src: distinguish refed/unrefed threadsafe Immediates

In some situations, it can be useful to use threadsafe callbacks
on an `Environment` to perform cleanup operations that should run
even when the process would otherwise be ending.

PR-URL: #33320
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

src/env-inl.h

@@ -752,9 +752,9 @@ void Environment::SetUnrefImmediate(Fn&& cb) {
 }
 
 template <typename Fn>
-void Environment::SetImmediateThreadsafe(Fn&& cb) {
+void Environment::SetImmediateThreadsafe(Fn&& cb, bool refed) {
   auto callback =
-      native_immediates_threadsafe_.CreateCallback(std::move(cb), false);
+      native_immediates_threadsafe_.CreateCallback(std::move(cb), refed);
   {
     Mutex::ScopedLock lock(native_immediates_threadsafe_mutex_);
     native_immediates_threadsafe_.Push(std::move(callback));

src/env.cc

@@ -704,19 +704,10 @@ void Environment::RunAndClearNativeImmediates(bool only_refed) {
   // exceptions, so we do not need to handle that.
   RunAndClearInterrupts();
 
-  // It is safe to check .size() first, because there is a causal relationship
-  // between pushes to the threadsafe and this function being called.
-  // For the common case, it's worth checking the size first before establishing
-  // a mutex lock.
-  if (native_immediates_threadsafe_.size() > 0) {
-    Mutex::ScopedLock lock(native_immediates_threadsafe_mutex_);
-    native_immediates_.ConcatMove(std::move(native_immediates_threadsafe_));
-  }
-
-  auto drain_list = [&]() {
+  auto drain_list = [&](NativeImmediateQueue* queue) {
     TryCatchScope try_catch(this);
     DebugSealHandleScope seal_handle_scope(isolate());
-    while (auto head = native_immediates_.Shift()) {
+    while (auto head = queue->Shift()) {
       if (head->is_refed())
         ref_count++;
 
@@ -734,12 +725,26 @@ void Environment::RunAndClearNativeImmediates(bool only_refed) {
     }
     return false;
   };
-  while (drain_list()) {}
+  while (drain_list(&native_immediates_)) {}
 
   immediate_info()->ref_count_dec(ref_count);
 
   if (immediate_info()->ref_count() == 0)
     ToggleImmediateRef(false);
+
+  // It is safe to check .size() first, because there is a causal relationship
+  // between pushes to the threadsafe immediate list and this function being
+  // called. For the common case, it's worth checking the size first before
+  // establishing a mutex lock.
+  // This is intentionally placed after the `ref_count` handling, because when
+  // refed threadsafe immediates are created, they are not counted towards the
+  // count in immediate_info() either.
+  NativeImmediateQueue threadsafe_immediates;
+  if (native_immediates_threadsafe_.size() > 0) {
+    Mutex::ScopedLock lock(native_immediates_threadsafe_mutex_);
+    threadsafe_immediates.ConcatMove(std::move(native_immediates_threadsafe_));
+  }
+  while (drain_list(&threadsafe_immediates)) {}
 }
 
 void Environment::RequestInterruptFromV8() {

src/env.h

@@ -1185,7 +1185,7 @@ class Environment : public MemoryRetainer {
   inline void SetUnrefImmediate(Fn&& cb);
   template <typename Fn>
   // This behaves like SetImmediate() but can be called from any thread.
-  inline void SetImmediateThreadsafe(Fn&& cb);
+  inline void SetImmediateThreadsafe(Fn&& cb, bool refed = true);
   // This behaves like V8's Isolate::RequestInterrupt(), but also accounts for
   // the event loop (i.e. combines the V8 function with SetImmediate()).
   // The passed callback may not throw exceptions.

src/node_worker.cc

@@ -764,7 +764,7 @@ void Worker::TakeHeapSnapshot(const FunctionCallbackInfo<Value>& args) {
               env, std::move(snapshot));
           Local<Value> args[] = { stream->object() };
           taker->MakeCallback(env->ondone_string(), arraysize(args), args);
-        });
+        }, /* refed */ false);
   });
   args.GetReturnValue().Set(scheduled ? taker->object() : Local<Object>());
 }