src: cancel pending delayed platform tasks on exit

Worker threads need an event loop without active libuv handles in
order to shut down. One source of handles that was previously
not accounted for were delayed V8 tasks; these create timers
that would be standing in the way of clearing the event loop.

To solve this, keep track of the scheduled tasks in a list
and close their timer handles before the corresponding isolate/loop
is removed from the platform.

It is not clear from the V8 documentation what the expectation is
with respect to pending background tasks at the end of the
isolate lifetime; however, an alternative approach of executing
these scheduled tasks when flushing them led to an infinite loop
of tasks scheduling each other; so it seems safe to assume that
the behaviour implemented in this patch is at least acceptable.

Original-PR-URL: ayojs/ayo#120
Original-Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
PR-URL: #16700
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node.cc

@@ -284,6 +284,10 @@ static struct {
     platform_->DrainBackgroundTasks(isolate);
   }
 
+  void CancelVMTasks(Isolate* isolate) {
+    platform_->CancelPendingDelayedTasks(isolate);
+  }
+
 #if HAVE_INSPECTOR
   bool StartInspector(Environment *env, const char* script_path,
                       const node::DebugOptions& options) {
@@ -316,6 +320,7 @@ static struct {
   void Initialize(int thread_pool_size) {}
   void Dispose() {}
   void DrainVMTasks(Isolate* isolate) {}
+  void CancelVMTasks(Isolate* isolate) {}
   bool StartInspector(Environment *env, const char* script_path,
                       const node::DebugOptions& options) {
     env->ThrowError("Node compiled with NODE_USE_V8_PLATFORM=0");
@@ -4891,6 +4896,7 @@ inline int Start(Isolate* isolate, IsolateData* isolate_data,
   uv_key_delete(&thread_local_env);
 
   v8_platform.DrainVMTasks(isolate);
+  v8_platform.CancelVMTasks(isolate);
   WaitForInspectorDisconnect(&env);
 #if defined(LEAK_SANITIZER)
   __lsan_do_leak_check();

src/node.h

@@ -214,6 +214,7 @@ class MultiIsolatePlatform : public v8::Platform {
  public:
   virtual ~MultiIsolatePlatform() { }
   virtual void DrainBackgroundTasks(v8::Isolate* isolate) = 0;
+  virtual void CancelPendingDelayedTasks(v8::Isolate* isolate) = 0;
 
   // These will be called by the `IsolateData` creation/destruction functions.
   virtual void RegisterIsolate(IsolateData* isolate_data,

src/node_platform.cc

@@ -4,6 +4,7 @@
 #include "env.h"
 #include "env-inl.h"
 #include "util.h"
+#include <algorithm>
 
 namespace node {
 
@@ -45,13 +46,17 @@ void PerIsolatePlatformData::CallOnForegroundThread(Task* task) {
 
 void PerIsolatePlatformData::CallDelayedOnForegroundThread(
     Task* task, double delay_in_seconds) {
-  auto pair = new std::pair<Task*, double>(task, delay_in_seconds);
-  foreground_delayed_tasks_.Push(pair);
+  auto delayed = new DelayedTask();
+  delayed->task = task;
+  delayed->platform_data = this;
+  delayed->timeout = delay_in_seconds;
+  foreground_delayed_tasks_.Push(delayed);
   uv_async_send(flush_tasks_);
 }
 
 PerIsolatePlatformData::~PerIsolatePlatformData() {
   FlushForegroundTasksInternal();
+  CancelPendingDelayedTasks();
 
   uv_close(reinterpret_cast<uv_handle_t*>(flush_tasks_),
            [](uv_handle_t* handle) {
@@ -120,7 +125,7 @@ size_t NodePlatform::NumberOfAvailableBackgroundThreads() {
   return threads_.size();
 }
 
-static void RunForegroundTask(Task* task) {
+void PerIsolatePlatformData::RunForegroundTask(Task* task) {
   Isolate* isolate = Isolate::GetCurrent();
   HandleScope scope(isolate);
   Environment* env = Environment::GetCurrent(isolate);
@@ -130,14 +135,29 @@ static void RunForegroundTask(Task* task) {
   delete task;
 }
 
-static void RunForegroundTask(uv_timer_t* handle) {
-  Task* task = static_cast<Task*>(handle->data);
-  RunForegroundTask(task);
-  uv_close(reinterpret_cast<uv_handle_t*>(handle), [](uv_handle_t* handle) {
-    delete reinterpret_cast<uv_timer_t*>(handle);
+void PerIsolatePlatformData::RunForegroundTask(uv_timer_t* handle) {
+  DelayedTask* delayed = static_cast<DelayedTask*>(handle->data);
+  auto& tasklist = delayed->platform_data->scheduled_delayed_tasks_;
+  auto it = std::find(tasklist.begin(), tasklist.end(), delayed);
+  CHECK_NE(it, tasklist.end());
+  tasklist.erase(it);
+  RunForegroundTask(delayed->task);
+  uv_close(reinterpret_cast<uv_handle_t*>(&delayed->timer),
+           [](uv_handle_t* handle) {
+    delete static_cast<DelayedTask*>(handle->data);
   });
 }
 
+void PerIsolatePlatformData::CancelPendingDelayedTasks() {
+  for (auto delayed : scheduled_delayed_tasks_) {
+    uv_close(reinterpret_cast<uv_handle_t*>(&delayed->timer),
+             [](uv_handle_t* handle) {
+      delete static_cast<DelayedTask*>(handle->data);
+    });
+  }
+  scheduled_delayed_tasks_.clear();
+}
+
 void NodePlatform::DrainBackgroundTasks(Isolate* isolate) {
   PerIsolatePlatformData* per_isolate = ForIsolate(isolate);
 
@@ -152,18 +172,18 @@ void NodePlatform::DrainBackgroundTasks(Isolate* isolate) {
 
 bool PerIsolatePlatformData::FlushForegroundTasksInternal() {
   bool did_work = false;
+
   while (auto delayed = foreground_delayed_tasks_.Pop()) {
     did_work = true;
     uint64_t delay_millis =
-        static_cast<uint64_t>(delayed->second + 0.5) * 1000;
-    uv_timer_t* handle = new uv_timer_t();
-    handle->data = static_cast<void*>(delayed->first);
-    uv_timer_init(loop_, handle);
+        static_cast<uint64_t>(delayed->timeout + 0.5) * 1000;
+    delayed->timer.data = static_cast<void*>(delayed);
+    uv_timer_init(loop_, &delayed->timer);
     // Timers may not guarantee queue ordering of events with the same delay if
     // the delay is non-zero. This should not be a problem in practice.
-    uv_timer_start(handle, RunForegroundTask, delay_millis, 0);
-    uv_unref(reinterpret_cast<uv_handle_t*>(handle));
-    delete delayed;
+    uv_timer_start(&delayed->timer, RunForegroundTask, delay_millis, 0);
+    uv_unref(reinterpret_cast<uv_handle_t*>(&delayed->timer));
+    scheduled_delayed_tasks_.push_back(delayed);
   }
   while (Task* task = foreground_tasks_.Pop()) {
     did_work = true;
@@ -199,6 +219,10 @@ void NodePlatform::FlushForegroundTasks(v8::Isolate* isolate) {
   ForIsolate(isolate)->FlushForegroundTasksInternal();
 }
 
+void NodePlatform::CancelPendingDelayedTasks(v8::Isolate* isolate) {
+  ForIsolate(isolate)->CancelPendingDelayedTasks();
+}
+
 bool NodePlatform::IdleTasksEnabled(Isolate* isolate) { return false; }
 
 double NodePlatform::MonotonicallyIncreasingTime() {

src/node_platform.h

@@ -14,6 +14,7 @@ namespace node {
 
 class NodePlatform;
 class IsolateData;
+class PerIsolatePlatformData;
 
 template <class T>
 class TaskQueue {
@@ -37,6 +38,13 @@ class TaskQueue {
   std::queue<T*> task_queue_;
 };
 
+struct DelayedTask {
+  v8::Task* task;
+  uv_timer_t timer;
+  double timeout;
+  PerIsolatePlatformData* platform_data;
+};
+
 class PerIsolatePlatformData {
  public:
   PerIsolatePlatformData(v8::Isolate* isolate, uv_loop_t* loop);
@@ -52,15 +60,20 @@ class PerIsolatePlatformData {
 
   // Returns true iff work was dispatched or executed.
   bool FlushForegroundTasksInternal();
+  void CancelPendingDelayedTasks();
+
  private:
   static void FlushTasks(uv_async_t* handle);
+  static void RunForegroundTask(v8::Task* task);
+  static void RunForegroundTask(uv_timer_t* timer);
 
   int ref_count_ = 1;
   v8::Isolate* isolate_;
   uv_loop_t* const loop_;
   uv_async_t* flush_tasks_ = nullptr;
   TaskQueue<v8::Task> foreground_tasks_;
-  TaskQueue<std::pair<v8::Task*, double>> foreground_delayed_tasks_;
+  TaskQueue<DelayedTask> foreground_delayed_tasks_;
+  std::vector<DelayedTask*> scheduled_delayed_tasks_;
 };
 
 class NodePlatform : public MultiIsolatePlatform {
@@ -69,6 +82,7 @@ class NodePlatform : public MultiIsolatePlatform {
   virtual ~NodePlatform() {}
 
   void DrainBackgroundTasks(v8::Isolate* isolate) override;
+  void CancelPendingDelayedTasks(v8::Isolate* isolate) override;
   void Shutdown();
 
   // v8::Platform implementation.