js/public/CallArgs.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * [SMDOC] JS::CallArgs API
   9  *
  10  * Helper classes encapsulating access to the callee, |this| value, arguments,
  11  * and argument count for a call/construct operation.
  12  *
  13  * JS::CallArgs encapsulates access to a JSNative's un-abstracted
  14  * |unsigned argc, Value* vp| arguments.  The principal way to create a
  15  * JS::CallArgs is using JS::CallArgsFromVp:
  16  *
  17  *   // If provided no arguments or a non-numeric first argument, return zero.
  18  *   // Otherwise return |this| exactly as given, without boxing.
  19  *   static bool
  20  *   Func(JSContext* cx, unsigned argc, JS::Value* vp)
  21  *   {
  22  *       JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
  23  *
  24  *       // Guard against no arguments or a non-numeric arg0.
  25  *       if (args.length() == 0 || !args[0].isNumber()) {
  26  *           args.rval().setInt32(0);
  27  *           return true;
  28  *       }
  29  *
  30  *       // Access to the callee must occur before accessing/setting
  31  *       // the return value.
  32  *       JSObject& callee = args.callee();
  33  *       args.rval().setObject(callee);
  34  *
  35  *       // callee() and calleev() will now assert.
  36  *
  37  *       // It's always fine to access thisv().
  38  *       HandleValue thisv = args.thisv();
  39  *       args.rval().set(thisv);
  40  *
  41  *       // As the return value was last set to |this|, returns |this|.
  42  *       return true;
  43  *   }
  44  *
  45  * CallArgs is exposed publicly and used internally.  Not all parts of its
  46  * public interface are meant to be used by embedders!  See inline comments to
  47  * for details.
  48  *
  49  * It's possible (albeit deprecated) to manually index into |vp| to access the
  50  * callee, |this|, and arguments of a function, and to set its return value.
  51  * This does not have the error-handling or moving-GC correctness of CallArgs.
  52  * New code should use CallArgs instead whenever possible.
  53  *
  54  * The eventual plan is to change JSNative to take |const CallArgs&| directly,
  55  * for automatic assertion of correct use and to make calling functions more
  56  * efficient.  Embedders should start internally switching away from using
  57  * |argc| and |vp| directly, except to create a |CallArgs|.  Then, when an
  58  * eventual release making that change occurs, porting efforts will require
  59  * changing methods' signatures but won't require invasive changes to the
  60  * methods' implementations, potentially under time pressure.
  61  */
  62
  63 #ifndef js_CallArgs_h
  64 #define js_CallArgs_h
  65
  66 #include "mozilla/Assertions.h"
  67 #include "mozilla/Attributes.h"
  68
  69 #include <type_traits>
  70
  71 #include "jstypes.h"
  72
  73 #include "js/RootingAPI.h"
  74 #include "js/Value.h"
  75
  76 /* Typedef for native functions called by the JS VM. */
  77 using JSNative = bool (*)(JSContext* cx, unsigned argc, JS::Value* vp);
  78
  79 namespace JS {
  80
  81 extern JS_PUBLIC_DATA const HandleValue UndefinedHandleValue;
  82
  83 namespace detail {
  84
  85 /*
  86  * Compute |this| for the |vp| inside a JSNative, either boxing primitives or
  87  * replacing with the global object as necessary.
  88  */
  89 extern JS_PUBLIC_API bool ComputeThis(JSContext* cx, JS::Value* vp,
  90                                       MutableHandleObject thisObject);
  91
  92 #ifdef JS_DEBUG
  93 extern JS_PUBLIC_API void CheckIsValidConstructible(const Value& v);
  94 #endif
  95
  96 class MOZ_STACK_CLASS IncludeUsedRval {
  97   mutable bool usedRval_ = false;
  98
  99  public:
 100   bool usedRval() const { return usedRval_; }
 101   void setUsedRval() const { usedRval_ = true; }
 102   void clearUsedRval() const { usedRval_ = false; }
 103   void assertUnusedRval() const { MOZ_ASSERT(!usedRval_); }
 104 };
 105
 106 class MOZ_STACK_CLASS NoUsedRval {
 107  public:
 108   bool usedRval() const { return false; }
 109   void setUsedRval() const {}
 110   void clearUsedRval() const {}
 111   void assertUnusedRval() const {}
 112 };
 113
 114 template <class WantUsedRval>
 115 class MOZ_STACK_CLASS CallArgsBase {
 116   static_assert(std::is_same_v<WantUsedRval, IncludeUsedRval> ||
 117                     std::is_same_v<WantUsedRval, NoUsedRval>,
 118                 "WantUsedRval can only be IncludeUsedRval or NoUsedRval");
 119
 120  protected:
 121   Value* argv_ = nullptr;
 122   unsigned argc_ = 0;
 123   bool constructing_ : 1;
 124
 125   // True if the caller does not use the return value.
 126   bool ignoresReturnValue_ : 1;
 127
 128 #ifdef JS_DEBUG
 129   WantUsedRval wantUsedRval_;
 130   bool usedRval() const { return wantUsedRval_.usedRval(); }
 131   void setUsedRval() const { wantUsedRval_.setUsedRval(); }
 132   void clearUsedRval() const { wantUsedRval_.clearUsedRval(); }
 133   void assertUnusedRval() const { wantUsedRval_.assertUnusedRval(); }
 134 #else
 135   bool usedRval() const { return false; }
 136   void setUsedRval() const {}
 137   void clearUsedRval() const {}
 138   void assertUnusedRval() const {}
 139 #endif
 140
 141   CallArgsBase() : constructing_(false), ignoresReturnValue_(false) {}
 142
 143  public:
 144   // CALLEE ACCESS
 145
 146   /*
 147    * Returns the function being called, as a value.  Must not be called after
 148    * rval() has been used!
 149    */
 150   HandleValue calleev() const {
 151     this->assertUnusedRval();
 152     return HandleValue::fromMarkedLocation(&argv_[-2]);
 153   }
 154
 155   /*
 156    * Returns the function being called, as an object.  Must not be called
 157    * after rval() has been used!
 158    */
 159   JSObject& callee() const { return calleev().toObject(); }
 160
 161   // CALLING/CONSTRUCTING-DIFFERENTIATIONS
 162
 163   bool isConstructing() const {
 164     if (!argv_[-1].isMagic()) {
 165       return false;
 166     }
 167
 168 #ifdef JS_DEBUG
 169     if (!this->usedRval()) {
 170       CheckIsValidConstructible(calleev());
 171     }
 172 #endif
 173
 174     return true;
 175   }
 176
 177   bool ignoresReturnValue() const { return ignoresReturnValue_; }
 178
 179   MutableHandleValue newTarget() const {
 180     MOZ_ASSERT(constructing_);
 181     return MutableHandleValue::fromMarkedLocation(&this->argv_[argc_]);
 182   }
 183
 184   /*
 185    * Returns the |this| value passed to the function.  This method must not
 186    * be called when the function is being called as a constructor via |new|.
 187    * The value may or may not be an object: it is the individual function's
 188    * responsibility to box the value if needed.
 189    */
 190   HandleValue thisv() const {
 191     // Some internal code uses thisv() in constructing cases, so don't do
 192     // this yet.
 193     // MOZ_ASSERT(!argv_[-1].isMagic(JS_IS_CONSTRUCTING));
 194     return HandleValue::fromMarkedLocation(&argv_[-1]);
 195   }
 196
 197   bool computeThis(JSContext* cx, MutableHandleObject thisObject) const {
 198     if (thisv().isObject()) {
 199       thisObject.set(&thisv().toObject());
 200       return true;
 201     }
 202
 203     return ComputeThis(cx, base(), thisObject);
 204   }
 205
 206   // ARGUMENTS
 207
 208   /* Returns the number of arguments. */
 209   unsigned length() const { return argc_; }
 210
 211   /* Returns the i-th zero-indexed argument. */
 212   MutableHandleValue operator[](unsigned i) const {
 213     MOZ_ASSERT(i < argc_);
 214     return MutableHandleValue::fromMarkedLocation(&this->argv_[i]);
 215   }
 216
 217   /*
 218    * Returns the i-th zero-indexed argument, or |undefined| if there's no
 219    * such argument.
 220    */
 221   HandleValue get(unsigned i) const {
 222     return i < length() ? HandleValue::fromMarkedLocation(&this->argv_[i])
 223                         : UndefinedHandleValue;
 224   }
 225
 226   /*
 227    * Returns true if the i-th zero-indexed argument is present and is not
 228    * |undefined|.
 229    */
 230   bool hasDefined(unsigned i) const {
 231     return i < argc_ && !this->argv_[i].isUndefined();
 232   }
 233
 234   // RETURN VALUE
 235
 236   /*
 237    * Returns the currently-set return value.  The initial contents of this
 238    * value are unspecified.  Once this method has been called, callee() and
 239    * calleev() can no longer be used.  (If you're compiling against a debug
 240    * build of SpiderMonkey, these methods will assert to aid debugging.)
 241    *
 242    * If the method you're implementing succeeds by returning true, you *must*
 243    * set this.  (SpiderMonkey doesn't currently assert this, but it will do
 244    * so eventually.)  You don't need to use or change this if your method
 245    * fails.
 246    */
 247   MutableHandleValue rval() const {
 248     this->setUsedRval();
 249     return MutableHandleValue::fromMarkedLocation(&argv_[-2]);
 250   }
 251
 252   /*
 253    * Returns true if there are at least |required| arguments passed in. If
 254    * false, it reports an error message on the context.
 255    */
 256   JS_PUBLIC_API inline bool requireAtLeast(JSContext* cx, const char* fnname,
 257                                            unsigned required) const;
 258
 259  public:
 260   // These methods are publicly exposed, but they are *not* to be used when
 261   // implementing a JSNative method and encapsulating access to |vp| within
 262   // it.  You probably don't want to use these!
 263
 264   void setCallee(const Value& aCalleev) const {
 265     this->clearUsedRval();
 266     argv_[-2] = aCalleev;
 267   }
 268
 269   void setThis(const Value& aThisv) const { argv_[-1] = aThisv; }
 270
 271   MutableHandleValue mutableThisv() const {
 272     return MutableHandleValue::fromMarkedLocation(&argv_[-1]);
 273   }
 274
 275  public:
 276   // These methods are publicly exposed, but we're unsure of the interfaces
 277   // (because they're hackish and drop assertions).  Avoid using these if you
 278   // can.
 279
 280   Value* array() const { return argv_; }
 281   Value* end() const { return argv_ + argc_ + constructing_; }
 282
 283  public:
 284   // These methods are only intended for internal use.  Embedders shouldn't
 285   // use them!
 286
 287   Value* base() const { return argv_ - 2; }
 288
 289   Value* spAfterCall() const {
 290     this->setUsedRval();
 291     return argv_ - 1;
 292   }
 293 };
 294
 295 }  // namespace detail
 296
 297 class MOZ_STACK_CLASS CallArgs
 298     : public detail::CallArgsBase<detail::IncludeUsedRval> {
 299  private:
 300   friend CallArgs CallArgsFromVp(unsigned argc, Value* vp);
 301   friend CallArgs CallArgsFromSp(unsigned stackSlots, Value* sp,
 302                                  bool constructing, bool ignoresReturnValue);
 303
 304   static CallArgs create(unsigned argc, Value* argv, bool constructing,
 305                          bool ignoresReturnValue = false) {
 306     CallArgs args;
 307     args.clearUsedRval();
 308     args.argv_ = argv;
 309     args.argc_ = argc;
 310     args.constructing_ = constructing;
 311     args.ignoresReturnValue_ = ignoresReturnValue;
 312 #ifdef DEBUG
 313     AssertValueIsNotGray(args.thisv());
 314     AssertValueIsNotGray(args.calleev());
 315     for (unsigned i = 0; i < argc; ++i) {
 316       AssertValueIsNotGray(argv[i]);
 317     }
 318 #endif
 319     return args;
 320   }
 321
 322  public:
 323   /*
 324    * Helper for requireAtLeast to report the actual exception.  Public
 325    * so we can call it from CallArgsBase and not need multiple
 326    * per-template instantiations of it.
 327    */
 328   static JS_PUBLIC_API void reportMoreArgsNeeded(JSContext* cx,
 329                                                  const char* fnname,
 330                                                  unsigned required,
 331                                                  unsigned actual);
 332 };
 333
 334 namespace detail {
 335 template <class WantUsedRval>
 336 JS_PUBLIC_API inline bool CallArgsBase<WantUsedRval>::requireAtLeast(
 337     JSContext* cx, const char* fnname, unsigned required) const {
 338   if (MOZ_LIKELY(required <= length())) {
 339     return true;
 340   }
 341
 342   CallArgs::reportMoreArgsNeeded(cx, fnname, required, length());
 343   return false;
 344 }
 345 }  // namespace detail
 346
 347 MOZ_ALWAYS_INLINE CallArgs CallArgsFromVp(unsigned argc, Value* vp) {
 348   return CallArgs::create(argc, vp + 2, vp[1].isMagic(JS_IS_CONSTRUCTING));
 349 }
 350
 351 // This method is only intended for internal use in SpiderMonkey.  We may
 352 // eventually move it to an internal header.  Embedders should use
 353 // JS::CallArgsFromVp!
 354 MOZ_ALWAYS_INLINE CallArgs CallArgsFromSp(unsigned stackSlots, Value* sp,
 355                                           bool constructing = false,
 356                                           bool ignoresReturnValue = false) {
 357   return CallArgs::create(stackSlots - constructing, sp - stackSlots,
 358                           constructing, ignoresReturnValue);
 359 }
 360
 361 }  // namespace JS
 362
 363 #endif /* js_CallArgs_h */