src: Node implementation of v8::Platform

Node.js currently uses the V8 implementation of the DefaultPlatform
which schedules VM tasks on a V8 managed thread pool. Since the Node.js
event loop is not aware of these tasks, the Node.js process may exit
while there are outstanding VM tasks. This will become problematic once
asynchronous wasm compilation lands in V8.

This PR introduces a Node.js specific implementation of the v8::Platform
on top of libuv so that the event loop is aware of outstanding VM tasks.

PR-URL: #14001
Fixes: #3665
Fixes: #8496
Fixes: #12980
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: Matt Loring

node.gyp

@@ -189,6 +189,7 @@
         'src/node_http_parser.cc',
         'src/node_main.cc',
         'src/node_os.cc',
+        'src/node_platform.cc',
         'src/node_revert.cc',
         'src/node_serdes.cc',
         'src/node_url.cc',
@@ -237,6 +238,7 @@
         'src/node_internals.h',
         'src/node_javascript.h',
         'src/node_mutex.h',
+        'src/node_platform.h',
         'src/node_root_certs.h',
         'src/node_version.h',
         'src/node_watchdog.h',
@@ -655,6 +657,8 @@
       'defines': [ 'NODE_WANT_INTERNALS=1' ],
 
       'sources': [
+        'src/node_platform.cc',
+        'src/node_platform.h',
         'test/cctest/test_base64.cc',
         'test/cctest/test_environment.cc',
         'test/cctest/test_util.cc',

src/inspector_agent.cc

@@ -495,11 +495,9 @@ class InspectorTimerHandle {
 
 class NodeInspectorClient : public V8InspectorClient {
  public:
-  NodeInspectorClient(node::Environment* env,
-                      v8::Platform* platform) : env_(env),
-                                                platform_(platform),
-                                                terminated_(false),
-                                                running_nested_loop_(false) {
+  NodeInspectorClient(node::Environment* env, node::NodePlatform* platform)
+      : env_(env), platform_(platform), terminated_(false),
+        running_nested_loop_(false) {
     client_ = V8Inspector::create(env->isolate(), this);
     contextCreated(env->context(), "Node.js Main Context");
   }
@@ -511,8 +509,7 @@ class NodeInspectorClient : public V8InspectorClient {
     terminated_ = false;
     running_nested_loop_ = true;
     while (!terminated_ && channel_->waitForFrontendMessage()) {
-      while (v8::platform::PumpMessageLoop(platform_, env_->isolate()))
-        {}
+      platform_->FlushForegroundTasksInternal();
     }
     terminated_ = false;
     running_nested_loop_ = false;
@@ -618,7 +615,7 @@ class NodeInspectorClient : public V8InspectorClient {
 
  private:
   node::Environment* env_;
-  v8::Platform* platform_;
+  node::NodePlatform* platform_;
   bool terminated_;
   bool running_nested_loop_;
   std::unique_ptr<V8Inspector> client_;
@@ -637,7 +634,7 @@ Agent::Agent(Environment* env) : parent_env_(env),
 Agent::~Agent() {
 }
 
-bool Agent::Start(v8::Platform* platform, const char* path,
+bool Agent::Start(node::NodePlatform* platform, const char* path,
                   const DebugOptions& options) {
   path_ = path == nullptr ? "" : path;
   debug_options_ = options;

src/inspector_agent.h

@@ -14,6 +14,7 @@
 // Forward declaration to break recursive dependency chain with src/env.h.
 namespace node {
 class Environment;
+class NodePlatform;
 }  // namespace node
 
 namespace v8 {
@@ -52,7 +53,7 @@ class Agent {
   ~Agent();
 
   // Create client_, may create io_ if option enabled
-  bool Start(v8::Platform* platform, const char* path,
+  bool Start(node::NodePlatform* platform, const char* path,
              const DebugOptions& options);
   // Stop and destroy io_
   void Stop();

src/node.cc

@@ -23,6 +23,7 @@
 #include "node_buffer.h"
 #include "node_constants.h"
 #include "node_javascript.h"
+#include "node_platform.h"
 #include "node_version.h"
 #include "node_internals.h"
 #include "node_revert.h"
@@ -250,25 +251,26 @@ node::DebugOptions debug_options;
 
 static struct {
 #if NODE_USE_V8_PLATFORM
-  void Initialize(int thread_pool_size) {
+  void Initialize(int thread_pool_size, uv_loop_t* loop) {
     tracing_agent_ =
-      trace_enabled ? new tracing::Agent() : nullptr;
-    platform_ = v8::platform::CreateDefaultPlatform(
-      thread_pool_size, v8::platform::IdleTaskSupport::kDisabled,
-      v8::platform::InProcessStackDumping::kDisabled,
-      trace_enabled ? tracing_agent_->GetTracingController() : nullptr);
+        trace_enabled ? new tracing::Agent() : nullptr;
+    platform_ = new NodePlatform(thread_pool_size, loop,
+        trace_enabled ? tracing_agent_->GetTracingController() : nullptr);
     V8::InitializePlatform(platform_);
     tracing::TraceEventHelper::SetTracingController(
-      trace_enabled ? tracing_agent_->GetTracingController() : nullptr);
-  }
-
-  void PumpMessageLoop(Isolate* isolate) {
-    v8::platform::PumpMessageLoop(platform_, isolate);
+        trace_enabled ? tracing_agent_->GetTracingController() : nullptr);
   }
 
   void Dispose() {
+    platform_->Shutdown();
     delete platform_;
     platform_ = nullptr;
+    delete tracing_agent_;
+    tracing_agent_ = nullptr;
+  }
+
+  void DrainVMTasks() {
+    platform_->DrainBackgroundTasks();
   }
 
 #if HAVE_INSPECTOR
@@ -293,12 +295,12 @@ static struct {
     tracing_agent_->Stop();
   }
 
-  v8::Platform* platform_;
   tracing::Agent* tracing_agent_;
+  NodePlatform* platform_;
 #else  // !NODE_USE_V8_PLATFORM
-  void Initialize(int thread_pool_size) {}
-  void PumpMessageLoop(Isolate* isolate) {}
+  void Initialize(int thread_pool_size, uv_loop_t* loop) {}
   void Dispose() {}
+  void DrainVMTasks() {}
   bool StartInspector(Environment *env, const char* script_path,
                       const node::DebugOptions& options) {
     env->ThrowError("Node compiled with NODE_USE_V8_PLATFORM=0");
@@ -4555,19 +4557,14 @@ inline int Start(Isolate* isolate, IsolateData* isolate_data,
     SealHandleScope seal(isolate);
     bool more;
     do {
-      v8_platform.PumpMessageLoop(isolate);
-      more = uv_run(env.event_loop(), UV_RUN_ONCE);
-
-      if (more == false) {
-        v8_platform.PumpMessageLoop(isolate);
-        EmitBeforeExit(&env);
-
-        // Emit `beforeExit` if the loop became alive either after emitting
-        // event, or after running some callbacks.
-        more = uv_loop_alive(env.event_loop());
-        if (uv_run(env.event_loop(), UV_RUN_NOWAIT) != 0)
-          more = true;
-      }
+      uv_run(env.event_loop(), UV_RUN_DEFAULT);
+
+      EmitBeforeExit(&env);
+
+      v8_platform.DrainVMTasks();
+      // Emit `beforeExit` if the loop became alive either after emitting
+      // event, or after running some callbacks.
+      more = uv_loop_alive(env.event_loop());
     } while (more == true);
   }
 
@@ -4577,6 +4574,7 @@ inline int Start(Isolate* isolate, IsolateData* isolate_data,
   RunAtExit(&env);
   uv_key_delete(&thread_local_env);
 
+  v8_platform.DrainVMTasks();
   WaitForInspectorDisconnect(&env);
 #if defined(LEAK_SANITIZER)
   __lsan_do_leak_check();
@@ -4665,7 +4663,7 @@ int Start(int argc, char** argv) {
   V8::SetEntropySource(crypto::EntropySource);
 #endif  // HAVE_OPENSSL
 
-  v8_platform.Initialize(v8_thread_pool_size);
+  v8_platform.Initialize(v8_thread_pool_size, uv_default_loop());
   // Enable tracing when argv has --trace-events-enabled.
   if (trace_enabled) {
     fprintf(stderr, "Warning: Trace event is an experimental feature "
@@ -4682,6 +4680,12 @@ int Start(int argc, char** argv) {
   v8_initialized = false;
   V8::Dispose();
 
+  // uv_run cannot be called from the time before the beforeExit callback
+  // runs until the program exits unless the event loop has any referenced
+  // handles after beforeExit terminates. This prevents unrefed timers
+  // that happen to terminate during shutdown from being run unsafely.
+  // Since uv_run cannot be called, uv_async handles held by the platform
+  // will never be fully cleaned up.
   v8_platform.Dispose();
 
   delete[] exec_argv;

src/node_platform.cc

@@ -0,0 +1,189 @@
+#include "node_platform.h"
+
+#include "util.h"
+
+namespace node {
+
+using v8::Isolate;
+using v8::Platform;
+using v8::Task;
+using v8::TracingController;
+
+static void FlushTasks(uv_async_t* handle) {
+  NodePlatform* platform = static_cast<NodePlatform*>(handle->data);
+  platform->FlushForegroundTasksInternal();
+}
+
+static void BackgroundRunner(void* data) {
+  TaskQueue<Task>* background_tasks = static_cast<TaskQueue<Task>*>(data);
+  while (Task* task = background_tasks->BlockingPop()) {
+    task->Run();
+    delete task;
+    background_tasks->NotifyOfCompletion();
+  }
+}
+
+NodePlatform::NodePlatform(int thread_pool_size, uv_loop_t* loop,
+                           TracingController* tracing_controller)
+    : loop_(loop) {
+  CHECK_EQ(0, uv_async_init(loop, &flush_tasks_, FlushTasks));
+  flush_tasks_.data = static_cast<void*>(this);
+  uv_unref(reinterpret_cast<uv_handle_t*>(&flush_tasks_));
+  if (tracing_controller) {
+    tracing_controller_.reset(tracing_controller);
+  } else {
+    TracingController* controller = new TracingController();
+    tracing_controller_.reset(controller);
+  }
+  for (int i = 0; i < thread_pool_size; i++) {
+    uv_thread_t* t = new uv_thread_t();
+    if (uv_thread_create(t, BackgroundRunner, &background_tasks_) != 0) {
+      delete t;
+      break;
+    }
+    threads_.push_back(std::unique_ptr<uv_thread_t>(t));
+  }
+}
+
+void NodePlatform::Shutdown() {
+  background_tasks_.Stop();
+  for (size_t i = 0; i < threads_.size(); i++) {
+    CHECK_EQ(0, uv_thread_join(threads_[i].get()));
+  }
+  // uv_run cannot be called from the time before the beforeExit callback
+  // runs until the program exits unless the event loop has any referenced
+  // handles after beforeExit terminates. This prevents unrefed timers
+  // that happen to terminate during shutdown from being run unsafely.
+  // Since uv_run cannot be called, this handle will never be fully cleaned
+  // up.
+  uv_close(reinterpret_cast<uv_handle_t*>(&flush_tasks_), nullptr);
+}
+
+size_t NodePlatform::NumberOfAvailableBackgroundThreads() {
+  return threads_.size();
+}
+
+static void RunForegroundTask(uv_timer_t* handle) {
+  Task* task = static_cast<Task*>(handle->data);
+  task->Run();
+  delete task;
+  uv_close(reinterpret_cast<uv_handle_t*>(handle), [](uv_handle_t* handle) {
+    delete reinterpret_cast<uv_timer_t*>(handle);
+  });
+}
+
+void NodePlatform::DrainBackgroundTasks() {
+  FlushForegroundTasksInternal();
+  background_tasks_.BlockingDrain();
+}
+
+void NodePlatform::FlushForegroundTasksInternal() {
+  while (auto delayed = foreground_delayed_tasks_.Pop()) {
+    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);
+    // 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;
+  }
+  while (Task* task = foreground_tasks_.Pop()) {
+    task->Run();
+    delete task;
+  }
+}
+
+void NodePlatform::CallOnBackgroundThread(Task* task,
+                                          ExpectedRuntime expected_runtime) {
+  background_tasks_.Push(task);
+}
+
+void NodePlatform::CallOnForegroundThread(Isolate* isolate, Task* task) {
+  foreground_tasks_.Push(task);
+  uv_async_send(&flush_tasks_);
+}
+
+void NodePlatform::CallDelayedOnForegroundThread(Isolate* isolate,
+                                                 Task* task,
+                                                 double delay_in_seconds) {
+  auto pair = new std::pair<Task*, double>(task, delay_in_seconds);
+  foreground_delayed_tasks_.Push(pair);
+  uv_async_send(&flush_tasks_);
+}
+
+bool NodePlatform::IdleTasksEnabled(Isolate* isolate) { return false; }
+
+double NodePlatform::MonotonicallyIncreasingTime() {
+  // Convert nanos to seconds.
+  return uv_hrtime() / 1e9;
+}
+
+TracingController* NodePlatform::GetTracingController() {
+  return tracing_controller_.get();
+}
+
+template <class T>
+TaskQueue<T>::TaskQueue()
+    : lock_(), tasks_available_(), tasks_drained_(),
+      outstanding_tasks_(0), stopped_(false), task_queue_() { }
+
+template <class T>
+void TaskQueue<T>::Push(T* task) {
+  Mutex::ScopedLock scoped_lock(lock_);
+  outstanding_tasks_++;
+  task_queue_.push(task);
+  tasks_available_.Signal(scoped_lock);
+}
+
+template <class T>
+T* TaskQueue<T>::Pop() {
+  Mutex::ScopedLock scoped_lock(lock_);
+  T* result = nullptr;
+  if (!task_queue_.empty()) {
+    result = task_queue_.front();
+    task_queue_.pop();
+  }
+  return result;
+}
+
+template <class T>
+T* TaskQueue<T>::BlockingPop() {
+  Mutex::ScopedLock scoped_lock(lock_);
+  while (task_queue_.empty() && !stopped_) {
+    tasks_available_.Wait(scoped_lock);
+  }
+  if (stopped_) {
+    return nullptr;
+  }
+  T* result = task_queue_.front();
+  task_queue_.pop();
+  return result;
+}
+
+template <class T>
+void TaskQueue<T>::NotifyOfCompletion() {
+  Mutex::ScopedLock scoped_lock(lock_);
+  if (--outstanding_tasks_ == 0) {
+    tasks_drained_.Broadcast(scoped_lock);
+  }
+}
+
+template <class T>
+void TaskQueue<T>::BlockingDrain() {
+  Mutex::ScopedLock scoped_lock(lock_);
+  while (outstanding_tasks_ > 0) {
+    tasks_drained_.Wait(scoped_lock);
+  }
+}
+
+template <class T>
+void TaskQueue<T>::Stop() {
+  Mutex::ScopedLock scoped_lock(lock_);
+  stopped_ = true;
+  tasks_available_.Broadcast(scoped_lock);
+}
+
+}  // namespace node