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node-api: make reference weak parameter an indirect link to references #38000

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98 changes: 81 additions & 17 deletions src/js_native_api_v8.cc
Original file line number Diff line number Diff line change
Expand Up @@ -314,16 +314,16 @@ class RefBase : protected Finalizer, RefTracker {
};

class Reference : public RefBase {
using SecondPassCallParameterRef = Reference*;

protected:
template <typename... Args>
Reference(napi_env env,
v8::Local<v8::Value> value,
Args&&... args)
Reference(napi_env env, v8::Local<v8::Value> value, Args&&... args)
: RefBase(env, std::forward<Args>(args)...),
_persistent(env->isolate, value) {
_persistent(env->isolate, value),
_secondPassParameter(new SecondPassCallParameterRef(this)) {
if (RefCount() == 0) {
_persistent.SetWeak(
this, FinalizeCallback, v8::WeakCallbackType::kParameter);
SetWeak();
}
}

Expand All @@ -344,10 +344,19 @@ class Reference : public RefBase {
finalize_hint);
}

virtual ~Reference() {
// If the second pass callback is scheduled, it will delete the
// parameter passed to it, otherwise it will never be scheduled
// and we need to delete it here.
if (_secondPassParameter != nullptr) {
delete _secondPassParameter;
}
}

inline uint32_t Ref() {
uint32_t refcount = RefBase::Ref();
if (refcount == 1) {
_persistent.ClearWeak();
ClearWeak();
}
return refcount;
}
Expand All @@ -356,8 +365,7 @@ class Reference : public RefBase {
uint32_t old_refcount = RefCount();
uint32_t refcount = RefBase::Unref();
if (old_refcount == 1 && refcount == 0) {
_persistent.SetWeak(
this, FinalizeCallback, v8::WeakCallbackType::kParameter);
SetWeak();
}
return refcount;
}
Expand All @@ -374,38 +382,94 @@ class Reference : public RefBase {
inline void Finalize(bool is_env_teardown = false) override {
// During env teardown, `~napi_env()` alone is responsible for finalizing.
// Thus, we don't want any stray gc passes to trigger a second call to
// `Finalize()`, so let's reset the persistent here if nothing is
// keeping it alive.
if (is_env_teardown && _persistent.IsWeak()) {
_persistent.ClearWeak();
// `RefBase::Finalize()`. ClearWeak will ensure that even if the
// gc is in progress no Finalization will be run for this Reference
// by the gc.
if (is_env_teardown) {
ClearWeak();
}

// Chain up to perform the rest of the finalization.
RefBase::Finalize(is_env_teardown);
}

private:
// ClearWeak is marking the Reference so that the gc should not
// collect it, but it is possible that a second pass callback
// may have been scheduled already if we are in shutdown. We clear
// the secondPassParameter so that even if it has been
// secheduled no Finalization will be run.
inline void ClearWeak() {
if (!_persistent.IsEmpty()) {
_persistent.ClearWeak();
}
if (_secondPassParameter != nullptr) {
*_secondPassParameter = nullptr;
}
}

// Mark the reference as weak and eligible for collection
// by the gc.
inline void SetWeak() {
if (_secondPassParameter == nullptr) {
// This means that the Reference has already been processed
// by the second pass calback, so its already been Finalized, do
// nothing
return;
}
_persistent.SetWeak(
_secondPassParameter, FinalizeCallback,
v8::WeakCallbackType::kParameter);
*_secondPassParameter = this;
}

// The N-API finalizer callback may make calls into the engine. V8's heap is
// not in a consistent state during the weak callback, and therefore it does
// not support calls back into it. However, it provides a mechanism for adding
// a finalizer which may make calls back into the engine by allowing us to
// attach such a second-pass finalizer from the first pass finalizer. Thus,
// we do that here to ensure that the N-API finalizer callback is free to call
// into the engine.
static void FinalizeCallback(const v8::WeakCallbackInfo<Reference>& data) {
Reference* reference = data.GetParameter();
static void FinalizeCallback(
const v8::WeakCallbackInfo<SecondPassCallParameterRef>& data) {
SecondPassCallParameterRef* parameter = data.GetParameter();
Reference* reference = *parameter;
if (reference == nullptr) {
return;
}

// The reference must be reset during the first pass.
reference->_persistent.Reset();
// Mark the parameter not delete-able until the second pass callback is
// invoked.
reference->_secondPassParameter = nullptr;

data.SetSecondPassCallback(SecondPassCallback);
}

static void SecondPassCallback(const v8::WeakCallbackInfo<Reference>& data) {
data.GetParameter()->Finalize();
// Second pass callbacks are scheduled with platform tasks. At env teardown,
// the tasks may have already be scheduled and we are unable to cancel the
// second pass callback task. We have to make sure that parameter is kept
// alive until the second pass callback is been invoked. In order to do
// this and still allow our code to Finalize/delete the Reference during
// shutdown we have to use a seperately allocated parameter instead
// of a parameter within the Reference object itself. This is what
// is stored in _secondPassParameter and it is alocated in the
// constructor for the Reference.
static void SecondPassCallback(
const v8::WeakCallbackInfo<SecondPassCallParameterRef>& data) {
SecondPassCallParameterRef* parameter = data.GetParameter();
Reference* reference = *parameter;
delete parameter;
if (reference == nullptr) {
// the reference itself has already been deleted so nothing to do
return;
}
reference->Finalize();
}

v8impl::Persistent<v8::Value> _persistent;
SecondPassCallParameterRef* _secondPassParameter;
};

enum UnwrapAction {
Expand Down