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inspector: make dispatching_messages_ atomic #56929

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inspector: make dispatching_messages_ atomic
lemire Feb 5, 2025
fcf6750
lint
lemire Feb 5, 2025
0b317a3
lint
lemire Feb 5, 2025
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46 changes: 26 additions & 20 deletions src/inspector/main_thread_interface.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -240,26 +240,32 @@ void MainThreadInterface::StopWaitingForFrontendEvent() {
}

void MainThreadInterface::DispatchMessages() {
if (dispatching_messages_)
return;
dispatching_messages_ = true;
bool had_messages = false;
do {
if (dispatching_message_queue_.empty()) {
Mutex::ScopedLock scoped_lock(requests_lock_);
requests_.swap(dispatching_message_queue_);
}
had_messages = !dispatching_message_queue_.empty();
while (!dispatching_message_queue_.empty()) {
MessageQueue::value_type task;
std::swap(dispatching_message_queue_.front(), task);
dispatching_message_queue_.pop_front();

v8::SealHandleScope seal_handle_scope(agent_->env()->isolate());
task->Call(this);
}
} while (had_messages);
dispatching_messages_ = false;
bool expected = false;
// compare_exchange_strong returns true if the value was successfully changed
// from false to true.
if (dispatching_messages_.compare_exchange_strong(
expected,
true,
std::memory_order_acquire,
std::memory_order_relaxed)) {
bool had_messages = false;
do {
if (dispatching_message_queue_.empty()) {
Mutex::ScopedLock scoped_lock(requests_lock_);
requests_.swap(dispatching_message_queue_);
}
had_messages = !dispatching_message_queue_.empty();
while (!dispatching_message_queue_.empty()) {
MessageQueue::value_type task;
std::swap(dispatching_message_queue_.front(), task);
dispatching_message_queue_.pop_front();

v8::SealHandleScope seal_handle_scope(agent_->env()->isolate());
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@legendecas legendecas Feb 6, 2025 •
edited
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MainThreadInterface::DispatchMessages accesses the Isolate and can only be accessed on the JavaScript thread. The dispatching_messages_ is only a guard for re-entrance and is not intended to be used across multiple threads.

node/src/inspector/main_thread_interface.cc

Lines 222 to 224 in 4355b3f

// We allow DispatchMessages reentry as we enter the pause. This is important
// to support debugging the code invoked by an inspector call, such
// as Runtime.evaluate

The MainThreadInterface::Post can be invoked from another IO thread, but not all methods are. I don't think making dispatching_messages_ is necessary.

task->Call(this);
}
} while (had_messages);
dispatching_messages_.store(false, std::memory_order_release);
}
}

std::shared_ptr<MainThreadHandle> MainThreadInterface::GetHandle() {
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2 changes: 1 addition & 1 deletion src/inspector/main_thread_interface.h
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Original file line number Diff line number Diff line change
Expand Up @@ -94,7 +94,7 @@ class MainThreadInterface :
// This queue is to maintain the order of the messages for the cases
// when we reenter the DispatchMessages function.
MessageQueue dispatching_message_queue_;
bool dispatching_messages_ = false;
std::atomic<bool> dispatching_messages_ = false;
// This flag indicates an internal request to exit the loop in
// WaitForFrontendEvent(). It's set to true by calling
// StopWaitingForFrontendEvent().
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