src: cleanup per-isolate state on platform on isolate unregister

Clean up once all references to an `Isolate*` are gone from the
`NodePlatform`, rather than waiting for the `PerIsolatePlatformData`
struct to be deleted since there may be cyclic references between
that struct and the individual tasks.

PR-URL: #20876
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>

src/node_platform.cc

@@ -82,12 +82,14 @@ void PerIsolatePlatformData::PostIdleTask(std::unique_ptr<v8::IdleTask> task) {
 }
 
 void PerIsolatePlatformData::PostTask(std::unique_ptr<Task> task) {
+  CHECK_NE(flush_tasks_, nullptr);
   foreground_tasks_.Push(std::move(task));
   uv_async_send(flush_tasks_);
 }
 
 void PerIsolatePlatformData::PostDelayedTask(
     std::unique_ptr<Task> task, double delay_in_seconds) {
+  CHECK_NE(flush_tasks_, nullptr);
   std::unique_ptr<DelayedTask> delayed(new DelayedTask());
   delayed->task = std::move(task);
   delayed->platform_data = shared_from_this();
@@ -97,13 +99,21 @@ void PerIsolatePlatformData::PostDelayedTask(
 }
 
 PerIsolatePlatformData::~PerIsolatePlatformData() {
+  Shutdown();
+}
+
+void PerIsolatePlatformData::Shutdown() {
+  if (flush_tasks_ == nullptr)
+    return;
+
   while (FlushForegroundTasksInternal()) {}
   CancelPendingDelayedTasks();
 
   uv_close(reinterpret_cast<uv_handle_t*>(flush_tasks_),
            [](uv_handle_t* handle) {
     delete reinterpret_cast<uv_async_t*>(handle);
   });
+  flush_tasks_ = nullptr;
 }
 
 void PerIsolatePlatformData::ref() {
@@ -144,6 +154,7 @@ void NodePlatform::UnregisterIsolate(IsolateData* isolate_data) {
   std::shared_ptr<PerIsolatePlatformData> existing = per_isolate_[isolate];
   CHECK(existing);
   if (existing->unref() == 0) {
+    existing->Shutdown();
     per_isolate_.erase(isolate);
   }
 }