ErrorObject.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sw=2 et tw=80:
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * 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/. */

#include "vm/ErrorObject-inl.h"

#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/DebugOnly.h"

#include <utility>

#include "jsapi.h"
#include "jsexn.h"
#include "jsfriendapi.h"
#include "jsnum.h"
#include "jspubtd.h"
#include "NamespaceImports.h"

#include "builtin/Array.h"
#include "gc/AllocKind.h"
#include "gc/FreeOp.h"
#include "gc/Rooting.h"
#include "js/CallArgs.h"
#include "js/CallNonGenericMethod.h"
#include "js/CharacterEncoding.h"
#include "js/Class.h"
#include "js/Conversions.h"
#include "js/ErrorReport.h"
#include "js/ForOfIterator.h"
#include "js/PropertySpec.h"
#include "js/RootingAPI.h"
#include "js/TypeDecls.h"
#include "js/Utility.h"
#include "js/Value.h"
#include "js/Wrapper.h"
#include "util/StringBuffer.h"
#include "vm/GlobalObject.h"
#include "vm/JSAtom.h"
#include "vm/JSFunction.h"
#include "vm/JSObject.h"
#include "vm/NativeObject.h"
#include "vm/ObjectGroup.h"
#include "vm/ObjectOperations.h"
#include "vm/SavedStacks.h"
#include "vm/SelfHosting.h"
#include "vm/Shape.h"
#include "vm/Stack.h"
#include "vm/StringType.h"
#include "vm/ToSource.h"  // js::ValueToSource

#include "vm/ArrayObject-inl.h"
#include "vm/JSContext-inl.h"
#include "vm/JSObject-inl.h"
#include "vm/NativeObject-inl.h"
#include "vm/ObjectOperations-inl.h"
#include "vm/SavedStacks-inl.h"
#include "vm/Shape-inl.h"

using namespace js;

#define IMPLEMENT_ERROR_PROTO_CLASS(name)                         \
  {                                                               \
#    name ".prototype", JSCLASS_HAS_CACHED_PROTO(JSProto_##name), \
        JS_NULL_CLASS_OPS,                                        \
        &ErrorObject::classSpecs[JSProto_##name - JSProto_Error]  \
  }

const JSClass ErrorObject::protoClasses[JSEXN_ERROR_LIMIT] = {
    IMPLEMENT_ERROR_PROTO_CLASS(Error),

    IMPLEMENT_ERROR_PROTO_CLASS(InternalError),
    IMPLEMENT_ERROR_PROTO_CLASS(AggregateError),
    IMPLEMENT_ERROR_PROTO_CLASS(EvalError),
    IMPLEMENT_ERROR_PROTO_CLASS(RangeError),
    IMPLEMENT_ERROR_PROTO_CLASS(ReferenceError),
    IMPLEMENT_ERROR_PROTO_CLASS(SyntaxError),
    IMPLEMENT_ERROR_PROTO_CLASS(TypeError),
    IMPLEMENT_ERROR_PROTO_CLASS(URIError),

    IMPLEMENT_ERROR_PROTO_CLASS(DebuggeeWouldRun),
    IMPLEMENT_ERROR_PROTO_CLASS(CompileError),
    IMPLEMENT_ERROR_PROTO_CLASS(LinkError),
    IMPLEMENT_ERROR_PROTO_CLASS(RuntimeError)};

static bool exn_toSource(JSContext* cx, unsigned argc, Value* vp);

static const JSFunctionSpec error_methods[] = {
    JS_FN(js_toSource_str, exn_toSource, 0, 0),
    JS_SELF_HOSTED_FN(js_toString_str, "ErrorToString", 0, 0), JS_FS_END};

// Error.prototype and NativeError.prototype have own .message and .name
// properties.
#define COMMON_ERROR_PROPERTIES(name) \
  JS_STRING_PS("message", "", 0), JS_STRING_PS("name", #name, 0)

static const JSPropertySpec error_properties[] = {
    COMMON_ERROR_PROPERTIES(Error),
    // Only Error.prototype has .stack!
    JS_PSGS("stack", ErrorObject::getStack, ErrorObject::setStack, 0),
    JS_PS_END};

#define IMPLEMENT_NATIVE_ERROR_PROPERTIES(name)       \
  static const JSPropertySpec name##_properties[] = { \
      COMMON_ERROR_PROPERTIES(name), JS_PS_END};

IMPLEMENT_NATIVE_ERROR_PROPERTIES(InternalError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(AggregateError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(EvalError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(RangeError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(ReferenceError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(SyntaxError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(TypeError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(URIError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(DebuggeeWouldRun)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(CompileError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(LinkError)
IMPLEMENT_NATIVE_ERROR_PROPERTIES(RuntimeError)

#define IMPLEMENT_NATIVE_ERROR_SPEC(name)                              \
  {                                                                    \
    ErrorObject::createConstructor, ErrorObject::createProto, nullptr, \
        nullptr, nullptr, name##_properties, nullptr, JSProto_Error    \
  }

#define IMPLEMENT_NONGLOBAL_ERROR_SPEC(name)                           \
  {                                                                    \
    ErrorObject::createConstructor, ErrorObject::createProto, nullptr, \
        nullptr, nullptr, name##_properties, nullptr,                  \
        JSProto_Error | ClassSpec::DontDefineConstructor               \
  }

const ClassSpec ErrorObject::classSpecs[JSEXN_ERROR_LIMIT] = {
    {ErrorObject::createConstructor, ErrorObject::createProto, nullptr, nullptr,
     error_methods, error_properties},

    IMPLEMENT_NATIVE_ERROR_SPEC(InternalError),
    IMPLEMENT_NATIVE_ERROR_SPEC(AggregateError),
    IMPLEMENT_NATIVE_ERROR_SPEC(EvalError),
    IMPLEMENT_NATIVE_ERROR_SPEC(RangeError),
    IMPLEMENT_NATIVE_ERROR_SPEC(ReferenceError),
    IMPLEMENT_NATIVE_ERROR_SPEC(SyntaxError),
    IMPLEMENT_NATIVE_ERROR_SPEC(TypeError),
    IMPLEMENT_NATIVE_ERROR_SPEC(URIError),

    IMPLEMENT_NONGLOBAL_ERROR_SPEC(DebuggeeWouldRun),
    IMPLEMENT_NONGLOBAL_ERROR_SPEC(CompileError),
    IMPLEMENT_NONGLOBAL_ERROR_SPEC(LinkError),
    IMPLEMENT_NONGLOBAL_ERROR_SPEC(RuntimeError)};

#define IMPLEMENT_ERROR_CLASS(name)                                   \
  {                                                                   \
    js_Error_str, /* yes, really, e.g. devtools depends on this. */   \
        JSCLASS_HAS_CACHED_PROTO(JSProto_##name) |                    \
            JSCLASS_HAS_RESERVED_SLOTS(ErrorObject::RESERVED_SLOTS) | \
            JSCLASS_BACKGROUND_FINALIZE,                              \
        &ErrorObjectClassOps,                                         \
        &ErrorObject::classSpecs[JSProto_##name - JSProto_Error]      \
  }

static void exn_finalize(JSFreeOp* fop, JSObject* obj);

static const JSClassOps ErrorObjectClassOps = {
    nullptr,       // addProperty
    nullptr,       // delProperty
    nullptr,       // enumerate
    nullptr,       // newEnumerate
    nullptr,       // resolve
    nullptr,       // mayResolve
    exn_finalize,  // finalize
    nullptr,       // call
    nullptr,       // hasInstance
    nullptr,       // construct
    nullptr,       // trace
};

const JSClass ErrorObject::classes[JSEXN_ERROR_LIMIT] = {
    IMPLEMENT_ERROR_CLASS(Error), IMPLEMENT_ERROR_CLASS(InternalError),
    IMPLEMENT_ERROR_CLASS(AggregateError), IMPLEMENT_ERROR_CLASS(EvalError),
    IMPLEMENT_ERROR_CLASS(RangeError), IMPLEMENT_ERROR_CLASS(ReferenceError),
    IMPLEMENT_ERROR_CLASS(SyntaxError), IMPLEMENT_ERROR_CLASS(TypeError),
    IMPLEMENT_ERROR_CLASS(URIError),
    // These Error subclasses are not accessible via the global object:
    IMPLEMENT_ERROR_CLASS(DebuggeeWouldRun),
    IMPLEMENT_ERROR_CLASS(CompileError), IMPLEMENT_ERROR_CLASS(LinkError),
    IMPLEMENT_ERROR_CLASS(RuntimeError)};

static void exn_finalize(JSFreeOp* fop, JSObject* obj) {
  MOZ_ASSERT(fop->maybeOnHelperThread());
  if (JSErrorReport* report = obj->as<ErrorObject>().getErrorReport()) {
    // Bug 1560019: This allocation is not currently tracked.
    fop->deleteUntracked(report);
  }
}

static ErrorObject* CreateErrorObject(JSContext* cx, const CallArgs& args,
                                      unsigned messageArg, JSExnType exnType,
                                      HandleObject proto) {
  // Compute the error message, if any.
  RootedString message(cx, nullptr);
  if (args.hasDefined(messageArg)) {
    message = ToString<CanGC>(cx, args[messageArg]);
    if (!message) {
      return nullptr;
    }
  }

  // Find the scripted caller, but only ones we're allowed to know about.
  NonBuiltinFrameIter iter(cx, cx->realm()->principals());

  RootedString fileName(cx);
  uint32_t sourceId = 0;
  if (args.length() > messageArg + 1) {
    fileName = ToString<CanGC>(cx, args[messageArg + 1]);
  } else {
    fileName = cx->runtime()->emptyString;
    if (!iter.done()) {
      if (const char* cfilename = iter.filename()) {
        fileName = JS_NewStringCopyZ(cx, cfilename);
      }
      if (iter.hasScript()) {
        sourceId = iter.script()->scriptSource()->id();
      }
    }
  }
  if (!fileName) {
    return nullptr;
  }

  uint32_t lineNumber, columnNumber = 0;
  if (args.length() > messageArg + 2) {
    if (!ToUint32(cx, args[messageArg + 2], &lineNumber)) {
      return nullptr;
    }
  } else {
    lineNumber = iter.done() ? 0 : iter.computeLine(&columnNumber);
    columnNumber = FixupColumnForDisplay(columnNumber);
  }

  RootedObject stack(cx);
  if (!CaptureStack(cx, &stack)) {
    return nullptr;
  }

  return ErrorObject::create(cx, exnType, stack, fileName, sourceId, lineNumber,
                             columnNumber, nullptr, message, proto);
}

static bool Error(JSContext* cx, unsigned argc, Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);

  // ECMA ed. 3, 15.11.1 requires Error, etc., to construct even when
  // called as functions, without operator new.  But as we do not give
  // each constructor a distinct JSClass, we must get the exception type
  // ourselves.
  JSExnType exnType =
      JSExnType(args.callee().as<JSFunction>().getExtendedSlot(0).toInt32());

  MOZ_ASSERT(exnType != JSEXN_AGGREGATEERR,
             "AggregateError has its own constructor function");

  JSProtoKey protoKey =
      JSCLASS_CACHED_PROTO_KEY(&ErrorObject::classes[exnType]);

  // ES6 19.5.1.1 mandates the .prototype lookup happens before the toString
  RootedObject proto(cx);
  if (!GetPrototypeFromBuiltinConstructor(cx, args, protoKey, &proto)) {
    return false;
  }

  auto* obj = CreateErrorObject(cx, args, 0, exnType, proto);
  if (!obj) {
    return false;
  }

  args.rval().setObject(*obj);
  return true;
}

static ArrayObject* IterableToArray(JSContext* cx, HandleValue iterable) {
  JS::ForOfIterator iterator(cx);
  if (!iterator.init(iterable, JS::ForOfIterator::ThrowOnNonIterable)) {
    return nullptr;
  }

  RootedArrayObject array(cx, NewDenseEmptyArray(cx));

  RootedValue nextValue(cx);
  while (true) {
    bool done;
    if (!iterator.next(&nextValue, &done)) {
      return nullptr;
    }
    if (done) {
      return array;
    }

    if (!NewbornArrayPush(cx, array, nextValue)) {
      return nullptr;
    }
  }
}

// AggregateError ( errors, message )
static bool AggregateError(JSContext* cx, unsigned argc, Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);

  mozilla::DebugOnly<JSExnType> exnType =
      JSExnType(args.callee().as<JSFunction>().getExtendedSlot(0).toInt32());

  MOZ_ASSERT(exnType == JSEXN_AGGREGATEERR);

  // Steps 1-2. (9.1.13 OrdinaryCreateFromConstructor, steps 1-2).
  RootedObject proto(cx);
  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_AggregateError,
                                          &proto)) {
    return false;
  }

  // TypeError anyway, but this gives a better error message.
  if (!args.requireAtLeast(cx, "AggregateError", 1)) {
    return false;
  }

  // 9.1.13 OrdinaryCreateFromConstructor, step 3.
  // Step 3.
  Rooted<ErrorObject*> obj(
      cx, CreateErrorObject(cx, args, 1, JSEXN_AGGREGATEERR, proto));
  if (!obj) {
    return false;
  }

  // Step 4.

  RootedArrayObject errorsList(cx, IterableToArray(cx, args.get(0)));
  if (!errorsList) {
    return false;
  }

  // Step 5.
  RootedValue errorsVal(cx, JS::ObjectValue(*errorsList));
  if (!NativeDefineDataProperty(cx, obj, cx->names().errors, errorsVal, 0)) {
    return false;
  }

  // Step 6.
  args.rval().setObject(*obj);
  return true;
}

/* static */
JSObject* ErrorObject::createProto(JSContext* cx, JSProtoKey key) {
  JSExnType type = ExnTypeFromProtoKey(key);

  if (type == JSEXN_ERR) {
    return GlobalObject::createBlankPrototype(
        cx, cx->global(), &ErrorObject::protoClasses[JSEXN_ERR]);
  }

  RootedObject protoProto(
      cx, GlobalObject::getOrCreateErrorPrototype(cx, cx->global()));
  if (!protoProto) {
    return nullptr;
  }

  return GlobalObject::createBlankPrototypeInheriting(
      cx, &ErrorObject::protoClasses[type], protoProto);
}

/* static */
JSObject* ErrorObject::createConstructor(JSContext* cx, JSProtoKey key) {
  JSExnType type = ExnTypeFromProtoKey(key);
  RootedObject ctor(cx);

  if (type == JSEXN_ERR) {
    ctor = GenericCreateConstructor<Error, 1, gc::AllocKind::FUNCTION_EXTENDED>(
        cx, key);
  } else {
    RootedFunction proto(
        cx, GlobalObject::getOrCreateErrorConstructor(cx, cx->global()));
    if (!proto) {
      return nullptr;
    }

    Native native;
    unsigned nargs;
    if (type == JSEXN_AGGREGATEERR) {
      native = AggregateError;
      nargs = 2;
    } else {
      native = Error;
      nargs = 1;
    }

    ctor =
        NewFunctionWithProto(cx, native, nargs, FunctionFlags::NATIVE_CTOR,
                             nullptr, ClassName(key, cx), proto,
                             gc::AllocKind::FUNCTION_EXTENDED, SingletonObject);
  }

  if (!ctor) {
    return nullptr;
  }

  ctor->as<JSFunction>().setExtendedSlot(0, Int32Value(type));
  return ctor;
}

/* static */
Shape* js::ErrorObject::assignInitialShape(JSContext* cx,
                                           Handle<ErrorObject*> obj) {
  MOZ_ASSERT(obj->empty());

  if (!NativeObject::addDataProperty(cx, obj, cx->names().fileName,
                                     FILENAME_SLOT, 0)) {
    return nullptr;
  }
  if (!NativeObject::addDataProperty(cx, obj, cx->names().lineNumber,
                                     LINENUMBER_SLOT, 0)) {
    return nullptr;
  }
  return NativeObject::addDataProperty(cx, obj, cx->names().columnNumber,
                                       COLUMNNUMBER_SLOT, 0);
}

/* static */
bool js::ErrorObject::init(JSContext* cx, Handle<ErrorObject*> obj,
                           JSExnType type, UniquePtr<JSErrorReport> errorReport,
                           HandleString fileName, HandleObject stack,
                           uint32_t sourceId, uint32_t lineNumber,
                           uint32_t columnNumber, HandleString message) {
  AssertObjectIsSavedFrameOrWrapper(cx, stack);
  cx->check(obj, stack);

  // Null out early in case of error, for exn_finalize's sake.
  obj->initReservedSlot(ERROR_REPORT_SLOT, PrivateValue(nullptr));

  if (!EmptyShape::ensureInitialCustomShape<ErrorObject>(cx, obj)) {
    return false;
  }

  // The .message property isn't part of the initial shape because it's
  // present in some error objects -- |Error.prototype|, |new Error("f")|,
  // |new Error("")| -- but not in others -- |new Error(undefined)|,
  // |new Error()|.
  RootedShape messageShape(cx);
  if (message) {
    messageShape = NativeObject::addDataProperty(cx, obj, cx->names().message,
                                                 MESSAGE_SLOT, 0);
    if (!messageShape) {
      return false;
    }
    MOZ_ASSERT(messageShape->slot() == MESSAGE_SLOT);
  }

  MOZ_ASSERT(obj->lookupPure(NameToId(cx->names().fileName))->slot() ==
             FILENAME_SLOT);
  MOZ_ASSERT(obj->lookupPure(NameToId(cx->names().lineNumber))->slot() ==
             LINENUMBER_SLOT);
  MOZ_ASSERT(obj->lookupPure(NameToId(cx->names().columnNumber))->slot() ==
             COLUMNNUMBER_SLOT);
  MOZ_ASSERT_IF(
      message,
      obj->lookupPure(NameToId(cx->names().message))->slot() == MESSAGE_SLOT);

  MOZ_ASSERT(JSEXN_ERR <= type && type < JSEXN_LIMIT);

  JSErrorReport* report = errorReport.release();
  obj->initReservedSlot(EXNTYPE_SLOT, Int32Value(type));
  obj->initReservedSlot(STACK_SLOT, ObjectOrNullValue(stack));
  obj->setReservedSlot(ERROR_REPORT_SLOT, PrivateValue(report));
  obj->initReservedSlot(FILENAME_SLOT, StringValue(fileName));
  obj->initReservedSlot(LINENUMBER_SLOT, Int32Value(lineNumber));
  obj->initReservedSlot(COLUMNNUMBER_SLOT, Int32Value(columnNumber));
  if (message) {
    obj->setSlotWithType(cx, messageShape, StringValue(message));
  }
  obj->initReservedSlot(SOURCEID_SLOT, Int32Value(sourceId));

  return true;
}

/* static */
ErrorObject* js::ErrorObject::create(JSContext* cx, JSExnType errorType,
                                     HandleObject stack, HandleString fileName,
                                     uint32_t sourceId, uint32_t lineNumber,
                                     uint32_t columnNumber,
                                     UniquePtr<JSErrorReport> report,
                                     HandleString message,
                                     HandleObject protoArg /* = nullptr */) {
  AssertObjectIsSavedFrameOrWrapper(cx, stack);

  RootedObject proto(cx, protoArg);
  if (!proto) {
    proto = GlobalObject::getOrCreateCustomErrorPrototype(cx, cx->global(),
                                                          errorType);
    if (!proto) {
      return nullptr;
    }
  }

  Rooted<ErrorObject*> errObject(cx);
  {
    const JSClass* clasp = ErrorObject::classForType(errorType);
    JSObject* obj = NewObjectWithGivenProto(cx, clasp, proto);
    if (!obj) {
      return nullptr;
    }
    errObject = &obj->as<ErrorObject>();
  }

  if (!ErrorObject::init(cx, errObject, errorType, std::move(report), fileName,
                         stack, sourceId, lineNumber, columnNumber, message)) {
    return nullptr;
  }

  return errObject;
}

JSErrorReport* js::ErrorObject::getOrCreateErrorReport(JSContext* cx) {
  if (JSErrorReport* r = getErrorReport()) {
    return r;
  }

  // We build an error report on the stack and then use CopyErrorReport to do
  // the nitty-gritty malloc stuff.
  JSErrorReport report;

  // Type.
  JSExnType type_ = type();
  report.exnType = type_;

  // Filename.
  UniqueChars filenameStr = JS_EncodeStringToLatin1(cx, fileName(cx));
  if (!filenameStr) {
    return nullptr;
  }
  report.filename = filenameStr.get();

  // Coordinates.
  report.sourceId = sourceId();
  report.lineno = lineNumber();
  report.column = columnNumber();

  // Message. Note that |new Error()| will result in an undefined |message|
  // slot, so we need to explicitly substitute the empty string in that case.
  RootedString message(cx, getMessage());
  if (!message) {
    message = cx->runtime()->emptyString;
  }

  UniqueChars utf8 = StringToNewUTF8CharsZ(cx, *message);
  if (!utf8) {
    return nullptr;
  }
  report.initOwnedMessage(utf8.release());

  // Cache and return.
  UniquePtr<JSErrorReport> copy = CopyErrorReport(cx, &report);
  if (!copy) {
    return nullptr;
  }
  setReservedSlot(ERROR_REPORT_SLOT, PrivateValue(copy.get()));
  return copy.release();
}

static bool FindErrorInstanceOrPrototype(JSContext* cx, HandleObject obj,
                                         MutableHandleObject result) {
  // Walk up the prototype chain until we find an error object instance or
  // prototype object. This allows code like:
  //  Object.create(Error.prototype).stack
  // or
  //   function NYI() { }
  //   NYI.prototype = new Error;
  //   (new NYI).stack
  // to continue returning stacks that are useless, but at least don't throw.

  RootedObject target(cx, CheckedUnwrapStatic(obj));
  if (!target) {
    ReportAccessDenied(cx);
    return false;
  }

  RootedObject proto(cx);
  while (!IsErrorProtoKey(StandardProtoKeyOrNull(target))) {
    if (!GetPrototype(cx, target, &proto)) {
      return false;
    }

    if (!proto) {
      // We walked the whole prototype chain and did not find an Error
      // object.
      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
                                JSMSG_INCOMPATIBLE_PROTO, js_Error_str,
                                "(get stack)", obj->getClass()->name);
      return false;
    }

    target = CheckedUnwrapStatic(proto);
    if (!target) {
      ReportAccessDenied(cx);
      return false;
    }
  }

  result.set(target);
  return true;
}

static MOZ_ALWAYS_INLINE bool IsObject(HandleValue v) { return v.isObject(); }

/* static */
bool js::ErrorObject::getStack(JSContext* cx, unsigned argc, Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);
  // We accept any object here, because of poor-man's subclassing of Error.
  return CallNonGenericMethod<IsObject, getStack_impl>(cx, args);
}

/* static */
bool js::ErrorObject::getStack_impl(JSContext* cx, const CallArgs& args) {
  RootedObject thisObj(cx, &args.thisv().toObject());

  RootedObject obj(cx);
  if (!FindErrorInstanceOrPrototype(cx, thisObj, &obj)) {
    return false;
  }

  if (!obj->is<ErrorObject>()) {
    args.rval().setString(cx->runtime()->emptyString);
    return true;
  }

  // Do frame filtering based on the ErrorObject's principals. This ensures we
  // don't see chrome frames when chrome code accesses .stack over Xrays.
  JSPrincipals* principals = obj->as<ErrorObject>().realm()->principals();

  RootedObject savedFrameObj(cx, obj->as<ErrorObject>().stack());
  RootedString stackString(cx);
  if (!BuildStackString(cx, principals, savedFrameObj, &stackString)) {
    return false;
  }

  if (cx->runtime()->stackFormat() == js::StackFormat::V8) {
    // When emulating V8 stack frames, we also need to prepend the
    // stringified Error to the stack string.
    HandlePropertyName name = cx->names().ErrorToStringWithTrailingNewline;
    FixedInvokeArgs<0> args2(cx);
    RootedValue rval(cx);
    if (!CallSelfHostedFunction(cx, name, args.thisv(), args2, &rval)) {
      return false;
    }

    if (!rval.isString()) {
      args.rval().setString(cx->runtime()->emptyString);
      return true;
    }

    RootedString stringified(cx, rval.toString());
    stackString = ConcatStrings<CanGC>(cx, stringified, stackString);
  }

  args.rval().setString(stackString);
  return true;
}

/* static */
bool js::ErrorObject::setStack(JSContext* cx, unsigned argc, Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);
  // We accept any object here, because of poor-man's subclassing of Error.
  return CallNonGenericMethod<IsObject, setStack_impl>(cx, args);
}

/* static */
bool js::ErrorObject::setStack_impl(JSContext* cx, const CallArgs& args) {
  RootedObject thisObj(cx, &args.thisv().toObject());

  if (!args.requireAtLeast(cx, "(set stack)", 1)) {
    return false;
  }
  RootedValue val(cx, args[0]);

  return DefineDataProperty(cx, thisObj, cx->names().stack, val);
}

JSString* js::ErrorToSource(JSContext* cx, HandleObject obj) {
  RootedValue nameVal(cx);
  RootedString name(cx);
  if (!GetProperty(cx, obj, obj, cx->names().name, &nameVal) ||
      !(name = ToString<CanGC>(cx, nameVal))) {
    return nullptr;
  }

  RootedValue messageVal(cx);
  RootedString message(cx);
  if (!GetProperty(cx, obj, obj, cx->names().message, &messageVal) ||
      !(message = ValueToSource(cx, messageVal))) {
    return nullptr;
  }

  RootedValue filenameVal(cx);
  RootedString filename(cx);
  if (!GetProperty(cx, obj, obj, cx->names().fileName, &filenameVal) ||
      !(filename = ValueToSource(cx, filenameVal))) {
    return nullptr;
  }

  RootedValue linenoVal(cx);
  uint32_t lineno;
  if (!GetProperty(cx, obj, obj, cx->names().lineNumber, &linenoVal) ||
      !ToUint32(cx, linenoVal, &lineno)) {
    return nullptr;
  }

  JSStringBuilder sb(cx);
  if (!sb.append("(new ") || !sb.append(name) || !sb.append("(")) {
    return nullptr;
  }

  if (!sb.append(message)) {
    return nullptr;
  }

  if (!filename->empty()) {
    if (!sb.append(", ") || !sb.append(filename)) {
      return nullptr;
    }
  }
  if (lineno != 0) {
    /* We have a line, but no filename, add empty string */
    if (filename->empty() && !sb.append(", \"\"")) {
      return nullptr;
    }

    JSString* linenumber = ToString<CanGC>(cx, linenoVal);
    if (!linenumber) {
      return nullptr;
    }
    if (!sb.append(", ") || !sb.append(linenumber)) {
      return nullptr;
    }
  }

  if (!sb.append("))")) {
    return nullptr;
  }

  return sb.finishString();
}

/*
 * Return a string that may eval to something similar to the original object.
 */
static bool exn_toSource(JSContext* cx, unsigned argc, Value* vp) {
  if (!CheckRecursionLimit(cx)) {
    return false;
  }
  CallArgs args = CallArgsFromVp(argc, vp);

  RootedObject obj(cx, ToObject(cx, args.thisv()));
  if (!obj) {
    return false;
  }

  JSString* str = ErrorToSource(cx, obj);
  if (!str) {
    return false;
  }

  args.rval().setString(str);
  return true;
}