LibCore: Allow event loops on other threads to wake up

Because the wake pipe is thread-local, it was previously not possible
to wake an event loop across a thread. Therefore, this commit
rearchitects event loop waking by making the wake function a member of
the event loop itself and having it keep a pointer to its thread's wake
pipe. The global wake() function calls wake on the current thread's
event loop.

This also fixes a bug in BackgroundAction: it should wake the event loop
it was created on, instead of the current thread's event loop.

Userland/DevTools/HackStudio/HackStudioWidget.cpp

@@ -849,7 +849,7 @@ void HackStudioWidget::initialize_debugger()
  m_disassembly_widget->update_state(*Debugger::the().session(), regs);
  HackStudioWidget::reveal_action_tab(*m_debug_info_widget);
  });
- Core::EventLoop::wake();
+ Core::EventLoop::wake_current();
 
  return Debugger::HasControlPassedToUser::Yes;
  },
@@ -859,7 +859,7 @@ void HackStudioWidget::initialize_debugger()
  if (m_current_editor_in_execution)
  m_current_editor_in_execution->editor().clear_execution_position();
  });
- Core::EventLoop::wake();
+ Core::EventLoop::wake_current();
  },
  [this]() {
  deferred_invoke([this] {
@@ -879,7 +879,7 @@ void HackStudioWidget::initialize_debugger()
  HackStudioWidget::hide_action_tabs();
  GUI::MessageBox::show(window(), "Program Exited", "Debugger", GUI::MessageBox::Type::Information);
  });
- Core::EventLoop::wake();
+ Core::EventLoop::wake_current();
  });
 }
 

Userland/Libraries/LibCore/EventLoop.cpp

@@ -302,7 +302,8 @@ class InspectorServerConnection : public Object {
 };
 
 EventLoop::EventLoop([[maybe_unused]] MakeInspectable make_inspectable)
- : m_private(make<Private>())
+ : m_wake_pipe_fds(&s_wake_pipe_fds)
+ , m_private(make<Private>())
 {
 #ifdef __serenity__
  if (!s_global_initializers_ran) {
@@ -487,11 +488,13 @@ size_t EventLoop::pump(WaitMode mode)
  return processed_events;
 }
 
-void EventLoop::post_event(Object& receiver, NonnullOwnPtr<Event>&& event)
+void EventLoop::post_event(Object& receiver, NonnullOwnPtr<Event>&& event, ShouldWake should_wake)
 {
  Threading::MutexLocker lock(m_private->lock);
  dbgln_if(EVENTLOOP_DEBUG, "Core::EventLoop::post_event: ({}) << receiver={}, event={}", m_queued_events.size(), receiver, event);
  m_queued_events.empend(receiver, move(event));
+ if (should_wake == ShouldWake::Yes)
+ wake();
 }
 
 SignalHandlers::SignalHandlers(int signo, void (*handle_signal)(int))
@@ -839,10 +842,16 @@ void EventLoop::unregister_notifier(Badge<Notifier>, Notifier& notifier)
  s_notifiers->remove(&notifier);
 }
 
+void EventLoop::wake_current()
+{
+ EventLoop::current().wake();
+}
+
 void EventLoop::wake()
 {
+ dbgln_if(EVENTLOOP_DEBUG, "Core::EventLoop::wake()");
  int wake_event = 0;
- int nwritten = write(s_wake_pipe_fds[1], &wake_event, sizeof(wake_event));
+ int nwritten = write((*m_wake_pipe_fds)[1], &wake_event, sizeof(wake_event));
  if (nwritten < 0) {
  perror("EventLoop::wake: write");
  VERIFY_NOT_REACHED();

Userland/Libraries/LibCore/EventLoop.h

@@ -34,6 +34,11 @@ class EventLoop {
  Yes,
  };
 
+ enum class ShouldWake {
+ No,
+ Yes
+ };
+
  explicit EventLoop(MakeInspectable = MakeInspectable::No);
  ~EventLoop();
  static void initialize_wake_pipes();
@@ -51,7 +56,7 @@ class EventLoop {
 
  void spin_until(Function<bool()>);
 
- void post_event(Object& receiver, NonnullOwnPtr<Event>&&);
+ void post_event(Object& receiver, NonnullOwnPtr<Event>&&, ShouldWake = ShouldWake::No);
 
  template<typename Callback>
  static decltype(auto) with_main_locked(Callback callback)
@@ -79,7 +84,8 @@ class EventLoop {
  m_queued_events.extend(move(other.m_queued_events));
  }
 
- static void wake();
+ static void wake_current();
+ void wake();
 
  static int register_signal(int signo, Function<void(int)> handler);
  static void unregister_signal(int handler_id);
@@ -126,6 +132,9 @@ class EventLoop {
  static thread_local int s_wake_pipe_fds[2];
  static thread_local bool s_wake_pipe_initialized;
 
+ // The wake pipe of this event loop needs to be accessible from other threads.
+ int (*m_wake_pipe_fds)[2];
+
  struct Private;
  NonnullOwnPtr<Private> m_private;
 };

Userland/Libraries/LibThreading/BackgroundAction.h

@@ -63,7 +63,7 @@ class BackgroundAction final : public Core::Object
  m_on_complete(m_result.release_value());
  remove_from_parent();
  });
- Core::EventLoop::wake();
+ origin_event_loop->wake();
  } else {
  this->remove_from_parent();
  }