Bug 839838 - Implement DOMRequest.then; r=sicking,bzbarsky

This is implemented by creating a Promise object internally and
forwarding the .then() call to it. Any further callbacks passed to
future .then() calls will be added as callbacks on the same internal
promise object. We also take care of resolving or rejecting the promise
if the success/error event of the DOMRequest object has been fired
before .then() is called.

dom/base/DOMCursor.h

@@ -43,6 +43,11 @@ protected:
 
 private:
   DOMCursor() MOZ_DELETE;
+  // Calling Then() on DOMCursor is a mistake, since the DOMCursor object
+  // should not have a .then() method from JS' point of view.
+  already_AddRefed<mozilla::dom::Promise>
+  Then(JSContext* aCx, AnyCallback* aResolveCallback,
+       AnyCallback* aRejectCallback, ErrorResult& aRv) MOZ_DELETE;
 
   nsCOMPtr<nsICursorContinueCallback> mCallback;
   bool mFinished;

dom/base/DOMRequest.cpp

@@ -10,11 +10,14 @@
 #include "nsThreadUtils.h"
 #include "DOMCursor.h"
 #include "nsIDOMEvent.h"
+#include "mozilla/dom/Promise.h"
 
+using mozilla::dom::AnyCallback;
 using mozilla::dom::DOMError;
 using mozilla::dom::DOMRequest;
 using mozilla::dom::DOMRequestService;
 using mozilla::dom::DOMCursor;
+using mozilla::dom::Promise;
 using mozilla::AutoSafeJSContext;
 
 DOMRequest::DOMRequest(nsPIDOMWindow* aWindow)
@@ -25,16 +28,24 @@ DOMRequest::DOMRequest(nsPIDOMWindow* aWindow)
 {
 }
 
+DOMRequest::~DOMRequest()
+{
+  mResult.setUndefined();
+  mozilla::DropJSObjects(this);
+}
+
 NS_IMPL_CYCLE_COLLECTION_CLASS(DOMRequest)
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(DOMRequest,
                                                   DOMEventTargetHelper)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mError)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPromise)
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(DOMRequest,
                                                 DOMEventTargetHelper)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mError)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mPromise)
   tmp->mResult = JSVAL_VOID;
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
@@ -107,6 +118,10 @@ DOMRequest::FireSuccess(JS::Handle<JS::Value> aResult)
   mResult = aResult;
 
   FireEvent(NS_LITERAL_STRING("success"), false, false);
+
+  if (mPromise) {
+    mPromise->MaybeResolve(mResult);
+  }
 }
 
 void
@@ -120,6 +135,10 @@ DOMRequest::FireError(const nsAString& aError)
   mError = new DOMError(GetOwner(), aError);
 
   FireEvent(NS_LITERAL_STRING("error"), true, true);
+
+  if (mPromise) {
+    mPromise->MaybeRejectBrokenly(mError);
+  }
 }
 
 void
@@ -133,6 +152,10 @@ DOMRequest::FireError(nsresult aError)
   mError = new DOMError(GetOwner(), aError);
 
   FireEvent(NS_LITERAL_STRING("error"), true, true);
+
+  if (mPromise) {
+    mPromise->MaybeRejectBrokenly(mError);
+  }
 }
 
 void
@@ -147,6 +170,10 @@ DOMRequest::FireDetailedError(DOMError* aError)
   mError = aError;
 
   FireEvent(NS_LITERAL_STRING("error"), true, true);
+
+  if (mPromise) {
+    mPromise->MaybeRejectBrokenly(mError);
+  }
 }
 
 void
@@ -175,6 +202,31 @@ DOMRequest::RootResultVal()
   mozilla::HoldJSObjects(this);
 }
 
+already_AddRefed<Promise>
+DOMRequest::Then(JSContext* aCx, AnyCallback* aResolveCallback,
+                 AnyCallback* aRejectCallback, ErrorResult& aRv)
+{
+  if (!mPromise) {
+    mPromise = Promise::Create(DOMEventTargetHelper::GetParentObject(), aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+    if (mDone) {
+      // Since we create mPromise lazily, it's possible that the DOMRequest object
+      // has already fired its success/error event.  In that case we should
+      // manually resolve/reject mPromise here.  mPromise will take care of
+      // calling the callbacks on |promise| as needed.
+      if (mError) {
+        mPromise->MaybeRejectBrokenly(mError);
+      } else {
+        mPromise->MaybeResolve(mResult);
+      }
+    }
+  }
+
+  return mPromise->Then(aCx, aResolveCallback, aRejectCallback, aRv);
+}
+
 NS_IMPL_ISUPPORTS(DOMRequestService, nsIDOMRequestService)
 
 NS_IMETHODIMP
@@ -253,7 +305,7 @@ public:
            const JS::Value& aResult)
   {
     NS_ASSERTION(NS_IsMainThread(), "Wrong thread!");
-    AutoSafeJSContext cx;
+    ThreadsafeAutoSafeJSContext cx;
     nsRefPtr<FireSuccessAsyncTask> asyncTask = new FireSuccessAsyncTask(cx, aRequest, aResult);
     if (NS_FAILED(NS_DispatchToMainThread(asyncTask))) {
       NS_WARNING("Failed to dispatch to main thread!");

dom/base/DOMRequest.h

@@ -18,12 +18,16 @@
 namespace mozilla {
 namespace dom {
 
+class AnyCallback;
+class Promise;
+
 class DOMRequest : public DOMEventTargetHelper,
                    public nsIDOMDOMRequest
 {
 protected:
   JS::Heap<JS::Value> mResult;
   nsRefPtr<DOMError> mError;
+  nsRefPtr<Promise> mPromise;
   bool mDone;
 
 public:
@@ -67,6 +71,9 @@ public:
   IMPL_EVENT_HANDLER(success)
   IMPL_EVENT_HANDLER(error)
 
+  already_AddRefed<mozilla::dom::Promise>
+  Then(JSContext* aCx, AnyCallback* aResolveCallback,
+       AnyCallback* aRejectCallback, ErrorResult& aRv);
 
   void FireSuccess(JS::Handle<JS::Value> aResult);
   void FireError(const nsAString& aError);
@@ -76,11 +83,7 @@ public:
   explicit DOMRequest(nsPIDOMWindow* aWindow);
 
 protected:
-  virtual ~DOMRequest()
-  {
-    mResult = JSVAL_VOID;
-    mozilla::DropJSObjects(this);
-  }
+  virtual ~DOMRequest();
 
   void FireEvent(const nsAString& aType, bool aBubble, bool aCancelable);
 

dom/base/test/test_domcursor.html

@@ -51,6 +51,7 @@ var tests = [
     ok("readyState" in req, "cursor has readyState");
     ok("done" in req, "cursor has finished");
     ok("continue" in req, "cursor has continue");
+    ok(!("then" in req), "cursor should not have a then method");
 
     is(req.readyState, "pending", "readyState is pending");
     is(req.result, undefined, "result is undefined");

dom/base/test/test_domrequest.html

@@ -17,59 +17,212 @@
 var reqserv = SpecialPowers.getDOMRequestService();
 ok("createRequest" in reqserv, "appears to be a service");
 
-// create a request
-var req = reqserv.createRequest(window);
-ok("result" in req, "request has result");
-ok("error" in req, "request has error");
-ok("onsuccess" in req, "request has onsuccess");
-ok("onerror" in req, "request has onerror");
-ok("readyState" in req, "request has readyState");
+function testBasics() {
+  // create a request
+  var req = reqserv.createRequest(window);
+  ok("result" in req, "request has result");
+  ok("error" in req, "request has error");
+  ok("onsuccess" in req, "request has onsuccess");
+  ok("onerror" in req, "request has onerror");
+  ok("readyState" in req, "request has readyState");
+  ok("then" in req, "request has then");
 
-is(req.readyState, "pending", "readyState is pending");
-is(req.result, undefined, "result is undefined");
-is(req.onsuccess, null, "onsuccess is null");
-is(req.onerror, null, "onerror is null");
+  is(req.readyState, "pending", "readyState is pending");
+  is(req.result, undefined, "result is undefined");
+  is(req.onsuccess, null, "onsuccess is null");
+  is(req.onerror, null, "onerror is null");
 
-// fire success
-var ev = null;
-req.onsuccess = function(e) {
-  ev = e;
+  runTest();
 }
-reqserv.fireSuccess(req, "my result");
-ok(ev, "got success event");
-is(ev.type, "success", "correct type during success");
-is(ev.target, req, "correct target during success");
-is(req.readyState, "done", "correct readyState after success");
-is(req.error, null, "correct error after success");
-is(req.result, "my result", "correct result after success");
 
-// fire error
-req = reqserv.createRequest(window);
-ev = null;
-req.onerror = function(e) {
+function testSuccess() {
+  // fire success
+  var req = reqserv.createRequest(window);
+  var ev = null;
+  req.onsuccess = function(e) {
     ev = e;
+  }
+  var result = null;
+  var promise = req.then(function(r) {
+    is(r, "my result", "correct result when resolving the promise");
+    result = r;
+    runTest();
+  }, function(e) {
+    ok(false, "promise should not be rejected");
+    runTest();
+  });
+  ok(promise instanceof Promise, "then() should return a Promise");
+  reqserv.fireSuccess(req, "my result");
+  ok(ev, "got success event");
+  is(ev.type, "success", "correct type during success");
+  is(ev.target, req, "correct target during success");
+  is(req.readyState, "done", "correct readyState after success");
+  is(req.error, null, "correct error after success");
+  is(req.result, "my result", "correct result after success");
+  is(result, null, "Promise should not be resolved synchronously");
 }
-reqserv.fireError(req, "OhMyError");
-ok(ev, "got error event");
-is(ev.type, "error", "correct type during error");
-is(ev.target, req, "correct target during error");
-is(req.readyState, "done", "correct readyState after error");
-is(req.error.name, "OhMyError", "correct error after error");
-is(req.result, undefined, "correct result after error");
 
-// fire detailed error
-req = reqserv.createRequest(window);
-ev = null;
-req.onerror = function(e) {
+function testError() {
+  // fire error
+  var req = reqserv.createRequest(window);
+  var ev = null;
+  req.onerror = function(e) {
     ev = e;
-};
-reqserv.fireDetailedError(req, new DOMError("detailedError"));
-ok(ev, "got error event");
-is(ev.type, "error", "correct type during error");
-is(ev.target, req, "correct target during error");
-is(req.readyState, "done", "correct readyState after error");
-is(req.error.name, "detailedError", "correct error after error");
-is(req.result, undefined, "correct result after error");
+  }
+  var error = null;
+  var promise = req.then(function(r) {
+    ok(false, "promise should not be resolved");
+    runTest();
+  }, function(e) {
+    ok(e instanceof DOMError, "got error rejection");
+    ok(e === req.error, "got correct error when rejecting the promise");
+    error = e;
+    runTest();
+  });
+  ok(promise instanceof Promise, "then() should return a Promise");
+  reqserv.fireError(req, "OhMyError");
+  ok(ev, "got error event");
+  is(ev.type, "error", "correct type during error");
+  is(ev.target, req, "correct target during error");
+  is(req.readyState, "done", "correct readyState after error");
+  is(req.error.name, "OhMyError", "correct error after error");
+  is(req.result, undefined, "correct result after error");
+  is(error, null, "Promise should not be rejected synchronously");
+}
+
+function testDetailedError() {
+  // fire detailed error
+  var req = reqserv.createRequest(window);
+  var ev = null;
+  req.onerror = function(e) {
+    ev = e;
+  };
+  var error = null;
+  var promise = req.then(function(r) {
+    ok(false, "promise should not be resolved");
+    runTest();
+  }, function(e) {
+    ok(e instanceof DOMError, "got error rejection");
+    ok(e === req.error, "got correct error when rejecting the promise");
+    error = e;
+    runTest();
+  });
+  ok(promise instanceof Promise, "then() should return a Promise");
+  reqserv.fireDetailedError(req, new DOMError("detailedError"));
+  ok(ev, "got error event");
+  is(ev.type, "error", "correct type during error");
+  is(ev.target, req, "correct target during error");
+  is(req.readyState, "done", "correct readyState after error");
+  is(req.error.name, "detailedError", "correct error after error");
+  is(req.result, undefined, "correct result after error");
+  is(error, null, "Promise should not be rejected synchronously");
+}
+
+function testThenAfterSuccess() {
+  // fire success
+  var req = reqserv.createRequest(window);
+  var ev = null;
+  req.onsuccess = function(e) {
+    ev = e;
+  }
+  reqserv.fireSuccess(req, "my result");
+  ok(ev, "got success event");
+  is(ev.type, "success", "correct type during success");
+  is(ev.target, req, "correct target during success");
+  is(req.readyState, "done", "correct readyState after success");
+  is(req.error, null, "correct error after success");
+  is(req.result, "my result", "correct result after success");
+  var result = null;
+  var promise = req.then(function(r) {
+    is(r, "my result", "correct result when resolving the promise");
+    result = r;
+    runTest();
+  }, function(e) {
+    ok(false, "promise should not be rejected");
+    runTest();
+  });
+  ok(promise instanceof Promise, "then() should return a Promise");
+  is(result, null, "Promise should not be resolved synchronously");
+}
+
+function testThenAfterError() {
+  // fire error
+  var req = reqserv.createRequest(window);
+  var ev = null;
+  req.onerror = function(e) {
+    ev = e;
+  }
+  reqserv.fireError(req, "OhMyError");
+  ok(ev, "got error event");
+  is(ev.type, "error", "correct type during error");
+  is(ev.target, req, "correct target during error");
+  is(req.readyState, "done", "correct readyState after error");
+  is(req.error.name, "OhMyError", "correct error after error");
+  is(req.result, undefined, "correct result after error");
+  var error = null;
+  var promise = req.then(function(r) {
+    ok(false, "promise should not be resolved");
+    runTest();
+  }, function(e) {
+    ok(e instanceof DOMError, "got error rejection");
+    ok(e === req.error, "got correct error when rejecting the promise");
+    error = e;
+    runTest();
+  });
+  ok(promise instanceof Promise, "then() should return a Promise");
+  is(error, null, "Promise should not be rejected synchronously");
+}
+
+function testDetailedError() {
+  // fire detailed error
+  var req = reqserv.createRequest(window);
+  var ev = null;
+  req.onerror = function(e) {
+    ev = e;
+  };
+  var error = null;
+  var promise = req.then(function(r) {
+    ok(false, "promise should not be resolved");
+    runTest();
+  }, function(e) {
+    ok(e instanceof DOMError, "got error rejection");
+    ok(e === req.error, "got correct error when rejecting the promise");
+    error = e;
+    runTest();
+  });
+  ok(promise instanceof Promise, "then() should return a Promise");
+  reqserv.fireDetailedError(req, new DOMError("detailedError"));
+  ok(ev, "got error event");
+  is(ev.type, "error", "correct type during error");
+  is(ev.target, req, "correct target during error");
+  is(req.readyState, "done", "correct readyState after error");
+  is(req.error.name, "detailedError", "correct error after error");
+  is(req.result, undefined, "correct result after error");
+  is(error, null, "Promise should not be rejected synchronously");
+}
+
+var tests = [
+  testBasics,
+  testSuccess,
+  testError,
+  testDetailedError,
+  testThenAfterSuccess,
+  testThenAfterError,
+];
+
+function runTest() {
+  if (!tests.length) {
+    SimpleTest.finish();
+    return;
+  }
+
+  var test = tests.shift();
+  test();
+}
+
+SimpleTest.waitForExplicitFinish();
+runTest();
+
 </script>
 </pre>
 </body>

dom/bindings/Bindings.conf

@@ -352,6 +352,10 @@ DOMInterfaces = {
     'headerFile': 'mozilla/dom/DOMRect.h',
 },
 
+'DOMRequest': {
+    'implicitJSContext': [ 'then' ],
+},
+
 'DOMSettableTokenList': {
     'nativeType': 'nsDOMSettableTokenList',
 },

dom/bindings/ToJSValue.h

@@ -236,6 +236,24 @@ ToJSValue(JSContext* aCx, JS::Handle<JS::Value> aArgument,
   return MaybeWrapValue(aCx, aValue);
 }
 
+// Accept existing JS values on the Heap (which may not be same-compartment with us
+inline bool
+ToJSValue(JSContext* aCx, JS::Heap<JS::Value> aArgument,
+          JS::MutableHandle<JS::Value> aValue)
+{
+  aValue.set(aArgument);
+  return MaybeWrapValue(aCx, aValue);
+}
+
+// Accept existing rooted JS values (which may not be same-compartment with us
+inline bool
+ToJSValue(JSContext* aCx, JS::Rooted<JS::Value> aArgument,
+          JS::MutableHandle<JS::Value> aValue)
+{
+  aValue.set(aArgument);
+  return MaybeWrapValue(aCx, aValue);
+}
+
 // Accept nsresult, for use in rejections, and create an XPCOM
 // exception object representing that nsresult.
 bool

dom/webidl/DOMCursor.webidl

@@ -3,8 +3,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-interface DOMCursor : DOMRequest {
+interface DOMCursor : EventTarget {
   readonly attribute boolean done;
   [Throws]
   void continue();
 };
+
+DOMCursor implements DOMRequestShared;

dom/webidl/DOMRequest.webidl

@@ -5,7 +5,8 @@
 
 enum DOMRequestReadyState { "pending", "done" };
 
-interface DOMRequest : EventTarget {
+[NoInterfaceObject]
+interface DOMRequestShared {
   readonly attribute DOMRequestReadyState readyState;
 
   readonly attribute any result;
@@ -14,3 +15,13 @@ interface DOMRequest : EventTarget {
   attribute EventHandler onsuccess;
   attribute EventHandler onerror;
 };
+
+interface DOMRequest : EventTarget {
+  // The [TreatNonCallableAsNull] annotation is required since then() should do
+  // nothing instead of throwing errors when non-callable arguments are passed.
+  [NewObject, Throws]
+  Promise<any> then([TreatNonCallableAsNull] optional AnyCallback? fulfillCallback = null,
+                    [TreatNonCallableAsNull] optional AnyCallback? rejectCallback = null);
+};
+
+DOMRequest implements DOMRequestShared;

js/public/RootingAPI.h

@@ -220,7 +220,7 @@ class Heap : public js::HeapBase<T>
      * that will be used for both lvalue and rvalue copies, so we can simply
      * omit the rvalue variant.
      */
-    explicit Heap(const Heap<T> &p) { init(p.ptr); }
+    Heap(const Heap<T> &p) { init(p.ptr); }
 
     ~Heap() {
         if (js::GCMethods<T>::needsPostBarrier(ptr))