From ba54513f5ca9f6279b1066b320cf6ec57b271c85 Mon Sep 17 00:00:00 2001
From: Kun Zhang <zhangkun@google.com>
Date: Thu, 28 Jan 2016 11:01:22 -0800
Subject: [PATCH] Fix a concurrent modification in BlankFutureProvider.

The Set from Collections.synchronizedSet() is not protected against
concurrent modification during iteration. We copy an ArrayList out of it
for iteration.
---
 .../java/io/grpc/internal/BlankFutureProvider.java   | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/core/src/main/java/io/grpc/internal/BlankFutureProvider.java b/core/src/main/java/io/grpc/internal/BlankFutureProvider.java
index 704f4d5d3b..bbb8aed20d 100644
--- a/core/src/main/java/io/grpc/internal/BlankFutureProvider.java
+++ b/core/src/main/java/io/grpc/internal/BlankFutureProvider.java
@@ -39,8 +39,10 @@ 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.Collections;
 import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Set;
 import java.util.concurrent.CancellationException;
 
@@ -122,7 +124,7 @@ public final class BlankFutureProvider<T> {
      * they were created.
      */
     public void link(Supplier<ListenableFuture<T>> source) {
-      for (final SettableFuture<T> future : futures) {
+      for (final SettableFuture<T> future : copyFutureList()) {
         ListenableFuture<T> sourceFuture = source.get();
         Futures.addCallback(sourceFuture, new FutureCallback<T>() {
           @Override public void onSuccess(T result) {
@@ -140,10 +142,16 @@ public final class BlankFutureProvider<T> {
      * Fails all futures with the given error.
      */
     public void fail(Throwable error) {
-      for (SettableFuture<T> future : futures) {
+      for (SettableFuture<T> future : copyFutureList()) {
         future.setException(error);
       }
     }
+
+    private List<SettableFuture<T>> copyFutureList() {
+      synchronized (futures) {
+        return new ArrayList<SettableFuture<T>>(futures);
+      }
+    }
   }
 
   private static <T> Set<SettableFuture<T>> createSet() {