"
 	"google.golang.org/grpc/internal/testutils"
 	"google.golang.org/grpc/internal/testutils/xds/e2e"
@@ -78,6 +79,11 @@ func waitForRPCsToReachBackend(ctx context.Context, client testgrpc.TestServiceC
 // to it. The test also verifies that when all requested resources are cached
 // from the primary, fallback is not triggered when the connection goes down.
 func (s) TestFallback_OnStartup(t *testing.T) {
+	// Enable fallback env var.
+	origFallbackEnv := envconfig.XDSFallbackSupport
+	envconfig.XDSFallbackSupport = true
+	defer func() { envconfig.XDSFallbackSupport = origFallbackEnv }()
+
 	ctx, cancel := context.WithTimeout(context.Background(), defaultFallbackTestTimeout)
 	defer cancel()
 
@@ -242,6 +248,11 @@ func (s) TestFallback_OnStartup(t *testing.T) {
 
 // Tests fallback when the primary management server fails during an update.
 func (s) TestFallback_MidUpdate(t *testing.T) {
+	// Enable fallback env var.
+	origFallbackEnv := envconfig.XDSFallbackSupport
+	envconfig.XDSFallbackSupport = true
+	defer func() { envconfig.XDSFallbackSupport = origFallbackEnv }()
+
 	ctx, cancel := context.WithTimeout(context.Background(), defaultFallbackTestTimeout)
 	defer cancel()
 
@@ -443,6 +454,11 @@ func (s) TestFallback_MidUpdate(t *testing.T) {
 
 // Tests fallback when the primary management server fails during startup.
 func (s) TestFallback_MidStartup(t *testing.T) {
+	// Enable fallback env var.
+	origFallbackEnv := envconfig.XDSFallbackSupport
+	envconfig.XDSFallbackSupport = true
+	defer func() { envconfig.XDSFallbackSupport = origFallbackEnv }()
+
 	ctx, cancel := context.WithTimeout(context.Background(), defaultFallbackTestTimeout)
 	defer cancel()