Source code
Revision control
Copy as Markdown
Other Tools
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 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
/*
* [SMDOC] JS::Result
*
* `Result` is used as the return type of many SpiderMonkey functions that
* can either succeed or fail. See "/mfbt/Result.h".
*
*
* ## Which return type to use
*
* `Result` is for return values. Obviously, if you're writing a function that
* can't fail, don't use Result. Otherwise:
*
* JS::Result<> - function can fail, doesn't return anything on success
* (defaults to `JS::Result<JS::Ok, JS::Error>`)
* JS::Result<JS::Ok, JS::OOM> - like JS::Result<>, but fails only on OOM
*
* JS::Result<Data> - function can fail, returns Data on success
* JS::Result<Data, JS::OOM> - returns Data, fails only on OOM
*
* mozilla::GenericErrorResult<JS::Error> - always fails
*
* That last type is like a Result with no success type. It's used for
* functions like `js::ReportNotFunction` that always return an error
* result. `GenericErrorResult<E>` implicitly converts to `Result<V, E>`,
* regardless of V.
*
*
* ## Checking Results when your return type is Result
*
* When you call a function that returns a `Result`, use the `MOZ_TRY` macro to
* check for errors:
*
* MOZ_TRY(DefenestrateObject(cx, obj));
*
* If `DefenestrateObject` returns a success result, `MOZ_TRY` is done, and
* control flows to the next statement. If `DefenestrateObject` returns an
* error result, `MOZ_TRY` will immediately return it, propagating the error to
* your caller. It's kind of like exceptions, but more explicit -- you can see
* in the code exactly where errors can happen.
*
* You can do a tail call instead of using `MOZ_TRY`:
*
* return DefenestrateObject(cx, obj);
*
* Indicate success with `return Ok();`.
*
* If the function returns a value on success, use `MOZ_TRY_VAR` to get it:
*
* RootedValue thrug(cx);
* MOZ_TRY_VAR(thrug, GetObjectThrug(cx, obj));
*
* This behaves the same as `MOZ_TRY` on error. On success, the success
* value of `GetObjectThrug(cx, obj)` is assigned to the variable `thrug`.
*
*
* ## GC safety
*
* When a function returns a `JS::Result<JSObject*>`, it is the program's
* responsibility to check for errors and root the object before continuing:
*
* RootedObject wrapper(cx);
* MOZ_TRY_VAR(wrapper, Enwrapify(cx, thing));
*
* This is ideal. On error, there is no object to root; on success, the
* assignment to wrapper roots it. GC safety is ensured.
*
* `Result` has methods .isOk(), .isErr(), .unwrap(), and .unwrapErr(), but if
* you're actually using them, it's possible to create a GC hazard. The static
* analysis will catch it if so, but that's hardly convenient. So try to stick
* to the idioms shown above.
*
*
* ## Future directions
*
* At present, JS::Error and JS::OOM are empty structs. The plan is to make them
* GC things that contain the actual error information (including the exception
* value and a saved stack).
*
* The long-term plan is to remove JS_IsExceptionPending and
* JS_GetPendingException in favor of JS::Error. Exception state will no longer
* exist.
*/
#ifndef js_Result_h
#define js_Result_h
#include "mozilla/Result.h"
namespace JS {
using mozilla::Ok;
template <typename T>
struct UnusedZero;
/**
* Type representing a JS error or exception. At the moment this only
* "represents" an error in a rather abstract way.
*/
struct Error {
// Since we claim UnusedZero<Error>::value and HasFreeLSB<Error>::value ==
// true below, we must only use positive even enum values.
enum class ErrorKind : uintptr_t { Unspecified = 2, OOM = 4 };
const ErrorKind kind = ErrorKind::Unspecified;
Error() = default;
protected:
friend struct UnusedZero<Error>;
constexpr MOZ_IMPLICIT Error(ErrorKind aKind) : kind(aKind) {}
};
struct OOM : Error {
constexpr OOM() : Error(ErrorKind::OOM) {}
protected:
friend struct UnusedZero<OOM>;
using Error::Error;
};
template <typename T>
struct UnusedZero {
using StorageType = std::underlying_type_t<Error::ErrorKind>;
static constexpr bool value = true;
static constexpr StorageType nullValue = 0;
static constexpr void AssertValid(StorageType aValue) {}
static constexpr T Inspect(const StorageType& aValue) {
return static_cast<Error::ErrorKind>(aValue);
}
static constexpr T Unwrap(StorageType aValue) {
return static_cast<Error::ErrorKind>(aValue);
}
static constexpr StorageType Store(T aValue) {
return static_cast<StorageType>(aValue.kind);
}
};
} // namespace JS
namespace mozilla::detail {
template <>
struct UnusedZero<JS::Error> : JS::UnusedZero<JS::Error> {};
template <>
struct UnusedZero<JS::OOM> : JS::UnusedZero<JS::OOM> {};
template <>
struct HasFreeLSB<JS::Error> {
static const bool value = true;
};
template <>
struct HasFreeLSB<JS::OOM> {
static const bool value = true;
};
} // namespace mozilla::detail
namespace JS {
/**
* `Result` is intended to be the return type of JSAPI calls and internal
* functions that can run JS code or allocate memory from the JS GC heap. Such
* functions can:
*
* - succeed, possibly returning a value;
*
* - fail with a JS exception (out-of-memory falls in this category); or
*
* - fail because JS execution was terminated, which occurs when e.g. a
* user kills a script from the "slow script" UI. This is also how we
* unwind the stack when the debugger forces the current function to
* return. JS `catch` blocks can't catch this kind of failure,
* and JS `finally` blocks don't execute.
*/
template <typename V = Ok, typename E = Error>
using Result = mozilla::Result<V, E>;
static_assert(sizeof(Result<>) == sizeof(uintptr_t),
"Result<> should be pointer-sized");
static_assert(sizeof(Result<int*, Error>) == sizeof(uintptr_t),
"Result<V*, Error> should be pointer-sized");
} // namespace JS
#endif // js_Result_h