Bug 1441333: Part 3 - Add helper to create a JS error with a saved stack. r=bz

There's no standard way to create a JS error with full stack and location
information from a saved stack. Since we need this functionality in order to
reject promises with useful Error objects, this patch adds a simple helper to
make that possible.

MozReview-Commit-ID: FyGuo4UjfsQ

--HG--
extra : rebase_source : 65ef11c56f23e04ea5eeb87b972bfc8e4867fdd2

dom/base/ChromeUtils.cpp

@@ -679,5 +679,68 @@ ChromeUtils::GetCallerLocation(const GlobalObject& aGlobal, nsIPrincipal* aPrinc
   aRetval.set(js::GetFirstSubsumedSavedFrame(cx, principals, frame, kSkipSelfHosted));
 }
 
+/* static */ void
+ChromeUtils::CreateError(const GlobalObject& aGlobal, const nsAString& aMessage,
+                         JS::Handle<JSObject*> aStack,
+                         JS::MutableHandle<JSObject*> aRetVal, ErrorResult& aRv)
+{
+  if (aStack && !JS::IsSavedFrame(aStack)) {
+    aRv.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  JSContext* cx = aGlobal.Context();
+
+  auto cleanup = MakeScopeExit([&]() {
+    aRv.NoteJSContextException(cx);
+  });
+
+  JS::RootedObject retVal(cx);
+  {
+    JS::RootedString fileName(cx, JS_GetEmptyString(cx));
+    uint32_t line = 0;
+    uint32_t column = 0;
+
+    Maybe<JSAutoCompartment> ac;
+    JS::RootedObject stack(cx);
+    if (aStack) {
+      stack = UncheckedUnwrap(aStack);
+      ac.emplace(cx, stack);
+
+      if (JS::GetSavedFrameLine(cx, stack, &line) != JS::SavedFrameResult::Ok ||
+          JS::GetSavedFrameColumn(cx, stack, &column) != JS::SavedFrameResult::Ok ||
+          JS::GetSavedFrameSource(cx, stack, &fileName) != JS::SavedFrameResult::Ok) {
+        return;
+      }
+    }
+
+    JS::RootedString message(cx);
+    {
+      JS::RootedValue msgVal(cx);
+      if (!xpc::NonVoidStringToJsval(cx, aMessage, &msgVal)) {
+        return;
+      }
+      message = msgVal.toString();
+    }
+
+    JS::Rooted<JS::Value> err(cx);
+    if (!JS::CreateError(cx, JSEXN_ERR, stack,
+                         fileName, line, column,
+                         nullptr, message, &err)) {
+      return;
+    }
+
+    MOZ_ASSERT(err.isObject());
+    retVal = &err.toObject();
+  }
+
+  if (aStack && !JS_WrapObject(cx, &retVal)) {
+    return;
+  }
+
+  cleanup.release();
+  aRetVal.set(retVal);
+}
+
 } // namespace dom
 } // namespace mozilla

dom/base/ChromeUtils.h

@@ -170,6 +170,11 @@ public:
   static void
   GetCallerLocation(const GlobalObject& global, nsIPrincipal* principal,
                     JS::MutableHandle<JSObject*> aRetval);
+
+  static void
+  CreateError(const GlobalObject& global, const nsAString& message,
+              JS::Handle<JSObject*> stack,
+              JS::MutableHandle<JSObject*> aRetVal, ErrorResult& aRv);
 };
 
 } // namespace dom

dom/webidl/ChromeUtils.webidl

@@ -301,6 +301,15 @@ partial namespace ChromeUtils {
    * exists on the call stack, returns null.
    */
   object? getCallerLocation(Principal principal);
+
+  /**
+   * Creates a JS Error object with the given message and stack.
+   *
+   * If a stack object is provided, the error object is created in the global
+   * that it belongs to.
+   */
+  [Throws]
+  object createError(DOMString message, optional object? stack = null);
 };
 
 /**

js/xpconnect/tests/unit/test_getCallerLocation.js

@@ -58,4 +58,27 @@ add_task(async function() {
   Assert.throws(() => { Cu.reportError("Meh", {}); },
                 err => err.result == Cr.NS_ERROR_INVALID_ARG,
                 "reportError should throw when passed a non-SavedFrame object");
+
+
+  // createError
+
+  Assert.throws(() => { ChromeUtils.createError("Meh", {}); },
+                err => err.result == Cr.NS_ERROR_INVALID_ARG,
+                "createError should throw when passed a non-SavedFrame object");
+
+  let cloned = Cu.cloneInto(frame, sandbox);
+  let error = ChromeUtils.createError("Meh", cloned);
+
+  equal(String(cloned), String(frame),
+        "Cloning a SavedStack preserves its stringification");
+
+  equal(Cu.getGlobalForObject(error), sandbox,
+        "createError creates errors in the global of the SavedFrame");
+  equal(error.stack, String(cloned),
+        "createError creates errors with the correct stack");
+
+  equal(error.message, "Meh", "Error message");
+  equal(error.fileName, "thing.js", "Error filename");
+  equal(error.lineNumber, 5, "Error line");
+  equal(error.columnNumber, 14, "Error column");
 });