grpclb: skip fallback if the LB is already in fallback mode (backport v1.38.x)

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

@@ -259,11 +259,19 @@ void handleAddresses(
  serviceName,
  newLbAddressGroups,
  newBackendServers);
+ fallbackBackendList = newBackendServers;
  if (newLbAddressGroups.isEmpty()) {
- // No balancer address: close existing balancer connection and enter fallback mode
- // immediately.
+ // No balancer address: close existing balancer connection and prepare to enter fallback
+ // mode. If there is no successful backend connection, it enters fallback mode immediately.
+ // Otherwise, fallback does not happen until backend connections are lost. This behavior
+ // might be different from other languages (e.g., existing balancer connection is not
+ // closed in C-core), but we aren't changing it at this time.
  shutdownLbComm();
- syncContext.execute(new FallbackModeTask(NO_LB_ADDRESS_PROVIDED_STATUS));
+ if (!usingFallbackBackends) {
+ fallbackReason = NO_LB_ADDRESS_PROVIDED_STATUS;
+ cancelFallbackTimer();
+ maybeUseFallbackBackends();
+ }
  } else {
  startLbComm(newLbAddressGroups);
  // Avoid creating a new RPC just because the addresses were updated, as it can cause a
@@ -281,7 +289,6 @@ void handleAddresses(
  TimeUnit.MILLISECONDS, timerService);
  }
  }
- fallbackBackendList = newBackendServers;
  if (usingFallbackBackends) {
  // Populate the new fallback backends to round-robin list.
  useFallbackBackends();

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

@@ -1452,8 +1452,11 @@ public void grpclbFallback_breakLbStreamBeforeFallbackTimerExpires() {
  public void grpclbFallback_noBalancerAddress() {
  InOrder inOrder = inOrder(helper, subchannelPool);
 
- // Create just backend addresses
- List<EquivalentAddressGroup> backendList = createResolvedBackendAddresses(2);
+ // Create 5 distinct backends
+ List<EquivalentAddressGroup> backends = createResolvedBackendAddresses(5);
+
+ // Name resolver gives the first two backend addresses
+ List<EquivalentAddressGroup> backendList = backends.subList(0, 2);
  deliverResolvedAddresses(backendList, Collections.<EquivalentAddressGroup>emptyList());
 
  assertThat(logs).containsAtLeast(
@@ -1474,6 +1477,28 @@ public void grpclbFallback_noBalancerAddress() {
  .createOobChannel(ArgumentMatchers.<EquivalentAddressGroup>anyList(), anyString());
  logs.clear();
 
+ /////////////////////////////////////////////////////////////////////////////////////////
+ // Name resolver sends new resolution results with new backend addr but no balancer addr
+ /////////////////////////////////////////////////////////////////////////////////////////
+ // Name resolver then gives the last three backends
+ backendList = backends.subList(2, 5);
+ deliverResolvedAddresses(backendList, Collections.<EquivalentAddressGroup>emptyList());
+
+ assertThat(logs).containsAtLeast(
+ "INFO: [grpclb-<api.google.com>] Using fallback backends",
+ "INFO: [grpclb-<api.google.com>] "
+ + "Using RR list=[[[FakeSocketAddress-fake-address-2]/{}], "
+ + "[[FakeSocketAddress-fake-address-3]/{}], "
+ + "[[FakeSocketAddress-fake-address-4]/{}]], drop=[null, null, null]",
+ "INFO: [grpclb-<api.google.com>] "
+ + "Update balancing state to CONNECTING: picks=[BUFFER_ENTRY], "
+ + "drops=[null, null, null]")
+ .inOrder();
+
+ // Shift to use updated backends
+ fallbackTestVerifyUseOfFallbackBackendLists(inOrder, backendList);
+ logs.clear();
+
  ///////////////////////////////////////////////////////////////////////////////////////
  // Name resolver sends new resolution results without any backend addr or balancer addr
  ///////////////////////////////////////////////////////////////////////////////////////