LibCore: Create wake pipe on each thread

After the previous change, the wake pipe was only being created on the
main thread by the main event loop. This change utilizes a flag to
always initialize the wake pipe on other threads. Because the pipe is
quite expensive (it will count towards the file descriptor limit, for
instance), we do the initialization "lazily": Only when an event loop is
constructed and it notices that there hasn't been a wake pipe created on
its thread, it will create the pipe. Conversely, this means that there
are no pipes on threads that never use an event loop.

Userland/Libraries/LibCore/EventLoop.cpp

@@ -66,6 +66,23 @@ static thread_local Vector<EventLoop&>* s_event_loop_stack;
 static thread_local HashMap<int, NonnullOwnPtr<EventLoopTimer>>* s_timers;
 static thread_local HashTable<Notifier*>* s_notifiers;
 thread_local int EventLoop::s_wake_pipe_fds[2];
+thread_local bool EventLoop::s_wake_pipe_initialized { false };
+
+void EventLoop::initialize_wake_pipes()
+{
+    if (!s_wake_pipe_initialized) {
+#if defined(SOCK_NONBLOCK)
+        int rc = pipe2(s_wake_pipe_fds, O_CLOEXEC);
+#else
+        int rc = pipe(s_wake_pipe_fds);
+        fcntl(s_wake_pipe_fds[0], F_SETFD, FD_CLOEXEC);
+        fcntl(s_wake_pipe_fds[1], F_SETFD, FD_CLOEXEC);
+
+#endif
+        VERIFY(rc == 0);
+        s_wake_pipe_initialized = true;
+    }
+}
 
 bool EventLoop::has_been_instantiated()
 {
@@ -273,19 +290,8 @@ EventLoop::EventLoop([[maybe_unused]] MakeInspectable make_inspectable)
     }
     s_main_event_loop.with_locked([&, this](auto*& main_event_loop) {
         if (main_event_loop == nullptr) {
-            // FIXME: The compiler complains that we don't use main_event_loop although we set it.
             main_event_loop = this;
             s_pid = getpid();
-            // FIXME: We only create the wake pipe for the main thread
-#if defined(SOCK_NONBLOCK)
-            int rc = pipe2(s_wake_pipe_fds, O_CLOEXEC);
-#else
-            int rc = pipe(s_wake_pipe_fds);
-            fcntl(s_wake_pipe_fds[0], F_SETFD, FD_CLOEXEC);
-            fcntl(s_wake_pipe_fds[1], F_SETFD, FD_CLOEXEC);
-
-#endif
-            VERIFY(rc == 0);
             s_event_loop_stack->append(*this);
 
 #ifdef __serenity__
@@ -300,6 +306,8 @@ EventLoop::EventLoop([[maybe_unused]] MakeInspectable make_inspectable)
         }
     });
 
+    initialize_wake_pipes();
+
     dbgln_if(EVENTLOOP_DEBUG, "{} Core::EventLoop constructed :)", getpid());
 }
 
@@ -587,6 +595,8 @@ void EventLoop::notify_forked(ForkEvent event)
         s_event_loop_stack->clear();
         s_timers->clear();
         s_notifiers->clear();
+        s_wake_pipe_initialized = false;
+        initialize_wake_pipes();
         if (auto* info = signals_info<false>()) {
             info->signal_handlers.clear();
             info->next_signal_id = 0;

Userland/Libraries/LibCore/EventLoop.h

@@ -36,6 +36,7 @@ public:
 
     explicit EventLoop(MakeInspectable = MakeInspectable::No);
     ~EventLoop();
+    static void initialize_wake_pipes();
 
     int exec();
 
@@ -123,6 +124,7 @@ private:
     int m_exit_code { 0 };
 
     static thread_local int s_wake_pipe_fds[2];
+    static thread_local bool s_wake_pipe_initialized;
 
     struct Private;
     NonnullOwnPtr<Private> m_private;