xds: add support for server side Listener watcher in XdsClient (#6801)

Author: sanjaypujare

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

@@ -159,6 +159,7 @@ public String toString() {
    * Corresponds to Envoy proto message {@link io.envoyproxy.envoy.api.v2.listener.FilterChain}.
    */
   static final class FilterChain {
+    // TODO(sanjaypujare): flatten structure by moving FilterChainMatch class members here.
     private final FilterChainMatch filterChainMatch;
     // TODO(sanjaypujare): remove dependency on envoy data type along with rest of the code.
     private final io.envoyproxy.envoy.api.v2.auth.DownstreamTlsContext downstreamTlsContext;

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

@@ -62,22 +62,15 @@ abstract class XdsClient {
    */
   static final class ConfigUpdate {
     private final String clusterName;
-    private final Listener listener;
 
-    private ConfigUpdate(String clusterName, @Nullable Listener listener) {
+    private ConfigUpdate(String clusterName) {
       this.clusterName = clusterName;
-      this.listener = listener;
     }
 
     String getClusterName() {
       return clusterName;
     }
 
-    @Nullable
-    public Listener getListener() {
-      return listener;
-    }
-
     @Override
     public String toString() {
       return
@@ -93,7 +86,6 @@ static Builder newBuilder() {
 
     static final class Builder {
       private String clusterName;
-      @Nullable private Listener listener;
 
       // Use ConfigUpdate.newBuilder().
       private Builder() {
@@ -104,14 +96,9 @@ Builder setClusterName(String clusterName) {
         return this;
       }
 
-      Builder setListener(Listener listener) {
-        this.listener = listener;
-        return this;
-      }
-
       ConfigUpdate build() {
         Preconditions.checkState(clusterName != null, "clusterName is not set");
-        return new ConfigUpdate(clusterName, listener);
+        return new ConfigUpdate(clusterName);
       }
     }
   }
@@ -352,6 +339,52 @@ EndpointUpdate build() {
     }
   }
 
+  /**
+   * Updates via resource discovery RPCs using LDS. Includes {@link Listener} object containing
+   * config for security, RBAC or other server side features such as rate limit.
+   */
+  static final class ListenerUpdate {
+    // TODO(sanjaypujare): flatten structure by moving Listener class members here.
+    private final Listener listener;
+
+    private ListenerUpdate(Listener listener) {
+      this.listener = listener;
+    }
+
+    public Listener getListener() {
+      return listener;
+    }
+
+    @Override
+    public String toString() {
+      return MoreObjects.toStringHelper(this)
+          .add("listener", listener)
+          .toString();
+    }
+
+    static Builder newBuilder() {
+      return new Builder();
+    }
+
+    static final class Builder {
+      private Listener listener;
+
+      // Use ListenerUpdate.newBuilder().
+      private Builder() {
+      }
+
+      Builder setListener(Listener listener) {
+        this.listener = listener;
+        return this;
+      }
+
+      ListenerUpdate build() {
+        Preconditions.checkState(listener != null, "listener is not set");
+        return new ListenerUpdate(listener);
+      }
+    }
+  }
+
   /**
    * Config watcher interface. To be implemented by the xDS resolver.
    */
@@ -385,6 +418,19 @@ interface EndpointWatcher {
     void onError(Status error);
   }
 
+  /**
+   * Listener watcher interface. To be used by {@link io.grpc.xds.internal.sds.XdsServerBuilder}.
+   */
+  interface ListenerWatcher {
+
+    /**
+     * Called when receiving an update on Listener configuration.
+     */
+    void onListenerChanged(ListenerUpdate update);
+
+    void onError(Status error);
+  }
+
   /**
    * Shutdown this {@link XdsClient} and release resources.
    */
@@ -430,6 +476,12 @@ void watchEndpointData(String clusterName, EndpointWatcher watcher) {
   void cancelEndpointDataWatch(String clusterName, EndpointWatcher watcher) {
   }
 
+  /**
+   * Registers a watcher for a Listener with the given port.
+   */
+  void watchListenerData(int port, ListenerWatcher watcher) {
+  }
+
   /**
    * Report client load stats to a remote server for the given cluster:cluster_service.
    *