refactor: move internal.go and internal_test.go into xds.go/xds_test.go

Per code review feedback, consolidated the contents of
internal/xds/internal.go into internal/xds/xds.go and
internal/xds/internal_test.go into internal/xds/xds_test.go
to keep related functionality in a single file and improve
code organization.

Author: CynicDog

internal/xds/internal.go

@@ -14,12 +14,17 @@
  * limitations under the License.
  */
 
-// Package xds contains methods to Get/Set handshake cluster names. It is separated
-// out from the top level /internal package to avoid circular dependencies.
+// Package xds contains functions, structs, and utilities for working with
+// handshake cluster names, as well as shared components used by xds balancers
+// and resolvers. It is separated from the top-level /internal package to
+// avoid circular dependencies.
 package xds
 
 import (
+	"fmt"
+
 	"google.golang.org/grpc/attributes"
+	"google.golang.org/grpc/internal/xds/clients"
 	"google.golang.org/grpc/resolver"
 )
 
@@ -40,3 +45,60 @@ func GetXDSHandshakeClusterName(attr *attributes.Attributes) (string, bool) {
 	name, ok := v.(string)
 	return name, ok
 }
+
+// LocalityString generates a string representation of clients.Locality in the
+// format specified in gRFC A76.
+func LocalityString(l clients.Locality) string {
+	return fmt.Sprintf("{region=%q, zone=%q, sub_zone=%q}", l.Region, l.Zone, l.SubZone)
+}
+
+// IsLocalityEqual allows the values to be compared by Attributes.Equal.
+func IsLocalityEqual(l clients.Locality, o any) bool {
+	ol, ok := o.(clients.Locality)
+	if !ok {
+		return false
+	}
+	return l.Region == ol.Region && l.Zone == ol.Zone && l.SubZone == ol.SubZone
+}
+
+// LocalityFromString converts a string representation of clients.locality as
+// specified in gRFC A76, into a LocalityID struct.
+func LocalityFromString(s string) (ret clients.Locality, _ error) {
+	_, err := fmt.Sscanf(s, "{region=%q, zone=%q, sub_zone=%q}", &ret.Region, &ret.Zone, &ret.SubZone)
+	if err != nil {
+		return clients.Locality{}, fmt.Errorf("%s is not a well formatted locality ID, error: %v", s, err)
+	}
+	return ret, nil
+}
+
+type localityKeyType string
+
+const localityKey = localityKeyType("grpc.xds.internal.address.locality")
+
+// GetLocalityID returns the locality ID of addr.
+func GetLocalityID(addr resolver.Address) clients.Locality {
+	path, _ := addr.BalancerAttributes.Value(localityKey).(clients.Locality)
+	return path
+}
+
+// SetLocalityID sets locality ID in addr to l.
+func SetLocalityID(addr resolver.Address, l clients.Locality) resolver.Address {
+	addr.BalancerAttributes = addr.BalancerAttributes.WithValue(localityKey, l)
+	return addr
+}
+
+// SetLocalityIDInEndpoint sets locality ID in endpoint to l.
+func SetLocalityIDInEndpoint(endpoint resolver.Endpoint, l clients.Locality) resolver.Endpoint {
+	endpoint.Attributes = endpoint.Attributes.WithValue(localityKey, l)
+	return endpoint
+}
+
+// ResourceTypeMapForTesting maps TypeUrl to corresponding ResourceType.
+var ResourceTypeMapForTesting map[string]any
+
+// UnknownCSMLabels are TelemetryLabels emitted from CDS if CSM Telemetry Label
+// data is not present in the CDS Resource.
+var UnknownCSMLabels = map[string]string{
+	"csm.service_name":           "unknown",
+	"csm.service_namespace_name": "unknown",
+}