summaryrefslogtreecommitdiffstats
path: root/js/public/CallArgs.h
blob: 342ff6317815f1825bc00427e2075d61fa19a445 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * 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/. */

/*
 * 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.
 * It's also possible to use the supported API of JS_CALLEE, JS_THIS, JS_ARGV,
 * JS_RVAL, and JS_SET_RVAL to the same ends.
 *
 * But neither API has 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(Value)
ComputeThis(JSContext* cx, JS::Value* vp);

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

class MOZ_STACK_CLASS IncludeUsedRval
{
  protected:
#ifdef JS_DEBUG
    mutable bool usedRval_;
    void setUsedRval() const { usedRval_ = true; }
    void clearUsedRval() const { usedRval_ = false; }
    void assertUnusedRval() const { MOZ_ASSERT(!usedRval_); }
#else
    void setUsedRval() const {}
    void clearUsedRval() const {}
    void assertUnusedRval() const {}
#endif
};

class MOZ_STACK_CLASS NoUsedRval
{
  protected:
    void setUsedRval() const {}
    void clearUsedRval() const {}
    void assertUnusedRval() const {}
};

template<class WantUsedRval>
class MOZ_STACK_CLASS CallArgsBase : public WantUsedRval
{
    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;

  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]);
    }

    Value computeThis(JSContext* cx) const {
        if (thisv().isObject())
            return thisv();

        return ComputeThis(cx, base());
    }

    // 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
        for (unsigned i = 0; i < argc; ++i)
            MOZ_ASSERT_IF(argv[i].isGCThing(), !GCThingIsMarkedGray(GCCellPtr(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

/*
 * Macros to hide interpreter stack layout details from a JSNative using its
 * JS::Value* vp parameter.  DO NOT USE THESE!  Instead use JS::CallArgs and
 * friends, above.  These macros will be removed when we change JSNative to
 * take a const JS::CallArgs&.
 */

/*
 * Return |this| if |this| is an object.  Otherwise, return the global object
 * if |this| is null or undefined, and finally return a boxed version of any
 * other primitive.
 *
 * Note: if this method returns null, an error has occurred and must be
 * propagated or caught.
 */
MOZ_ALWAYS_INLINE JS::Value
JS_THIS(JSContext* cx, JS::Value* vp)
{
    return vp[1].isPrimitive() ? JS::detail::ComputeThis(cx, vp) : vp[1];
}

/*
 * A note on JS_THIS_OBJECT: no equivalent method is part of the CallArgs
 * interface, and we're unlikely to add one (functions shouldn't be implicitly
 * exposing the global object to arbitrary callers).  Continue using |vp|
 * directly for this case, but be aware this API will eventually be replaced
 * with a function that operates directly upon |args.thisv()|.
 */
#define JS_THIS_OBJECT(cx,vp)   (JS_THIS(cx,vp).toObjectOrNull())

/*
 * |this| is passed to functions in ES5 without change.  Functions themselves
 * do any post-processing they desire to box |this|, compute the global object,
 * &c.  This macro retrieves a function's unboxed |this| value.
 *
 * This macro must not be used in conjunction with JS_THIS or JS_THIS_OBJECT,
 * or vice versa.  Either use the provided this value with this macro, or
 * compute the boxed |this| value using those.  JS_THIS_VALUE must not be used
 * if the function is being called as a constructor.
 *
 * But: DO NOT USE THIS!  Instead use JS::CallArgs::thisv(), above.
 *
 */
#define JS_THIS_VALUE(cx,vp)    ((vp)[1])

#endif /* js_CallArgs_h */