xdsclient/xdsresource: add AutoHostRewrite and Endpoint Hostname support (grpc#8728)

This PR implements the validation logic and extracting per endpoint
Hostname attributes from xDS resources for [gRFC
A81](https://github.com/grpc/proposal/blob/master/A81-xds-authority-rewriting.md)

### Key Changes:

1.  **RDS Resource Validation :**
* The boolean value of `RouteAction.auto_host_rewrite` is extracted from
the RDS resource and stored in route struct
* This field is only set to `true` in the parsed route struct if the
`trusted_xds_server` option is present in the `ServerConfig` and the
global environment variable for authority overriding is enabled.

2.  **EDS Resource Validation:**
* The `Endpoint.hostname` field is extracted from the EDS resource and
will be stored as a `hostname` string in parsed endpoint struct. It will
be changed to be an per-endpoint resolver attribute in a follow-up PR.

RELEASE NOTES: None

Author: Pranjali-2501

internal/envconfig/envconfig.go

@@ -77,6 +77,11 @@ var (
 	//   - Target resolution is disabled.
 	//   - The DNS resolver is being used.
 	EnableDefaultPortForProxyTarget = boolFromEnv("GRPC_EXPERIMENTAL_ENABLE_DEFAULT_PORT_FOR_PROXY_TARGET", true)
+
+	// XDSAuthorityRewrite indicates whether xDS authority rewriting is enabled.
+	// This feature is defined in gRFC A81 and is enabled by setting the
+	// environment variable GRPC_EXPERIMENTAL_XDS_AUTHORITY_REWRITE to "true".
+	XDSAuthorityRewrite = boolFromEnv("GRPC_EXPERIMENTAL_XDS_AUTHORITY_REWRITE", false)
 )
 
 func boolFromEnv(envVar string, def bool) bool {

internal/xds/xdsclient/xdsresource/filter_chain.go

@@ -680,7 +680,7 @@ func processNetworkFilters(filters []*v3listenerpb.Filter) (*FilterChain, error)
 					// server-side." - A36
 					// Can specify v3 here, as will never get to this function
 					// if v2.
-					routeU, err := generateRDSUpdateFromRouteConfiguration(hcm.GetRouteConfig())
+					routeU, err := generateRDSUpdateFromRouteConfiguration(hcm.GetRouteConfig(), nil)
 					if err != nil {
 						return nil, fmt.Errorf("failed to parse inline RDS resp: %v", err)
 					}

internal/xds/xdsclient/xdsresource/listener_resource_type.go

@@ -35,8 +35,8 @@ type listenerResourceDecoder struct {
 	bootstrapConfig *bootstrap.Config
 }
 
-func (d *listenerResourceDecoder) Decode(resource *xdsclient.AnyProto, _ xdsclient.DecodeOptions) (*xdsclient.DecodeResult, error) {
-	name, listener, err := unmarshalListenerResource(resource.ToAny())
+func (d *listenerResourceDecoder) Decode(resource *xdsclient.AnyProto, opts xdsclient.DecodeOptions) (*xdsclient.DecodeResult, error) {
+	name, listener, err := unmarshalListenerResource(resource.ToAny(), &opts)
 	if name == "" {
 		// Name is unset only when protobuf deserialization fails.
 		return nil, err

internal/xds/xdsclient/xdsresource/route_config_resource_type.go

@@ -37,8 +37,8 @@ type routeConfigResourceDecoder struct {
 	bootstrapConfig *bootstrap.Config
 }
 
-func (d *routeConfigResourceDecoder) Decode(resource *xdsclient.AnyProto, _ xdsclient.DecodeOptions) (*xdsclient.DecodeResult, error) {
-	name, rc, err := unmarshalRouteConfigResource(resource.ToAny())
+func (d *routeConfigResourceDecoder) Decode(resource *xdsclient.AnyProto, opts xdsclient.DecodeOptions) (*xdsclient.DecodeResult, error) {
+	name, rc, err := unmarshalRouteConfigResource(resource.ToAny(), &opts)
 	if name == "" {
 		// Name is unset only when protobuf deserialization fails.
 		return nil, err

internal/xds/xdsclient/xdsresource/type_eds.go

@@ -54,6 +54,7 @@ type Endpoint struct {
 	Weight       uint32
 	HashKey      string
 	Metadata     map[string]any
+	Hostname     string
 }
 
 // Locality contains information of a locality.

internal/xds/xdsclient/xdsresource/type_rds.go

@@ -150,6 +150,9 @@ type Route struct {
 	// ClusterSpecifierPlugin is the name of the Cluster Specifier Plugin that
 	// this Route is linked to, if specified by xDS.
 	ClusterSpecifierPlugin string
+	// AutoHostRewrite indicates that the ":authority" header can be rewritten
+	// to the hostname of the upstream endpoint.
+	AutoHostRewrite bool
 }
 
 // WeightedCluster contains settings for an xds ActionType.WeightedCluster.

internal/xds/xdsclient/xdsresource/unmarshal_eds.go

@@ -132,6 +132,7 @@ func parseEndpoints(lbEndpoints []*v3endpointpb.LbEndpoint, uniqueEndpointAddrs
 			Weight:       weight,
 			HashKey:      hashKey,
 			Metadata:     endpointMetadata,
+			Hostname:     lbEndpoint.GetEndpoint().GetHostname(),
 		})
 	}
 	return endpoints, nil