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ClassBuilder.cpp

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#include "ClassBuilder.h"

#include <mutex>

#include "UnfairLock.h"

namespace tns {

namespace {

// objc_allocateClassPair only rejects names that are already *registered*, so

// two threads extending the same class name concurrently can both allocate it

// and register duplicate same-named classes (objc keeps both and name lookups

// become ambiguous). Serialize the whole allocate -> register window.

// This lock is a leaf — the section never acquires a v8::Locker — so it cannot

// form a cycle with the isolate locks.

UnfairMutex extendedClassRegistrationMutex;

// Registered objc class names are never reclaimed and the ladder below

// allocates suffixes densely under the lock, so a name's taken suffixes form a

// contiguous prefix. Gallop + binary-search with objc_getClass probes to find

// its end, so heavy same-name reuse (worker tests, HMR re-extends of one

// class) stays O(log N) per extend with no side state to grow.

int FirstFreeSuffix(const std::string& initialName) {

auto taken = [&initialName](int i) {

return objc_getClass((initialName + std::to_string(i)).c_str()) != nil;

};

int hi = 1;

while (taken(hi)) {

hi *= 2;

}

int lo = hi / 2;

while (lo + 1 < hi) {

int mid = lo + (hi - lo) / 2;

if (taken(mid)) {

lo = mid;

} else {

hi = mid;

}

return hi;

}

// Bounds only *consecutive* failures past the probed free position, i.e.

// allocation failing for reasons other than an ordinary name collision (which

// would otherwise loop forever holding the mutex).

constexpr int kMaxConsecutiveAllocFailures = 100;

} // namespace

// Moved this method in a separate .cpp file because ARC destroys the class

// created with objc_allocateClassPair when the control leaves this method scope

Class ClassBuilder::GetExtendedClass(const std::string& baseClassName,

const std::string& staticClassName,

int isolateId) {

Class baseClass = objc_getClass(baseClassName.c_str());

std::string name = staticClassName;

if (name.empty()) {

name = baseClassName;

name += std::to_string(isolateId);

name += "__";

name += std::to_string(++ClassBuilder::classNameCounter_);

}

// Allocation failure is the collision signal (objc_getClass beforehand

// would race), but that only detects *registered* names — hence the lock

// spanning allocate -> register.

std::lock_guard<UnfairMutex> lock(extendedClassRegistrationMutex);

Class clazz = objc_allocateClassPair(baseClass, name.c_str(), 0);

if (clazz == nil) {

std::string initialName = name;

int next = FirstFreeSuffix(initialName);

for (int attempts = 0;

clazz == nil && attempts < kMaxConsecutiveAllocFailures; attempts++) {

name = initialName + std::to_string(next++);

clazz = objc_allocateClassPair(baseClass, name.c_str(), 0);

}

if (clazz == nil) {

return nil;

}

objc_registerClassPair(clazz);

return clazz;

}

} // namespace tns