util: Remove deactivation and GracefulSwitchLb from MultiChildLb

It is easy to manage these things outside of MultiChildLb and it makes
the shared code easier and use less memory. In particular, we don't want
to use many instances of GracefulSwitchLb in virtually every policy
simply because it was needed in one or two cases.

util/src/main/java/io/grpc/util/MultiChildLoadBalancer.java

@@ -234,36 +234,21 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
       return new AcceptResolvedAddrRetVal(unavailableStatus, null);
     }
 
-    Collection<ChildLbState> reusedChildren = addMissingChildrenAndIdReuse(newChildren);
-
-    // Raactivate deactivated children
-    for (ChildLbState reusedChild : reusedChildren) {
-      reusedChild.reactivate(reusedChild.getPolicyFactory());
-    }
+    addMissingChildren(newChildren);
 
     updateChildrenWithResolvedAddresses(resolvedAddresses, newChildren);
 
     return new AcceptResolvedAddrRetVal(Status.OK, getRemovedChildren(newChildren.keySet()));
   }
 
-  protected final Collection<ChildLbState> addMissingChildrenAndIdReuse(
-      Map<Object, ChildLbState> newChildren) {
-    Collection<ChildLbState> reusedChildren = new ArrayList<>();
-
+  protected final void addMissingChildren(Map<Object, ChildLbState> newChildren) {
     // Do adds and identify reused children
     for (Map.Entry<Object, ChildLbState> entry : newChildren.entrySet()) {
       final Object key = entry.getKey();
       if (!childLbStates.containsKey(key)) {
         childLbStates.put(key, entry.getValue());
-      } else {
-        // Reuse the existing one
-        ChildLbState existingChildLbState = childLbStates.get(key);
-        if (existingChildLbState.isDeactivated()) {
-          reusedChildren.add(existingChildLbState);
-        }
       }
     }
-    return reusedChildren;
   }
 
   protected final void updateChildrenWithResolvedAddresses(ResolvedAddresses resolvedAddresses,
@@ -274,9 +259,7 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
       ResolvedAddresses childAddresses =
           getChildAddresses(entry.getKey(), resolvedAddresses, childConfig);
       childLbState.setResolvedAddresses(childAddresses); // update child
-      if (!childLbState.deactivated) {
-        childLbState.lb.handleResolvedAddresses(childAddresses); // update child LB
-      }
+      childLbState.lb.handleResolvedAddresses(childAddresses); // update child LB
     }
   }
 
@@ -288,8 +271,7 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
     // Do removals
     for (Object key : ImmutableList.copyOf(childLbStates.keySet())) {
       if (!newChildKeys.contains(key)) {
-        ChildLbState childLbState = childLbStates.get(key);
-        childLbState.deactivate();
+        ChildLbState childLbState = childLbStates.remove(key);
         removedChildren.add(childLbState);
       }
     }
@@ -326,10 +308,6 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
     return helper;
   }
 
-  protected final void removeChild(Object key) {
-    childLbStates.remove(key);
-  }
-
   @VisibleForTesting
   public final ImmutableMap<Object, ChildLbState> getImmutableChildMap() {
     return ImmutableMap.copyOf(childLbStates);
@@ -357,12 +335,12 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
   }
 
   /**
-   * Filters out non-ready and deactivated child load balancers (subchannels).
+   * Filters out non-ready child load balancers (subchannels).
    */
   protected final List<ChildLbState> getReadyChildren() {
     List<ChildLbState> activeChildren = new ArrayList<>();
     for (ChildLbState child : getChildLbStates()) {
-      if (!child.isDeactivated() && child.getCurrentState() == READY) {
+      if (child.getCurrentState() == READY) {
         activeChildren.add(child);
       }
     }
@@ -372,9 +350,7 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
   /**
    * This represents the state of load balancer children.  Each endpoint (represented by an
    * EquivalentAddressGroup or EDS string) will have a separate ChildLbState which in turn will
-   * define a GracefulSwitchLoadBalancer.  When the GracefulSwitchLoadBalancer is activated, a
-   * single PickFirstLoadBalancer will be created which will then create a subchannel and start
-   * trying to connect to it.
+   * have a single child LoadBalancer created from the provided factory.
    *
    * <p>A ChildLbStateHelper is the glue between ChildLbState and the helpers associated with the
    * petiole policy above and the PickFirstLoadBalancer's helper below.
@@ -387,65 +363,23 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
     private ResolvedAddresses resolvedAddresses;
     private final Object config;
 
-    private final GracefulSwitchLoadBalancer lb;
-    private final LoadBalancer.Factory policyFactory;
+    private final LoadBalancer lb;
     private ConnectivityState currentState;
     private SubchannelPicker currentPicker;
-    private boolean deactivated;
 
     public ChildLbState(Object key, LoadBalancer.Factory policyFactory, Object childConfig,
-        SubchannelPicker initialPicker) {
-      this(key, policyFactory, childConfig, initialPicker, null, false);
-    }
-
-    public ChildLbState(Object key, LoadBalancer.Factory policyFactory, Object childConfig,
-          SubchannelPicker initialPicker, ResolvedAddresses resolvedAddrs, boolean deactivated) {
+          SubchannelPicker initialPicker) {
       this.key = key;
-      this.policyFactory = policyFactory;
-      this.deactivated = deactivated;
       this.currentPicker = initialPicker;
       this.config = childConfig;
-      this.lb = new GracefulSwitchLoadBalancer(createChildHelper());
-      this.currentState = deactivated ? IDLE : CONNECTING;
-      this.resolvedAddresses = resolvedAddrs;
-      if (!deactivated) {
-        lb.switchTo(policyFactory);
-      }
+      this.lb = policyFactory.newLoadBalancer(createChildHelper());
+      this.currentState = CONNECTING;
     }
 
     protected ChildLbStateHelper createChildHelper() {
       return new ChildLbStateHelper();
     }
 
-    /**
-     * The default implementation. This not only marks the lb policy as not active, it also removes
-     * this child from the map of children maintained by the petiole policy.
-     *
-     * <p>Note that this does not explicitly shutdown this child.  That will generally be done by
-     * acceptResolvedAddresses on the LB, but can also be handled by an override such as is done
-     * in <a href=" https://github.com/grpc/grpc-java/blob/master/xds/src/main/java/io/grpc/xds/ClusterManagerLoadBalancer.java">ClusterManagerLoadBalancer</a>.
-     *
-     * <p>If you plan to reactivate, you will probably want to override this to not call
-     * childLbStates.remove() and handle that cleanup another way.
-     */
-    protected void deactivate() {
-      if (deactivated) {
-        return;
-      }
-
-      childLbStates.remove(key); // This means it can't be reactivated again
-      deactivated = true;
-      logger.log(Level.FINE, "Child balancer {0} deactivated", key);
-    }
-
-    /**
-     * This base implementation does nothing but reset the flag.  If you really want to both
-     * deactivate and reactivate you should override them both.
-     */
-    protected void reactivate(LoadBalancer.Factory policyFactory) {
-      deactivated = false;
-    }
-
     /**
      * Override for unique behavior such as delayed shutdowns of subchannels.
      */
@@ -460,8 +394,7 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
       return "Address = " + key
           + ", state = " + currentState
           + ", picker type: " + currentPicker.getClass()
-          + ", lb: " + lb.delegate().getClass()
-          + (deactivated ? ", deactivated" : "");
+          + ", lb: " + lb;
     }
 
     public final Object getKey() {
@@ -469,7 +402,7 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
     }
 
     @VisibleForTesting
-    public final GracefulSwitchLoadBalancer getLb() {
+    public final LoadBalancer getLb() {
       return lb;
     }
 
@@ -478,10 +411,6 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
       return currentPicker;
     }
 
-    protected final LoadBalancer.Factory getPolicyFactory() {
-      return policyFactory;
-    }
-
     protected final Subchannel getSubchannels(PickSubchannelArgs args) {
       if (getCurrentPicker() == null) {
         return null;
@@ -508,18 +437,6 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
       return resolvedAddresses.getAddresses().get(0);
     }
 
-    public final boolean isDeactivated() {
-      return deactivated;
-    }
-
-    protected final void setDeactivated() {
-      deactivated = true;
-    }
-
-    protected final void markReactivated() {
-      deactivated = false;
-    }
-
     protected final void setResolvedAddresses(ResolvedAddresses newAddresses) {
       checkNotNull(newAddresses, "Missing address list for child");
       resolvedAddresses = newAddresses;
@@ -552,17 +469,15 @@ public abstract class MultiChildLoadBalancer extends LoadBalancer {
       @Override
       public void updateBalancingState(final ConnectivityState newState,
           final SubchannelPicker newPicker) {
-        // If we are already in the process of resolving addresses, the overall balancing state
-        // will be updated at the end of it, and we don't need to trigger that update here.
         if (!childLbStates.containsKey(key)) {
           return;
         }
 
-        // Subchannel picker and state are saved, but will only be propagated to the channel
-        // when the child instance exits deactivated state.
         currentState = newState;
         currentPicker = newPicker;
-        if (!deactivated && !resolvingAddresses) {
+        // If we are already in the process of resolving addresses, the overall balancing state
+        // will be updated at the end of it, and we don't need to trigger that update here.
+        if (!resolvingAddresses) {
           if (newState == IDLE) {
             lb.requestConnection();
           }

util/src/test/java/io/grpc/util/MultiChildLoadBalancerTest.java

@@ -358,9 +358,6 @@ public class MultiChildLoadBalancerTest {
       ConnectivityState overallState = null;
       final Map<Object, SubchannelPicker> childPickers = new HashMap<>();
       for (ChildLbState childLbState : getChildLbStates()) {
-        if (childLbState.isDeactivated()) {
-          continue;
-        }
         childPickers.put(childLbState.getKey(), childLbState.getCurrentPicker());
         overallState = aggregateState(overallState, childLbState.getCurrentState());
       }

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

@@ -99,8 +99,7 @@ final class RingHashLoadBalancer extends MultiChildLoadBalancer {
         return addressValidityStatus;
       }
 
-      // We don't care about reuse because we don't want to activate them
-      addMissingChildrenAndIdReuse(newChildren);
+      addMissingChildren(newChildren);
       updateChildrenWithResolvedAddresses(resolvedAddresses, newChildren);
 
       // Now do the ringhash specific logic with weights and building the ring