xds: remove xds client fallback environment variable

internal/envconfig/envconfig.go

@@ -52,11 +52,6 @@ var (
 	// or "false".
 	EnforceALPNEnabled = boolFromEnv("GRPC_ENFORCE_ALPN_ENABLED", true)
 
-	// 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)
-
 	// NewPickFirstEnabled is set if the new pickfirst leaf policy is to be used
 	// instead of the exiting pickfirst implementation. This can be disabled by
 	// setting the environment variable "GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST"

internal/xds/bootstrap/bootstrap.go

@@ -106,12 +106,6 @@ func (scs *ServerConfigs) UnmarshalJSON(data []byte) error {
 	if err := json.Unmarshal(data, &servers); err != nil {
 		return fmt.Errorf("xds: failed to JSON unmarshal server configurations during bootstrap: %v, config:\n%s", err, string(data))
 	}
-	// Only use the first server config if fallback support is disabled.
-	if !envconfig.XDSFallbackSupport {
-		if len(servers) > 1 {
-			servers = servers[:1]
-		}
-	}
 	*scs = servers
 	return nil
 }

internal/xds/bootstrap/bootstrap_test.go

@@ -1201,78 +1201,3 @@ 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 }()
-
-	cancel := setupBootstrapOverride(map[string]string{
-		"multipleXDSServers": `
-		{
-			"node": {
-				"id": "ENVOY_NODE_ID",
-				"metadata": {
-				    "TRAFFICDIRECTOR_GRPC_HOSTNAME": "trafficdirector"
-			    }
-			},
-			"xds_servers" : [
-				{
-					"server_uri": "trafficdirector.googleapis.com:443",
-					"channel_creds": [{ "type": "google_default" }],
-					"server_features": ["xds_v3"]
-				},
-				{
-					"server_uri": "backup.never.use.com:1234",
-					"channel_creds": [{ "type": "google_default" }]
-				}
-			],
-			"authorities": {
-				"xds.td.com": {
-					"xds_servers": [
-						{
-							"server_uri": "td.com",
-							"channel_creds": [ { "type": "google_default" } ],
-							"server_features" : ["xds_v3"]
-						},
-						{
-							"server_uri": "backup.never.use.com:1234",
-							"channel_creds": [{ "type": "google_default" }]
-						}
-					]
-				}
-			}
-		}`,
-	})
-	defer cancel()
-
-	wantConfig := &Config{
-		xDSServers: []*ServerConfig{{
-			serverURI:      "trafficdirector.googleapis.com:443",
-			channelCreds:   []ChannelCreds{{Type: "google_default"}},
-			serverFeatures: []string{"xds_v3"},
-			selectedCreds:  ChannelCreds{Type: "google_default"},
-		}},
-		node: v3Node,
-		clientDefaultListenerResourceNameTemplate: "%s",
-		authorities: map[string]*Authority{
-			"xds.td.com": {
-				ClientListenerResourceNameTemplate: "xdstp://xds.td.com/envoy.config.listener.v3.Listener/%s",
-				XDSServers: []*ServerConfig{{
-					serverURI:      "td.com",
-					channelCreds:   []ChannelCreds{{Type: "google_default"}},
-					serverFeatures: []string{"xds_v3"},
-					selectedCreds:  ChannelCreds{Type: "google_default"},
-				}},
-			},
-		},
-	}
-	t.Run("bootstrap_file_name", func(t *testing.T) {
-		testGetConfigurationWithFileNameEnv(t, "multipleXDSServers", false, wantConfig)
-	})
-	t.Run("bootstrap_file_contents", func(t *testing.T) {
-		testGetConfigurationWithFileContentEnv(t, "multipleXDSServers", false, wantConfig)
-	})
-}