js/public/CallArgs.h
author Narcis Beleuzu <nbeleuzu@mozilla.com>
Tue, 25 Sep 2018 16:26:04 +0300
changeset 438081 d03b538b6b41
parent 435753 f2bedf1fe932
permissions -rw-r--r--
Backed out 13 changesets (bug 1492014) for multiple failures. a=backout Backed out changeset f629b66a235f (bug 1492014) Backed out changeset 0a8510efbba3 (bug 1492014) Backed out changeset 2798792623cf (bug 1492014) Backed out changeset 8818f5ae0d13 (bug 1492014) Backed out changeset 0c6b469c9618 (bug 1492014) Backed out changeset d1654231ffb8 (bug 1492014) Backed out changeset 5a3967f37d3f (bug 1492014) Backed out changeset ac095c2fdd8b (bug 1492014) Backed out changeset 4d1ba44e30ab (bug 1492014) Backed out changeset e34938a11fab (bug 1492014) Backed out changeset 3e142ea44140 (bug 1492014) Backed out changeset b0f2fafdba8c (bug 1492014) Backed out changeset d77330a1cbfa (bug 1492014)

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * 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/. */

/*
 * [SMDOC] JS::CallArgs API
 *
 * Helper classes encapsulating access to the callee, |this| value, arguments,
 * and argument count for a call/construct operation.
 *
 * JS::CallArgs encapsulates access to a JSNative's un-abstracted
 * |unsigned argc, Value* vp| arguments.  The principal way to create a
 * JS::CallArgs is using JS::CallArgsFromVp:
 *
 *   // If provided no arguments or a non-numeric first argument, return zero.
 *   // Otherwise return |this| exactly as given, without boxing.
 *   static bool
 *   Func(JSContext* cx, unsigned argc, JS::Value* vp)
 *   {
 *       JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
 *
 *       // Guard against no arguments or a non-numeric arg0.
 *       if (args.length() == 0 || !args[0].isNumber()) {
 *           args.rval().setInt32(0);
 *           return true;
 *       }
 *
 *       // Access to the callee must occur before accessing/setting
 *       // the return value.
 *       JSObject& callee = args.callee();
 *       args.rval().setObject(callee);
 *
 *       // callee() and calleev() will now assert.
 *
 *       // It's always fine to access thisv().
 *       HandleValue thisv = args.thisv();
 *       args.rval().set(thisv);
 *
 *       // As the return value was last set to |this|, returns |this|.
 *       return true;
 *   }
 *
 * CallArgs is exposed publicly and used internally.  Not all parts of its
 * public interface are meant to be used by embedders!  See inline comments to
 * for details.
 *
 * It's possible (albeit deprecated) to manually index into |vp| to access the
 * callee, |this|, and arguments of a function, and to set its return value.
 * This does not have the error-handling or moving-GC correctness of CallArgs.
 * New code should use CallArgs instead whenever possible.
 *
 * The eventual plan is to change JSNative to take |const CallArgs&| directly,
 * for automatic assertion of correct use and to make calling functions more
 * efficient.  Embedders should start internally switching away from using
 * |argc| and |vp| directly, except to create a |CallArgs|.  Then, when an
 * eventual release making that change occurs, porting efforts will require
 * changing methods' signatures but won't require invasive changes to the
 * methods' implementations, potentially under time pressure.
 */

#ifndef js_CallArgs_h
#define js_CallArgs_h

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

#include "jstypes.h"

#include "js/RootingAPI.h"
#include "js/Value.h"

/* Typedef for native functions called by the JS VM. */
typedef bool
(* JSNative)(JSContext* cx, unsigned argc, JS::Value* vp);

namespace JS {

extern JS_PUBLIC_DATA(const HandleValue) UndefinedHandleValue;

namespace detail {

/*
 * Compute |this| for the |vp| inside a JSNative, either boxing primitives or
 * replacing with the global object as necessary.
 */
extern JS_PUBLIC_API(bool)
ComputeThis(JSContext* cx, JS::Value* vp, MutableHandleObject thisObject);

#ifdef JS_DEBUG
extern JS_PUBLIC_API(void)
CheckIsValidConstructible(const Value& v);
#endif

class MOZ_STACK_CLASS IncludeUsedRval
{
    mutable bool usedRval_;

  public:
    bool usedRval() const { return usedRval_; }
    void setUsedRval() const { usedRval_ = true; }
    void clearUsedRval() const { usedRval_ = false; }
    void assertUnusedRval() const { MOZ_ASSERT(!usedRval_); }
};

class MOZ_STACK_CLASS NoUsedRval
{
  public:
    bool usedRval() const { return false; }
    void setUsedRval() const {}
    void clearUsedRval() const {}
    void assertUnusedRval() const {}
};

template<class WantUsedRval>
class MOZ_STACK_CLASS CallArgsBase
{
    static_assert(mozilla::IsSame<WantUsedRval, IncludeUsedRval>::value ||
                  mozilla::IsSame<WantUsedRval, NoUsedRval>::value,
                  "WantUsedRval can only be IncludeUsedRval or NoUsedRval");

  protected:
    Value* argv_;
    unsigned argc_;
    bool constructing_:1;

    // True if the caller does not use the return value.
    bool ignoresReturnValue_:1;

#ifdef JS_DEBUG
    WantUsedRval wantUsedRval_;
    bool usedRval() const { return wantUsedRval_.usedRval(); }
    void setUsedRval() const { wantUsedRval_.setUsedRval(); }
    void clearUsedRval() const { wantUsedRval_.clearUsedRval(); }
    void assertUnusedRval() const { wantUsedRval_.assertUnusedRval(); }
#else
    bool usedRval() const { return false; }
    void setUsedRval() const {}
    void clearUsedRval() const {}
    void assertUnusedRval() const {}
#endif

  public:
    // CALLEE ACCESS

    /*
     * Returns the function being called, as a value.  Must not be called after
     * rval() has been used!
     */
    HandleValue calleev() const {
        this->assertUnusedRval();
        return HandleValue::fromMarkedLocation(&argv_[-2]);
    }

    /*
     * Returns the function being called, as an object.  Must not be called
     * after rval() has been used!
     */
    JSObject& callee() const {
        return calleev().toObject();
    }

    // CALLING/CONSTRUCTING-DIFFERENTIATIONS

    bool isConstructing() const {
        if (!argv_[-1].isMagic()) {
            return false;
        }

#ifdef JS_DEBUG
        if (!this->usedRval()) {
            CheckIsValidConstructible(calleev());
        }
#endif

        return true;
    }

    bool ignoresReturnValue() const {
        return ignoresReturnValue_;
    }

    MutableHandleValue newTarget() const {
        MOZ_ASSERT(constructing_);
        return MutableHandleValue::fromMarkedLocation(&this->argv_[argc_]);
    }

    /*
     * Returns the |this| value passed to the function.  This method must not
     * be called when the function is being called as a constructor via |new|.
     * The value may or may not be an object: it is the individual function's
     * responsibility to box the value if needed.
     */
    HandleValue thisv() const {
        // Some internal code uses thisv() in constructing cases, so don't do
        // this yet.
        // MOZ_ASSERT(!argv_[-1].isMagic(JS_IS_CONSTRUCTING));
        return HandleValue::fromMarkedLocation(&argv_[-1]);
    }

    bool computeThis(JSContext* cx, MutableHandleObject thisObject) const {
        if (thisv().isObject()) {
            thisObject.set(&thisv().toObject());
            return true;
        }

        return ComputeThis(cx, base(), thisObject);
    }

    // ARGUMENTS

        /* Returns the number of arguments. */
    unsigned length() const { return argc_; }

    /* Returns the i-th zero-indexed argument. */
    MutableHandleValue operator[](unsigned i) const {
        MOZ_ASSERT(i < argc_);
        return MutableHandleValue::fromMarkedLocation(&this->argv_[i]);
    }

    /*
     * Returns the i-th zero-indexed argument, or |undefined| if there's no
     * such argument.
     */
    HandleValue get(unsigned i) const {
        return i < length()
               ? HandleValue::fromMarkedLocation(&this->argv_[i])
               : UndefinedHandleValue;
    }

    /*
     * Returns true if the i-th zero-indexed argument is present and is not
     * |undefined|.
     */
    bool hasDefined(unsigned i) const {
        return i < argc_ && !this->argv_[i].isUndefined();
    }

    // RETURN VALUE

    /*
     * Returns the currently-set return value.  The initial contents of this
     * value are unspecified.  Once this method has been called, callee() and
     * calleev() can no longer be used.  (If you're compiling against a debug
     * build of SpiderMonkey, these methods will assert to aid debugging.)
     *
     * If the method you're implementing succeeds by returning true, you *must*
     * set this.  (SpiderMonkey doesn't currently assert this, but it will do
     * so eventually.)  You don't need to use or change this if your method
     * fails.
     */
    MutableHandleValue rval() const {
        this->setUsedRval();
        return MutableHandleValue::fromMarkedLocation(&argv_[-2]);
    }

  public:
    // These methods are publicly exposed, but they are *not* to be used when
    // implementing a JSNative method and encapsulating access to |vp| within
    // it.  You probably don't want to use these!

    void setCallee(const Value& aCalleev) const {
        this->clearUsedRval();
        argv_[-2] = aCalleev;
    }

    void setThis(const Value& aThisv) const {
        argv_[-1] = aThisv;
    }

    MutableHandleValue mutableThisv() const {
        return MutableHandleValue::fromMarkedLocation(&argv_[-1]);
    }

  public:
    // These methods are publicly exposed, but we're unsure of the interfaces
    // (because they're hackish and drop assertions).  Avoid using these if you
    // can.

    Value* array() const { return argv_; }
    Value* end() const { return argv_ + argc_ + constructing_; }

  public:
    // These methods are only intended for internal use.  Embedders shouldn't
    // use them!

    Value* base() const { return argv_ - 2; }

    Value* spAfterCall() const {
        this->setUsedRval();
        return argv_ - 1;
    }
};

} // namespace detail

class MOZ_STACK_CLASS CallArgs : public detail::CallArgsBase<detail::IncludeUsedRval>
{
  private:
    friend CallArgs CallArgsFromVp(unsigned argc, Value* vp);
    friend CallArgs CallArgsFromSp(unsigned stackSlots, Value* sp, bool constructing,
                                   bool ignoresReturnValue);

    static CallArgs create(unsigned argc, Value* argv, bool constructing,
                           bool ignoresReturnValue = false) {
        CallArgs args;
        args.clearUsedRval();
        args.argv_ = argv;
        args.argc_ = argc;
        args.constructing_ = constructing;
        args.ignoresReturnValue_ = ignoresReturnValue;
#ifdef DEBUG
        MOZ_ASSERT(ValueIsNotGray(args.thisv()));
        MOZ_ASSERT(ValueIsNotGray(args.calleev()));
        for (unsigned i = 0; i < argc; ++i) {
            MOZ_ASSERT(ValueIsNotGray(argv[i]));
        }
#endif
        return args;
    }

  public:
    /*
     * Returns true if there are at least |required| arguments passed in. If
     * false, it reports an error message on the context.
     */
    JS_PUBLIC_API(bool) requireAtLeast(JSContext* cx, const char* fnname, unsigned required) const;

};

MOZ_ALWAYS_INLINE CallArgs
CallArgsFromVp(unsigned argc, Value* vp)
{
    return CallArgs::create(argc, vp + 2, vp[1].isMagic(JS_IS_CONSTRUCTING));
}

// This method is only intended for internal use in SpiderMonkey.  We may
// eventually move it to an internal header.  Embedders should use
// JS::CallArgsFromVp!
MOZ_ALWAYS_INLINE CallArgs
CallArgsFromSp(unsigned stackSlots, Value* sp, bool constructing = false,
               bool ignoresReturnValue = false)
{
    return CallArgs::create(stackSlots - constructing, sp - stackSlots, constructing,
                            ignoresReturnValue);
}

} // namespace JS

#endif /* js_CallArgs_h */