Handle empty address list from NameResolver.

Passing an empty list to NameResolver.Listener.onUpdate() will trigger
onError(). It is documented in the javadoc and enforced by
ManagedChannelImpl.

Forbid empty address list in EquivalentAddressGroup.

Resolves #1657

core/src/main/java/io/grpc/EquivalentAddressGroup.java

@@ -31,6 +31,8 @@
 
 package io.grpc;
 
+import com.google.common.base.Preconditions;
+
 import java.net.SocketAddress;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -49,6 +51,7 @@ public final class EquivalentAddressGroup {
   private final List<SocketAddress> addrs;
 
   public EquivalentAddressGroup(List<SocketAddress> addrs) {
+    Preconditions.checkArgument(!addrs.isEmpty(), "addrs is empty");
     this.addrs = Collections.unmodifiableList(new ArrayList<SocketAddress>(addrs));
   }
 

core/src/main/java/io/grpc/LoadBalancer.java

@@ -68,6 +68,9 @@ public abstract class LoadBalancer<T> {
    * Handles newly resolved addresses and service config from name resolution system.
    *
    * <p>Implementations should not modify the given {@code servers}.
+   *
+   * @param servers the resolved server addresses. Never empty.
+   * @param config extra configuration data from naming system.
    */
   public void handleResolvedAddresses(List<ResolvedServerInfo> servers, Attributes config) { }
 

core/src/main/java/io/grpc/NameResolver.java

@@ -119,6 +119,9 @@ public abstract class NameResolver {
      * Handles updates on resolved addresses and config.
      *
      * <p>Implementations will not modify the given {@code servers}.
+     *
+     * @param servers the resolved server addresses. An empty list will trigger {@link #onError}
+     * @param config extra configuration data from naming system
      */
     void onUpdate(List<ResolvedServerInfo> servers, Attributes config);
 

core/src/main/java/io/grpc/internal/ManagedChannelImpl.java

@@ -168,7 +168,11 @@ public final class ManagedChannelImpl extends ManagedChannel implements WithLogI
     this.nameResolver.start(new NameResolver.Listener() {
       @Override
       public void onUpdate(List<ResolvedServerInfo> servers, Attributes config) {
-        loadBalancer.handleResolvedAddresses(servers, config);
+        if (servers.isEmpty()) {
+          onError(Status.UNAVAILABLE.withDescription("NameResolver returned an empty list"));
+        } else {
+          loadBalancer.handleResolvedAddresses(servers, config);
+        }
       }
 
       @Override

core/src/test/java/io/grpc/internal/ManagedChannelImplTest.java

@@ -362,6 +362,27 @@ public class ManagedChannelImplTest {
     Status status = statusCaptor.getValue();
     assertSame(error.getCode(), status.getCode());
     assertSame(error.getCause(), status.getCause());
+    assertEquals(1, loadBalancerFactory.balancers.size());
+    verify(loadBalancerFactory.balancers.get(0)).handleNameResolutionError(same(error));
+  }
+
+  @Test
+  public void nameResolverReturnsEmptyList() {
+    String errorDescription = "NameResolver returned an empty list";
+    ManagedChannel channel = createChannel(
+        new FakeNameResolverFactory(new ArrayList<ResolvedServerInfo>()), NO_INTERCEPTOR);
+    ClientCall<String, Integer> call = channel.newCall(method, CallOptions.DEFAULT);
+    call.start(mockCallListener, new Metadata());
+    ArgumentCaptor<Status> statusCaptor = ArgumentCaptor.forClass(Status.class);
+    verify(mockCallListener, timeout(1000)).onClose(statusCaptor.capture(), any(Metadata.class));
+    Status status = statusCaptor.getValue();
+    assertSame(Status.Code.UNAVAILABLE, status.getCode());
+    assertTrue(status.getDescription(), status.getDescription().contains(errorDescription));
+    assertEquals(1, loadBalancerFactory.balancers.size());
+    verify(loadBalancerFactory.balancers.get(0)).handleNameResolutionError(statusCaptor.capture());
+    status = statusCaptor.getValue();
+    assertSame(Status.Code.UNAVAILABLE, status.getCode());
+    assertEquals(errorDescription, status.getDescription());
   }
 
   @Test