Bug 1170760 part 4. Change Promise::Constructor to run in the Xray compartment when new Promise happens over Xrays. r=peterv

dom/bindings/Codegen.py

@@ -7057,13 +7057,44 @@ class CGPerSignatureCall(CGThing):
         needsUnwrap = False
         argsPost = []
         if isConstructor:
-            needsUnwrap = True
-            needsUnwrappedVar = False
-            unwrappedVar = "obj"
             if descriptor.name == "Promise" or descriptor.name == "MozAbortablePromise":
                 # Hack for Promise for now: pass in our desired proto so the
                 # implementation can create the reflector with the right proto.
                 argsPost.append("desiredProto")
+                # Also, we do not want to enter the content compartment when the
+                # Promise constructor is called via Xrays, because we want to
+                # create our callback functions that we will hand to our caller
+                # in the Xray compartment.  The reason we want to do that is the
+                # following situation, over Xrays:
+                #
+                #   contentWindow.Promise.race([Promise.resolve(5)])
+                #
+                # Ideally this would work.  Internally, race() does a
+                # contentWindow.Promise.resolve() on everything in the array.
+                # Per spec, to support subclassing,
+                # contentWindow.Promise.resolve has to do:
+                #
+                #   var resolve, reject;
+                #   var p = new contentWindow.Promise(function(a, b) {
+                #    resolve = a;
+                #    reject = b;
+                #  });
+                #  resolve(arg);
+                #  return p;
+                #
+                # where "arg" is, in this case, the chrome-side return value of
+                # Promise.resolve(5).  But if the "resolve" function in that
+                # case were created in the content compartment, then calling it
+                # would wrap "arg" in an opaque wrapper, and that function tries
+                # to get .then off the argument, which would throw.  So we need
+                # to create the "resolve" function in the chrome compartment,
+                # and hence want to be running the entire Promise constructor
+                # (which creates that function) in the chrome compartment in
+                # this case. So don't set needsUnwrap here.
+            else:
+                needsUnwrap = True
+                needsUnwrappedVar = False
+                unwrappedVar = "obj"
         elif descriptor.interface.isJSImplemented():
             if not idlNode.isStatic():
                 needsUnwrap = True

dom/bindings/test/TestInterfaceJS.js

@@ -124,12 +124,11 @@ TestInterfaceJS.prototype = {
   },
 
   testPromiseWithThrowingChromeThenable: function() {
-    // We need to produce a thing that has a "then" property in the page
-    // compartment, since we plan to call the page-provided resolve function.
-    var thenable = new this._win.Object();
-    Cu.waiveXrays(thenable).then = function() {
-      noSuchMethodExistsYo3()
-    }
+    var thenable =  {
+      then: function() {
+        noSuchMethodExistsYo3()
+      }
+    };
     return new this._win.Promise(function(resolve) {
       resolve(thenable)
     });
@@ -144,13 +143,12 @@ TestInterfaceJS.prototype = {
   },
 
   testPromiseWithDOMExceptionThrowingThenable: function() {
-    // We need to produce a thing that has a "then" property in the page
-    // compartment, since we plan to call the page-provided resolve function.
-    var thenable = new this._win.Object();
-    Cu.waiveXrays(thenable).then = () => {
-      throw new this._win.DOMException("We are a fourth DOMException",
-                                       "TypeMismatchError");
-    }
+    var thenable =  {
+      then: () => {
+        throw new this._win.DOMException("We are a fourth DOMException",
+                                         "TypeMismatchError");
+      }
+    };
     return new this._win.Promise(function(resolve) {
       resolve(thenable)
     });

dom/promise/Promise.cpp

@@ -798,17 +798,22 @@ Promise::CallInitFunction(const GlobalObject& aGlobal,
   aRv.WouldReportJSException();
 
   if (aRv.Failed()) {
-    // We want the same behavior as this JS implementation:
+    // There are two possibilities here.  Either we've got a rethrown exception,
+    // or we reported that already and synthesized a generic NS_ERROR_FAILURE on
+    // the ErrorResult.  In the former case, it doesn't much matter how we get
+    // the exception JS::Value from the ErrorResult to us, since we'll just end
+    // up wrapping it into the right compartment as needed if we hand it to
+    // someone.  But in the latter case we have to ensure that the new exception
+    // object we create is created in our reflector compartment, not in our
+    // current compartment, because in the case when we're a Promise constructor
+    // called over Xrays creating it in the current compartment would mean
+    // rejecting with a value that can't be accessed by code that can call
+    // then() on this Promise.
     //
-    // function Promise(arg) { try { arg(a, b); } catch (e) { this.reject(e); }}
-    //
-    // In particular, that means not using MaybeReject(aRv) here, since that
-    // would create the exception object in our reflector compartment, while we
-    // want to create it in whatever the current compartment on cx is.
-    JS::Rooted<JS::Value> value(cx);
-    DebugOnly<bool> conversionResult = ToJSValue(cx, aRv, &value);
-    MOZ_ASSERT(conversionResult);
-    MaybeRejectInternal(cx, value);
+    // Luckily, MaybeReject(aRv) does exactly what we want here: it enters our
+    // reflector compartment before trying to produce a JS::Value from the
+    // ErrorResult.
+    MaybeReject(aRv);
   }
 }