Promise.allOf should not block on empty input (#508)

Author: rlei

src/main/java/com/uber/cadence/internal/sync/AllOfFuture.java

@@ -23,6 +23,7 @@
 import com.uber.cadence.workflow.Workflow;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
@@ -34,25 +35,22 @@ class AllOfPromise<G> implements Promise<List<G>> {
 
   private int notReadyCount;
 
-  @SuppressWarnings("unchecked")
   AllOfPromise(Promise<G>[] promises) {
-    // Using array to initialize it to the desired size with nulls.
-    result = (G[]) new Object[promises.length];
-    int index = 0;
-    for (Promise<G> f : promises) {
-      addPromise(index, f);
-      index++;
-    }
+    this(Arrays.asList(promises));
   }
 
   @SuppressWarnings("unchecked")
   public AllOfPromise(Collection<Promise<G>> promises) {
-    // Using array to initialize it to the desired size with nulls.
-    result = (G[]) new Object[promises.size()];
-    int index = 0;
-    for (Promise<G> f : promises) {
-      addPromise(index, f);
-      index++;
+    if (promises.isEmpty()) {
+      impl.complete(Collections.emptyList());
+    } else {
+      // Using array to initialize it to the desired size with nulls.
+      result = (G[]) new Object[promises.size()];
+      int index = 0;
+      for (Promise<G> f : promises) {
+        addPromise(index, f);
+        index++;
+      }
     }
   }
 

src/test/java/com/uber/cadence/internal/sync/PromiseTest.java

@@ -17,6 +17,7 @@
 
 package com.uber.cadence.internal.sync;
 
+import static java.util.Collections.emptyList;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
@@ -438,6 +439,36 @@ public void testAllOf() throws Throwable {
     trace.setExpected(expected);
   }
 
+  @Test
+  public void testAllOf_shouldCompleteWithoutBlocking() throws Throwable {
+    DeterministicRunner r =
+        DeterministicRunner.newRunner(
+            () -> {
+              Promise<List<String>> allOf = Promise.allOf(emptyList());
+
+              assertTrue(allOf.isCompleted());
+
+              Promise<Boolean> thenApply =
+                  allOf.thenApply(result -> result.isEmpty() ? Boolean.TRUE : Boolean.FALSE);
+              assertTrue(thenApply.isCompleted());
+              assertTrue(thenApply.get());
+
+              assertTrue(allOf.get().isEmpty());
+              trace.add("done1");
+
+              @SuppressWarnings({"unchecked", "rawtypes"})
+              AllOfPromise<String> allOfEmptyArray = new AllOfPromise<>(new Promise[0]);
+
+              assertTrue(allOfEmptyArray.isCompleted());
+              assertTrue(allOfEmptyArray.get().isEmpty());
+              trace.add("done2");
+            });
+
+    r.runUntilAllBlocked();
+    trace.setExpected("done1", "done2");
+    trace.assertExpected();
+  }
+
   @Test
   public void testAnyOf() throws Throwable {
     DeterministicRunner r =