grpclb: use fallback addresses if no balancer address is given

grpclb/src/main/java/io/grpc/grpclb/GrpclbState.java

@@ -193,25 +193,27 @@ void handleSubchannelState(Subchannel subchannel, ConnectivityStateInfo newState
   void handleAddresses(
       List<LbAddressGroup> newLbAddressGroups, List<EquivalentAddressGroup> newBackendServers) {
     if (newLbAddressGroups.isEmpty()) {
-      propagateError(Status.UNAVAILABLE.withDescription(
-              "NameResolver returned no LB address while asking for GRPCLB"));
-      return;
-    }
-    LbAddressGroup newLbAddressGroup = flattenLbAddressGroups(newLbAddressGroups);
-    startLbComm(newLbAddressGroup);
-    // Avoid creating a new RPC just because the addresses were updated, as it can cause a
-    // stampeding herd. The current RPC may be on a connection to an address not present in
-    // newLbAddressGroups, but we're considering that "okay". If we detected the RPC is to an
-    // outdated backend, we could choose to re-create the RPC.
-    if (lbStream == null) {
-      startLbRpc();
+      // No balancer address: close existing balancer connection and enter fallback mode
+      // immediately.
+      shutdownLbComm();
+      syncContext.execute(new FallbackModeTask());
+    } else {
+      LbAddressGroup newLbAddressGroup = flattenLbAddressGroups(newLbAddressGroups);
+      startLbComm(newLbAddressGroup);
+      // Avoid creating a new RPC just because the addresses were updated, as it can cause a
+      // stampeding herd. The current RPC may be on a connection to an address not present in
+      // newLbAddressGroups, but we're considering that "okay". If we detected the RPC is to an
+      // outdated backend, we could choose to re-create the RPC.
+      if (lbStream == null) {
+        startLbRpc();
+      }
+      // Start the fallback timer if it's never started
+      if (fallbackTimer == null) {
+        fallbackTimer = syncContext.schedule(
+            new FallbackModeTask(), FALLBACK_TIMEOUT_MS, TimeUnit.MILLISECONDS, timerService);
+      }
     }
     fallbackBackendList = newBackendServers;
-    // Start the fallback timer if it's never started
-    if (fallbackTimer == null) {
-      fallbackTimer = syncContext.schedule(
-          new FallbackModeTask(), FALLBACK_TIMEOUT_MS, TimeUnit.MILLISECONDS, timerService);
-    }
     if (usingFallbackBackends) {
       // Populate the new fallback backends to round-robin list.
       useFallbackBackends();

grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

@@ -32,6 +32,7 @@
 import static org.junit.Assert.assertTrue;
 import static org.mockito.AdditionalAnswers.delegatesTo;
 import static org.mockito.Matchers.any;
+import static org.mockito.Matchers.anyString;
 import static org.mockito.Matchers.eq;
 import static org.mockito.Matchers.same;
 import static org.mockito.Mockito.atLeast;
@@ -1388,6 +1389,31 @@ public void grpclbFallback_breakLbStreamBeforeFallbackTimerExpires() {
             .build()));
   }
 
+  @Test
+  public void grpclbFallback_noBalancerAddress() {
+    long loadReportIntervalMillis = 1983;
+    InOrder inOrder = inOrder(helper, subchannelPool);
+
+    // Create a resolution list with just backend addresses
+    List<EquivalentAddressGroup> resolutionList = createResolvedServerAddresses(false, false);
+    Attributes resolutionAttrs = Attributes.EMPTY;
+    deliverResolvedAddresses(resolutionList, resolutionAttrs);
+
+    assertThat(logs).containsExactly(
+          "INFO: Using fallback backends",
+          "INFO: Using RR list=[[[FakeSocketAddress-fake-address-0]/{}], "
+              + "[[FakeSocketAddress-fake-address-1]/{}]], drop=[null, null]",
+          "INFO: CONNECTING: picks=[BUFFER_ENTRY], drops=[null, null]").inOrder();
+
+    // Fall back to the backends from resolver
+    fallbackTestVerifyUseOfFallbackBackendLists(inOrder, resolutionList);
+
+    // No fallback timeout timer scheduled.
+    assertEquals(0, fakeClock.numPendingTasks(FALLBACK_MODE_TASK_FILTER));
+    verify(helper, never())
+        .createOobChannel(any(EquivalentAddressGroup.class), anyString());
+  }
+
   @Test
   public void grpclbFallback_balancerLost() {
     subtestGrpclbFallbackConnectionLost(true, false);