xds: Avoid PriorityLb re-enabling timer on duplicate CONNECTING (#12289)

Since c4256ad we no longer fabricate a TRANSIENT_FAILURE update from
children. However, previously that would have set
seenReadyOrIdleSinceTransientFailure = false and prevented future timer
creation. If a LB policy gives extraneous updates with state CONNECTING,
then it was possible to re-create failOverTimer which would then wait
the 10 seconds for the child to finish CONNECTING. We only want to give
the child one opportunity after transitioning out of READY/IDLE.

grpc/proposal#509

Author: ejona86

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

@@ -320,13 +320,14 @@ public void updateBalancingState(final ConnectivityState newState,
         if (!children.containsKey(priority)) {
           return;
         }
+        ConnectivityState oldState = connectivityState;
         connectivityState = newState;
         picker = newPicker;
 
         if (deletionTimer != null && deletionTimer.isPending()) {
           return;
         }
-        if (newState.equals(CONNECTING)) {
+        if (newState.equals(CONNECTING) && !oldState.equals(newState)) {
           if (!failOverTimer.isPending() && seenReadyOrIdleSinceTransientFailure) {
             failOverTimer = syncContext.schedule(new FailOverTask(), 10, TimeUnit.SECONDS,
                 executor);

xds/src/test/java/io/grpc/xds/PriorityLoadBalancerTest.java

@@ -28,6 +28,7 @@
 import static org.mockito.Mockito.atLeastOnce;
 import static org.mockito.Mockito.clearInvocations;
 import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.inOrder;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.times;
@@ -70,6 +71,7 @@
 import org.junit.runners.JUnit4;
 import org.mockito.ArgumentCaptor;
 import org.mockito.Captor;
+import org.mockito.InOrder;
 import org.mockito.Mock;
 import org.mockito.junit.MockitoJUnit;
 import org.mockito.junit.MockitoRule;
@@ -554,6 +556,55 @@ public void connectingResetFailOverIfSeenReadyOrIdleSinceTransientFailure() {
     assertThat(fooHelpers).hasSize(2);
   }
 
+  @Test
+  public void failoverTimerNotRestartedOnDupConnecting() {
+    InOrder inOrder = inOrder(helper);
+    PriorityChildConfig priorityChildConfig0 =
+        new PriorityChildConfig(newChildConfig(fooLbProvider, new Object()), true);
+    PriorityChildConfig priorityChildConfig1 =
+        new PriorityChildConfig(newChildConfig(fooLbProvider, new Object()), true);
+    PriorityLbConfig priorityLbConfig =
+        new PriorityLbConfig(
+            ImmutableMap.of("p0", priorityChildConfig0, "p1", priorityChildConfig1),
+            ImmutableList.of("p0", "p1"));
+    priorityLb.acceptResolvedAddresses(
+        ResolvedAddresses.newBuilder()
+            .setAddresses(ImmutableList.<EquivalentAddressGroup>of())
+            .setLoadBalancingPolicyConfig(priorityLbConfig)
+            .build());
+    // Nothing important about this verify, other than to provide a baseline
+    inOrder.verify(helper)
+        .updateBalancingState(eq(CONNECTING), pickerReturns(PickResult.withNoResult()));
+    assertThat(fooBalancers).hasSize(1);
+    assertThat(fooHelpers).hasSize(1);
+    Helper helper0 = Iterables.getOnlyElement(fooHelpers);
+
+    // Cause seenReadyOrIdleSinceTransientFailure = true
+    helper0.updateBalancingState(IDLE, EMPTY_PICKER);
+    inOrder.verify(helper)
+        .updateBalancingState(eq(IDLE), pickerReturns(PickResult.withNoResult()));
+    helper0.updateBalancingState(CONNECTING, EMPTY_PICKER);
+
+    // p0 keeps repeating CONNECTING, failover happens
+    fakeClock.forwardTime(5, TimeUnit.SECONDS);
+    helper0.updateBalancingState(CONNECTING, EMPTY_PICKER);
+    fakeClock.forwardTime(5, TimeUnit.SECONDS);
+    assertThat(fooBalancers).hasSize(2);
+    assertThat(fooHelpers).hasSize(2);
+    inOrder.verify(helper, times(2))
+        .updateBalancingState(eq(CONNECTING), pickerReturns(PickResult.withNoResult()));
+    Helper helper1 = Iterables.getLast(fooHelpers);
+
+    // p0 keeps repeating CONNECTING, no reset of failover timer
+    helper1.updateBalancingState(IDLE, EMPTY_PICKER); // Stop timer for p1
+    inOrder.verify(helper)
+        .updateBalancingState(eq(IDLE), pickerReturns(PickResult.withNoResult()));
+    helper0.updateBalancingState(CONNECTING, EMPTY_PICKER);
+    fakeClock.forwardTime(10, TimeUnit.SECONDS);
+    inOrder.verify(helper, never())
+        .updateBalancingState(eq(CONNECTING), any());
+  }
+
   @Test
   public void readyToConnectDoesNotFailOverButUpdatesPicker() {
     PriorityChildConfig priorityChildConfig0 =