Bug 996831. Add a ToJSValue overload for nsresult, to allow rejecting promises from C++ more easily. r=bholley

dom/bindings/Exceptions.cpp

@@ -131,28 +131,7 @@ Throw(JSContext* aCx, nsresult aRv, const char* aMessage)
     }
   }
 
-  nsRefPtr<Exception> finalException;
-
-  // Do we use DOM exceptions for this error code?
-  switch (NS_ERROR_GET_MODULE(aRv)) {
-  case NS_ERROR_MODULE_DOM:
-  case NS_ERROR_MODULE_SVG:
-  case NS_ERROR_MODULE_DOM_XPATH:
-  case NS_ERROR_MODULE_DOM_INDEXEDDB:
-  case NS_ERROR_MODULE_DOM_FILEHANDLE:
-    finalException = DOMException::Create(aRv);
-    break;
-
-  default:
-      break;
-  }
-
-  // If not, use the default.
-  if (!finalException) {
-    // aMessage can be null.
-    finalException = new Exception(nsCString(aMessage), aRv,
-                                   EmptyCString(), nullptr, nullptr);
-  }
+  nsRefPtr<Exception> finalException = CreateException(aCx, aRv, aMessage);
 
   MOZ_ASSERT(finalException);
   if (!ThrowExceptionObject(aCx, finalException)) {
@@ -164,6 +143,29 @@ Throw(JSContext* aCx, nsresult aRv, const char* aMessage)
   return false;
 }
 
+already_AddRefed<Exception>
+CreateException(JSContext* aCx, nsresult aRv, const char* aMessage)
+{
+  // Do we use DOM exceptions for this error code?
+  switch (NS_ERROR_GET_MODULE(aRv)) {
+  case NS_ERROR_MODULE_DOM:
+  case NS_ERROR_MODULE_SVG:
+  case NS_ERROR_MODULE_DOM_XPATH:
+  case NS_ERROR_MODULE_DOM_INDEXEDDB:
+  case NS_ERROR_MODULE_DOM_FILEHANDLE:
+    return DOMException::Create(aRv);
+  default:
+    break;
+  }
+
+  // If not, use the default.
+  // aMessage can be null, so we can't use nsDependentCString on it.
+  nsRefPtr<Exception> exception =
+    new Exception(nsCString(aMessage), aRv,
+                  EmptyCString(), nullptr, nullptr);
+  return exception.forget();
+}
+
 already_AddRefed<nsIStackFrame>
 GetCurrentJSStack()
 {

dom/bindings/Exceptions.h

@@ -30,6 +30,11 @@ ThrowExceptionObject(JSContext* aCx, Exception* aException);
 bool
 ThrowExceptionObject(JSContext* aCx, nsIException* aException);
 
+// Create an exception object for the given nsresult and message but
+// don't set it pending on aCx.  This never returns null.
+already_AddRefed<Exception>
+CreateException(JSContext* aCx, nsresult aRv, const char* aMessage = nullptr);
+
 already_AddRefed<nsIStackFrame>
 GetCurrentJSStack();
 

dom/bindings/ToJSValue.cpp

@@ -5,6 +5,8 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/dom/ToJSValue.h"
+#include "mozilla/dom/DOMException.h"
+#include "mozilla/dom/Exceptions.h"
 #include "nsAString.h"
 #include "nsContentUtils.h"
 #include "nsStringBuffer.h"
@@ -49,5 +51,14 @@ ISupportsToJSValue(JSContext* aCx,
 
 } // namespace tojsvalue_detail
 
+bool
+ToJSValue(JSContext* aCx,
+          nsresult aArgument,
+          JS::MutableHandle<JS::Value> aValue)
+{
+  nsRefPtr<Exception> exception = CreateException(aCx, aArgument);
+  return ToJSValue(aCx, exception, aValue);
+}
+
 } // namespace dom
 } // namespace mozilla

dom/bindings/ToJSValue.h

@@ -54,6 +54,14 @@ ToJSValue(JSContext* aCx,
   return true;
 }
 
+// The uint32_t version is disabled for now because on the super-old b2g
+// compiler nsresult and uint32_t are the same type.  If someone needs this at
+// some point we'll need to figure out how to make it work (e.g. by switching to
+// traits structs and using the trick IPC's ParamTraits uses, where a traits
+// struct templated on the type inherits from a base traits struct of some sort,
+// templated on the same type, or something).  Maybe b2g will update to a modern
+// compiler before that happens....
+#if 0
 inline bool
 ToJSValue(JSContext* aCx,
           uint32_t aArgument,
@@ -65,6 +73,7 @@ ToJSValue(JSContext* aCx,
   aValue.setNumber(aArgument);
   return true;
 }
+#endif
 
 inline bool
 ToJSValue(JSContext* aCx,
@@ -212,6 +221,13 @@ ToJSValue(JSContext* aCx, JS::Handle<JS::Value> aArgument,
   return MaybeWrapValue(aCx, aValue);
 }
 
+// Accept nsresult, for use in rejections, and create an XPCOM
+// exception object representing that nsresult.
+bool
+ToJSValue(JSContext* aCx,
+          nsresult aArgument,
+          JS::MutableHandle<JS::Value> aValue);
+
 // Accept arrays of other things we accept
 template <typename T>
 bool

dom/promise/Promise.h

@@ -78,15 +78,17 @@ public:
                    JS::Handle<JS::Value> aValue);
 
   // Helpers for using Promise from C++.
-  // Most DOM objects are handled already.  To add a new type T, such as ints,
-  // or dictionaries, add a ToJSValue overload in ToJSValue.h.
+  // Most DOM objects are handled already.  To add a new type T, add a
+  // ToJSValue overload in ToJSValue.h.
+  // aArg is a const reference so we can pass rvalues like integer constants
   template <typename T>
-  void MaybeResolve(T& aArg) {
+  void MaybeResolve(const T& aArg) {
     MaybeSomething(aArg, &Promise::MaybeResolve);
   }
 
+  // aArg is a const reference so we can pass rvalues like NS_ERROR_*
   template <typename T>
-  void MaybeReject(T& aArg) {
+  void MaybeReject(const T& aArg) {
     MaybeSomething(aArg, &Promise::MaybeReject);
   }