xds: Wrap around the ring in RingHashLoadBalancer.getTargetIndex

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

@@ -396,6 +396,12 @@ private int getTargetIndex(long requestHash) {
         return 0;
       }
 
+      // A hash greater than the largest ring entry wraps clockwise back to the first entry;
+      // otherwise the binary search below stops at the last entry instead of wrapping around.
+      if (requestHash > ring.get(ring.size() - 1).hash) {
+        return 0;
+      }
+
       int low = 0;
       int high = ring.size() - 1;
       int mid = (low + high) / 2;

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

@@ -478,6 +478,32 @@ public void deterministicPickWithRequestHashHeader_multipleHeaderValues() {
     assertThat(result.getSubchannel().getAddresses()).isEqualTo(servers.get(1));
   }
 
+  @Test
+  public void pickWithHashAboveMaxRingEntry_wrapsAroundToFirstEntry() {
+    // Map each server address to exactly one ring entry, so the first and last ring entries
+    // belong to different hosts.
+    RingHashConfig config = new RingHashConfig(3, 3, "");
+    List<EquivalentAddressGroup> servers = createWeightedServerAddrs(1, 1, 1);
+    initializeLbSubchannels(config, servers);
+    InOrder inOrder = Mockito.inOrder(helper);
+
+    // Bring all subchannels to READY so that any pick resolves to its ring entry's host.
+    for (Subchannel subchannel : subchannels.values()) {
+      deliverSubchannelState(subchannel, CSI_READY);
+      inOrder.verify(helper).updateBalancingState(eq(READY), pickerCaptor.capture());
+    }
+    SubchannelPicker picker = pickerCaptor.getValue();
+
+    // Long.MIN_VALUE sorts below every ring entry, so it hits the first entry on the ring.
+    Subchannel firstEntryHost =
+        picker.pickSubchannel(getDefaultPickSubchannelArgs(Long.MIN_VALUE)).getSubchannel();
+    // Long.MAX_VALUE sorts above every ring entry; on a ring it must wrap clockwise back to the
+    // first entry rather than sticking to the last one.
+    Subchannel wrappedHost =
+        picker.pickSubchannel(getDefaultPickSubchannelArgs(Long.MAX_VALUE)).getSubchannel();
+    assertThat(wrappedHost).isSameInstanceAs(firstEntryHost);
+  }
+
   @Test
   public void pickWithRandomHash_allSubchannelsReady() {
     loadBalancer = new RingHashLoadBalancer(helper, new FakeRandom());