xds: Use XdsDependencyManager for XdsNameResolver

Contributes to the gRFC A74 effort.
https://github.com/grpc/proposal/blob/master/A74-xds-config-tears.md

Co-authored-by: Eric Anderson <ejona@google.com>

Author: larry-safran

xds/src/main/java/io/grpc/xds/XdsAttributes.java

@@ -36,6 +36,22 @@ final class XdsAttributes {
   static final Attributes.Key<ObjectPool<XdsClient>> XDS_CLIENT_POOL =
       Attributes.Key.create("io.grpc.xds.XdsAttributes.xdsClientPool");
 
+  /**
+   * Attribute key for passing around the latest XdsConfig across NameResolver/LoadBalancers.
+   */
+  @NameResolver.ResolutionResultAttr
+  static final Attributes.Key<XdsConfig> XDS_CONFIG =
+      Attributes.Key.create("io.grpc.xds.XdsAttributes.xdsConfig");
+
+
+  /**
+   * Attribute key for passing around the XdsDependencyManager across NameResolver/LoadBalancers.
+   */
+  @NameResolver.ResolutionResultAttr
+  static final Attributes.Key<XdsConfig.XdsClusterSubscriptionRegistry>
+      XDS_CLUSTER_SUBSCRIPT_REGISTRY =
+      Attributes.Key.create("io.grpc.xds.XdsAttributes.xdsConfig.XdsClusterSubscriptionRegistry");
+
   /**
    * Attribute key for obtaining the global provider that provides atomics for aggregating
    * outstanding RPCs sent to each cluster.

xds/src/main/java/io/grpc/xds/XdsConfig.java

@@ -26,9 +26,9 @@
 import io.grpc.xds.XdsRouteConfigureResource.RdsUpdate;
 import java.io.Closeable;
 import java.util.HashMap;
-import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Set;
 
 /**
  * Represents the xDS configuration tree for a specified Listener.
@@ -178,13 +178,22 @@ public boolean equals(Object obj) {
       public StatusOr<EdsUpdate> getEndpoint() {
         return endpoint;
       }
+
+      @Override
+      public String toString() {
+        if (endpoint.hasValue()) {
+          return "EndpointConfig{endpoint=" + endpoint.getValue() + "}";
+        } else {
+          return "EndpointConfig{error=" + endpoint.getStatus() + "}";
+        }
+      }
     }
 
     // The list of leaf clusters for an aggregate cluster.
     static final class AggregateConfig implements ClusterChild {
-      private final List<String> leafNames;
+      private final Set<String> leafNames;
 
-      public AggregateConfig(List<String> leafNames) {
+      public AggregateConfig(Set<String> leafNames) {
         this.leafNames = checkNotNull(leafNames, "leafNames");
       }
 
@@ -234,6 +243,7 @@ XdsConfigBuilder setVirtualHost(VirtualHost virtualHost) {
     XdsConfig build() {
       checkNotNull(listener, "listener");
       checkNotNull(route, "route");
+      checkNotNull(virtualHost, "virtualHost");
       return new XdsConfig(listener, route, clusters, virtualHost);
     }
   }