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

legendecas · legendecas · commit 4afcd55274c7 · 2021-04-20T00:11:33.000+08:00
As the cancellation of second pass callbacks are not reachable from the current v8 API, and the second pass callbacks are scheduled with NodePlatform's task runner, we have to ensure that the weak parameter holds indirect access to the v8impl::Reference object so that the object can be destroyed on addon env teardown before the whole node env is able to shutdown. PR-URL: #38000 Reviewed-By: Michael Dawson <midawson@redhat.com> Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
diff --git a/src/js_native_api_v8.cc b/src/js_native_api_v8.cc
@@ -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();
     }
   }
 
@@ -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;
   }
@@ -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;
   }
@@ -377,39 +385,95 @@ class Reference : public RefBase {
 
     // 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();
   }
 
   bool env_teardown_finalize_started_ = false;
   v8impl::Persistent<v8::Value> _persistent;
+  SecondPassCallParameterRef* _secondPassParameter;
 };
 
 enum UnwrapAction {
