xds: env var protection for xds fallback (#7483)

internal/envconfig/envconfig.go

@@ -46,6 +46,10 @@ var (
 	// by setting the environment variable "GRPC_ENFORCE_ALPN_ENABLED" to "true"
 	// or "false".
 	EnforceALPNEnabled = boolFromEnv("GRPC_ENFORCE_ALPN_ENABLED", false)
+	// 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", false)
 )
 
 func boolFromEnv(envVar string, def bool) bool {

internal/xds/bootstrap/bootstrap.go

@@ -84,6 +84,39 @@ func (cc ChannelCreds) String() string {
 	return cc.Type + "-" + string(b)
 }
 
+// ServerConfigs represents a collection of server configurations.
+type ServerConfigs []*ServerConfig
+
+// Equal returns true if scs equals other.
+func (scs *ServerConfigs) Equal(other *ServerConfigs) bool {
+	if len(*scs) != len(*other) {
+		return false
+	}
+	for i := range *scs {
+		if !(*scs)[i].Equal((*other)[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+// UnmarshalJSON takes the json data (a list of server configurations) and
+// unmarshals it to the struct.
+func (scs *ServerConfigs) UnmarshalJSON(data []byte) error {
+	servers := []*ServerConfig{}
+	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
+}
+
 // Authority contains configuration for an xDS control plane authority.
 //
 // This type does not implement custom JSON marshal/unmarshal logic because it
@@ -104,7 +137,7 @@ type Authority struct {
 	// "xdstp://<authority_name>/envoy.config.listener.v3.Listener/%s".
 	ClientListenerResourceNameTemplate string `json:"client_listener_resource_name_template,omitempty"`
 	// XDSServers contains the list of server configurations for this authority.
-	XDSServers []*ServerConfig `json:"xds_servers,omitempty"`
+	XDSServers ServerConfigs `json:"xds_servers,omitempty"`
 }
 
 // Equal returns true if a equals other.
@@ -116,7 +149,7 @@ func (a *Authority) Equal(other *Authority) bool {
 		return false
 	case a.ClientListenerResourceNameTemplate != other.ClientListenerResourceNameTemplate:
 		return false
-	case !slices.EqualFunc(a.XDSServers, other.XDSServers, func(a, b *ServerConfig) bool { return a.Equal(b) }):
+	case !a.XDSServers.Equal(&other.XDSServers):
 		return false
 	}
 	return true
@@ -315,7 +348,7 @@ func ServerConfigForTesting(opts ServerConfigTestingOptions) (*ServerConfig, err
 // Config is the internal representation of the bootstrap configuration provided
 // to the xDS client.
 type Config struct {
-	xDSServers                                []*ServerConfig
+	xDSServers                                ServerConfigs
 	cpcs                                      map[string]certproviderNameAndConfig
 	serverListenerResourceNameTemplate        string
 	clientDefaultListenerResourceNameTemplate string
@@ -405,7 +438,7 @@ func (c *Config) Equal(other *Config) bool {
 		return true
 	case (c != nil) != (other != nil):
 		return false
-	case !slices.EqualFunc(c.xDSServers, other.xDSServers, func(a, b *ServerConfig) bool { return a.Equal(b) }):
+	case !c.xDSServers.Equal(&other.xDSServers):
 		return false
 	case !maps.EqualFunc(c.certProviderConfigs, other.certProviderConfigs, func(a, b *certprovider.BuildableConfig) bool { return a.String() == b.String() }):
 		return false
@@ -429,7 +462,7 @@ func (c *Config) String() string {
 
 // The following fields correspond 1:1 with the JSON schema for Config.
 type configJSON struct {
-	XDSServers                                []*ServerConfig                      `json:"xds_servers,omitempty"`
+	XDSServers                                ServerConfigs                        `json:"xds_servers,omitempty"`
 	CertificateProviders                      map[string]certproviderNameAndConfig `json:"certificate_providers,omitempty"`
 	ServerListenerResourceNameTemplate        string                               `json:"server_listener_resource_name_template,omitempty"`
 	ClientDefaultListenerResourceNameTemplate string                               `json:"client_default_listener_resource_name_template,omitempty"`

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)
 		})
@@ -1194,3 +1198,78 @@ 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) {
+	// 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": `
+		{
+			"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)
+	})
+}