Fix exclusiveJoin() bug when both branches complete simultaneously.

c++/src/kj/async-test.c++

@@ -809,5 +809,22 @@ TEST(Async, Poll) {
   paf.promise.wait(waitScope);
 }
 
+KJ_TEST("exclusiveJoin both events complete simultaneously") {
+  // Previously, if both branches of an exclusiveJoin() completed simultaneously, then the parent
+  // event could be armed twice. This is an error, but the exact results of this error depend on
+  // the parent PromiseNode type. One case where it matters is ArrayJoinPromiseNode, which counts
+  // events and decides it is done when it has received exactly the number of events expected.
+
+  EventLoop loop;
+  WaitScope waitScope(loop);
+
+  auto builder = kj::heapArrayBuilder<kj::Promise<uint>>(2);
+  builder.add(kj::Promise<uint>(123).exclusiveJoin(kj::Promise<uint>(456)));
+  builder.add(kj::NEVER_DONE);
+  auto joined = kj::joinPromises(builder.finish());
+
+  KJ_EXPECT(!joined.poll(waitScope));
+}
+
 }  // namespace
 }  // namespace kj

c++/src/kj/async.c++

@@ -945,14 +945,19 @@ bool ExclusiveJoinPromiseNode::Branch::get(ExceptionOrValue& output) {
 }
 
 Maybe<Own<Event>> ExclusiveJoinPromiseNode::Branch::fire() {
-  // Cancel the branch that didn't return first.  Ignore exceptions caused by cancellation.
-  if (this == &joinNode.left) {
-    kj::runCatchingExceptions([&]() { joinNode.right.dependency = nullptr; });
+  if (dependency) {
+    // Cancel the branch that didn't return first.  Ignore exceptions caused by cancellation.
+    if (this == &joinNode.left) {
+      kj::runCatchingExceptions([&]() { joinNode.right.dependency = nullptr; });
+    } else {
+      kj::runCatchingExceptions([&]() { joinNode.left.dependency = nullptr; });
+    }
+
+    joinNode.onReadyEvent.arm();
   } else {
-    kj::runCatchingExceptions([&]() { joinNode.left.dependency = nullptr; });
+    // The other branch already fired, and this branch was canceled. It's possible for both
+    // branches to fire if both were armed simultaneously.
   }
-
-  joinNode.onReadyEvent.arm();
   return nullptr;
 }