Fail fast implementation for joinList (#68)

src/main/java/com/spotify/futures/CompletableFutures.java

@@ -53,7 +53,7 @@ private CompletableFutures() {
    * Returns a new {@link CompletableFuture} which completes to a list of all values of its input
    * stages, if all succeed.  The list of results is in the same order as the input stages.
    *
-   * <p> If any of the given stages complete exceptionally, then the returned future also does so,
+   * <p> As soon as any of the given stages complete exceptionally, then the returned future also does so,
    * with a {@link CompletionException} holding this exception as its cause.
    *
    * <p> If no stages are provided, returns a future holding an empty list.
@@ -75,7 +75,19 @@ public static <T> CompletableFuture<List<T>> allAsList(
     for (int i = 0; i < stages.size(); i++) {
       all[i] = stages.get(i).toCompletableFuture();
     }
-    return CompletableFuture.allOf(all)
+
+    CompletableFuture<Void> allOf = CompletableFuture.allOf(all);
+
+    for (int i = 0; i < all.length; i++) {
+      all[i].exceptionally(throwable -> {
+        if (!allOf.isDone()) {
+          allOf.completeExceptionally(throwable);
+        }
+        return null; // intentionally unused
+      });
+    }
+
+    return allOf
         .thenApply(ignored -> {
           final List<T> result = new ArrayList<>(all.length);
           for (int i = 0; i < all.length; i++) {

src/test/java/com/spotify/futures/CompletableFuturesTest.java

@@ -37,6 +37,8 @@
 import java.util.concurrent.ScheduledFuture;
 import java.util.function.Supplier;
 import java.util.stream.Stream;
+import java.util.concurrent.CompletionException;
+import java.util.concurrent.TimeoutException;
 
 import static com.spotify.futures.CompletableFutures.allAsList;
 import static com.spotify.futures.CompletableFutures.allAsMap;
@@ -64,13 +66,15 @@
 import static org.hamcrest.Matchers.contains;
 import static org.hamcrest.Matchers.hasProperty;
 import static org.hamcrest.Matchers.hasSize;
+import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.nullValue;
 import static org.hamcrest.core.Is.is;
 import static org.hamcrest.core.Is.isA;
 import static org.hamcrest.core.IsNot.not;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThat;
+import static org.junit.Assert.fail;
 import static org.mockito.Matchers.any;
 import static org.mockito.Matchers.anyLong;
 import static org.mockito.Matchers.eq;
@@ -125,6 +129,23 @@ public void allAsList_exceptional() throws Exception {
     allAsList(input).get();
   }
 
+  @Test
+  public void allAsList_exceptional_failFast() {
+    final CompletableFuture<String> incomplete = incompleteFuture();
+    final CompletableFuture<String> failed =
+        exceptionallyCompletedFuture(new TimeoutException());
+    final List<CompletionStage<String>> input =
+        asList(incomplete, failed);
+
+    try {
+      allAsList(input).join();
+      fail("Expected exception being thrown.");
+    } catch (Exception e) {
+      assertThat(e, instanceOf(CompletionException.class));
+      assertThat(e.getCause(), instanceOf(TimeoutException.class));
+    }
+  }
+
   @Test
   public void allAsList_null() throws Exception {
     exception.expect(NullPointerException.class);