Create BlankFutureProvider which is reused among LoadBalancers.

If a LoadBalancer is requested for a transport future before it can get
one from TransportManager, e.g., before name resolution is completed,
LoadBalancer will return a blank future created by BlankFutureProvider
and to be linked with real futures later.

core/src/main/java/io/grpc/SimpleLoadBalancerFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014, Google Inc. All rights reserved.
+ * Copyright 2015, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -31,12 +31,11 @@
 
 package io.grpc;
 
-import com.google.common.base.Preconditions;
-import com.google.common.util.concurrent.FutureCallback;
+import com.google.common.base.Supplier;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.ListenableFuture;
-import com.google.common.util.concurrent.SettableFuture;
 
+import io.grpc.internal.BlankFutureProvider;
 import io.grpc.internal.ClientTransport;
 
 import java.net.SocketAddress;
@@ -51,6 +50,7 @@ import javax.annotation.concurrent.GuardedBy;
  * addresses from the {@link NameResolver}.
  */
 // TODO(zhangkun83): Only pick-first is implemented. We need to implement round-robin.
+@ExperimentalApi
 public final class SimpleLoadBalancerFactory extends LoadBalancer.Factory {
 
   private static final SimpleLoadBalancerFactory instance = new SimpleLoadBalancerFactory();
@@ -72,10 +72,9 @@ public final class SimpleLoadBalancerFactory extends LoadBalancer.Factory {
     private final List<ResolvedServerInfo> servers = new ArrayList<ResolvedServerInfo>();
     @GuardedBy("servers")
     private int currentServerIndex;
-    // TODO(zhangkun83): virtually any LoadBalancer would need to handle picks before name
-    // resolution is done, we may want to move the related logic into ManagedChannelImpl.
     @GuardedBy("servers")
-    private List<SettableFuture<ClientTransport>> pendingPicks;
+    private final BlankFutureProvider<ClientTransport> pendingPicks =
+        new BlankFutureProvider<ClientTransport>();
     @GuardedBy("servers")
     private StatusException nameResolutionError;
 
@@ -93,12 +92,7 @@ public final class SimpleLoadBalancerFactory extends LoadBalancer.Factory {
           if (nameResolutionError != null) {
             return Futures.immediateFailedFuture(nameResolutionError);
           }
-          SettableFuture<ClientTransport> future = SettableFuture.create();
-          if (pendingPicks == null) {
-            pendingPicks = new ArrayList<SettableFuture<ClientTransport>>();
-          }
-          pendingPicks.add(future);
-          return future;
+          return pendingPicks.newBlankFuture();
         }
         currentServer = servers.get(currentServerIndex);
       }
@@ -108,56 +102,40 @@ public final class SimpleLoadBalancerFactory extends LoadBalancer.Factory {
     @Override
     public void handleResolvedAddresses(
         List<ResolvedServerInfo> updatedServers, Attributes config) {
-      List<SettableFuture<ClientTransport>> pendingPicksCopy = null;
-      ResolvedServerInfo currentServer = null;
+      BlankFutureProvider.FulfillmentBatch<ClientTransport> pendingPicksFulfillmentBatch;
+      final ResolvedServerInfo currentServer;
       synchronized (servers) {
         nameResolutionError = null;
         servers.clear();
         for (ResolvedServerInfo addr : updatedServers) {
           servers.add(addr);
         }
-        if (!servers.isEmpty()) {
-          pendingPicksCopy = pendingPicks;
-          pendingPicks = null;
-          if (currentServerIndex >= servers.size()) {
-            currentServerIndex = 0;
-          }
-          currentServer = servers.get(currentServerIndex);
+        if (servers.isEmpty()) {
+          return;
         }
-      }
-      if (pendingPicksCopy != null) {
-        // If pendingPicksCopy != null, then servers.isEmpty() == false, then
-        // currentServer must have been assigned.
-        Preconditions.checkState(currentServer != null, "currentServer is null");
-        for (final SettableFuture<ClientTransport> pendingPick : pendingPicksCopy) {
-          ListenableFuture<ClientTransport> future = tm.getTransport(currentServer.getAddress());
-          Futures.addCallback(future, new FutureCallback<ClientTransport>() {
-            @Override public void onSuccess(ClientTransport result) {
-              pendingPick.set(result);
-            }
-
-            @Override public void onFailure(Throwable t) {
-              pendingPick.setException(t);
-            }
-          });
+        pendingPicksFulfillmentBatch = pendingPicks.createFulfillmentBatch();
+        if (currentServerIndex >= servers.size()) {
+          currentServerIndex = 0;
         }
+        currentServer = servers.get(currentServerIndex);
       }
+      pendingPicksFulfillmentBatch.link(new Supplier<ListenableFuture<ClientTransport>>() {
+        @Override public ListenableFuture<ClientTransport> get() {
+          return tm.getTransport(currentServer.getAddress());
+        }
+      });
     }
 
     @Override
     public void handleNameResolutionError(Status error) {
-      List<SettableFuture<ClientTransport>> pendingPicksCopy = null;
-      StatusException statusException = error.asException();
+      BlankFutureProvider.FulfillmentBatch<ClientTransport> pendingPicksFulfillmentBatch;
+      StatusException statusException =
+          error.augmentDescription("Name resolution failed").asException();
       synchronized (servers) {
-        pendingPicksCopy = pendingPicks;
-        pendingPicks = null;
+        pendingPicksFulfillmentBatch = pendingPicks.createFulfillmentBatch();
         nameResolutionError = statusException;
       }
-      if (pendingPicksCopy != null) {
-        for (SettableFuture<ClientTransport> pendingPick : pendingPicksCopy) {
-          pendingPick.setException(statusException);
-        }
-      }
+      pendingPicksFulfillmentBatch.fail(statusException);
     }
 
     @Override

core/src/main/java/io/grpc/internal/BlankFutureProvider.java

@@ -0,0 +1,125 @@
+/*
+ * Copyright 2015, Google Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *    * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *    * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *
+ *    * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package io.grpc.internal;
+
+import com.google.common.base.Preconditions;
+import com.google.common.base.Supplier;
+import com.google.common.util.concurrent.FutureCallback;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.common.util.concurrent.SettableFuture;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import javax.annotation.concurrent.NotThreadSafe;
+
+/**
+ * Issues blank {@link ListenableFuture}s when requested, and later fulfills them, either by linking
+ * them with upstream {@code ListenableFuture}s that will eventually be completed, or by failing
+ * them immediately.
+ *
+ * <p>This is useful for {@code ListenableFuture} providers that may be requested at a moment when
+ * the necessary information for providing a {@code ListenableFuture} is not available, but will be
+ * later.
+ */
+@NotThreadSafe
+public final class BlankFutureProvider<T> {
+  private List<SettableFuture<T>> blankFutures = new ArrayList<SettableFuture<T>>();
+
+  /**
+   * Creates a blank future and track it.
+   */
+  public ListenableFuture<T> newBlankFuture() {
+    SettableFuture<T> future = SettableFuture.create();
+    blankFutures.add(future);
+    return future;
+  }
+
+  /**
+   * Creates a {@link FulfillmentBatch} that will be used to fulfill the currently tracked blank
+   * futures.
+   *
+   * <p>After this method has returned, the {@link BlankFutureProvider} will no longer track the
+   * previous blank futures, and can be used to create and track new blank futures.
+   */
+  public FulfillmentBatch<T> createFulfillmentBatch() {
+    List<SettableFuture<T>> blankFuturesCopy = blankFutures;
+    blankFutures = new ArrayList<SettableFuture<T>>();
+    return new FulfillmentBatch<T>(blankFuturesCopy);
+  }
+
+  /**
+   * A batch of blank futures that are going to be fulfilled, by either linking them with other
+   * futures, or failing them.
+   *
+   * <p>This object is independent from the {@link BlankFutureProvider} that created it. They don't
+   * need synchronization between them.
+   */
+  public static class FulfillmentBatch<T> {
+    private final List<SettableFuture<T>> futures;
+
+    private FulfillmentBatch(List<SettableFuture<T>> futures) {
+      this.futures = Preconditions.checkNotNull(futures, "futures");
+    }
+
+    /**
+     * Links the blank futures with futures that will be eventually completed.
+     *
+     * <p>For each blank future, this method calls {@link Supplier#get()} on {@code source} and link
+     * the returned future to the blank future.
+     */
+    public void link(Supplier<ListenableFuture<T>> source) {
+      for (final SettableFuture<T> future : futures) {
+        ListenableFuture<T> sourceFuture = source.get();
+        Futures.addCallback(sourceFuture, new FutureCallback<T>() {
+          @Override public void onSuccess(T result) {
+            future.set(result);
+          }
+
+          @Override public void onFailure(Throwable t) {
+            future.setException(t);
+          }
+        });
+      }
+    }
+
+    /**
+     * Fails all futures with the given error.
+     */
+    public void fail(Throwable error) {
+      for (SettableFuture<T> future : futures) {
+        future.setException(error);
+      }
+    }
+  }
+}

core/src/test/java/io/grpc/internal/ManagedChannelImplTest.java

@@ -311,7 +311,8 @@ public class ManagedChannelImplTest {
     ArgumentCaptor<Status> statusCaptor = ArgumentCaptor.forClass(Status.class);
     verify(mockCallListener, timeout(1000)).onClose(statusCaptor.capture(), any(Metadata.class));
     Status status = statusCaptor.getValue();
-    assertSame(error, status);
+    assertSame(error.getCode(), status.getCode());
+    assertSame(error.getCause(), status.getCause());
   }
 
   @Test