LibCore: Use IOCP for the event loop on Windows

This commit changes the event loop to use IOCPs instead of
WaitForMultipleObjects to wait on events. This is done through the Nt
kernel api NtAssociateWaitCompletionPacket which associates an event
with a completion packet. Each completion packet notifies only once, as
they are normally used to signal completion of an operation so to use
them for notifiers they are associated again after each time they are
triggered.
There are more optimizations that can be done, such as reusing the
EventLoopNotifier and EventLoopTimer structures to reduce the number of
allocations and context switches for timer and notifier registration.

AK/Windows.h

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2024, stasoid <stasoid@yahoo.com>
  * Copyright (c) 2025, ayeteadoe <ayeteadoe@gmail.com>
+ * Copyright (c) 2025, Ryszard Goc <ryszardgoc@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -11,11 +12,63 @@
 #pragma once
 
 #include <AK/Assertions.h>
+#include <AK/Diagnostics.h>
 #include <AK/Platform.h>
 
 #ifdef AK_OS_WINDOWS // needed for Swift
 #    define timeval dummy_timeval
+#    include <ntstatus.h>
 #    include <winsock2.h>
+#    include <winternl.h>
+
+extern "C" {
+// NOTE: These are documented here: https://learn.microsoft.com/en-us/windows/win32/devnotes/-win32-misclowlevelclientsupport
+// If the function signature changes, we should catch it with GetProcAddress failing in most cases.
+// None of these are marked deprecated and they seem to be used internally in the kernel.
+
+NTAPI NTSTATUS NtAssociateWaitCompletionPacket(
+    _In_ HANDLE WaitCompletionPacketHandle,
+    _In_ HANDLE IoCompletionHandle,
+    _In_ HANDLE TargetObjectHandle,
+    _In_opt_ PVOID KeyContext,
+    _In_opt_ PVOID ApcContext,
+    _In_ NTSTATUS IoStatus,
+    _In_ ULONG_PTR IoStatusInformation,
+    _Out_opt_ PBOOLEAN AlreadySignaled);
+
+NTAPI NTSTATUS NtCancelWaitCompletionPacket(
+    _In_ HANDLE WaitCompletionPacketHandle,
+    _In_ BOOLEAN RemoveSignaledPacket);
+
+NTAPI NTSTATUS NtCreateWaitCompletionPacket(
+    _Out_ PHANDLE WaitCompletionPacketHandle,
+    _In_ ACCESS_MASK DesiredAccess,
+    _In_opt_ POBJECT_ATTRIBUTES ObjectAttributes);
+}
+
+using PFN_NtCreateWaitCompletionPacket = decltype(&NtCreateWaitCompletionPacket);
+using PFN_NtCancelWaitCompletionPacket = decltype(&NtCancelWaitCompletionPacket);
+using PFN_NtAssociateWaitCompletionPacket = decltype(&NtAssociateWaitCompletionPacket);
+
+inline struct SystemApi {
+    PFN_NtAssociateWaitCompletionPacket NtAssociateWaitCompletionPacket = NULL;
+    PFN_NtCancelWaitCompletionPacket NtCancelWaitCompletionPacket = NULL;
+    PFN_NtCreateWaitCompletionPacket NtCreateWaitCompletionPacket = NULL;
+
+    SystemApi()
+    {
+        HMODULE hNtdll = GetModuleHandleW(L"ntdll.dll");
+        VERIFY(hNtdll);
+        AK_IGNORE_DIAGNOSTIC("-Wcast-function-type-mismatch",
+            NtAssociateWaitCompletionPacket = (PFN_NtAssociateWaitCompletionPacket)GetProcAddress(hNtdll, "NtAssociateWaitCompletionPacket");
+            NtCancelWaitCompletionPacket = (PFN_NtCancelWaitCompletionPacket)GetProcAddress(hNtdll, "NtCancelWaitCompletionPacket");
+            NtCreateWaitCompletionPacket = (PFN_NtCreateWaitCompletionPacket)GetProcAddress(hNtdll, "NtCreateWaitCompletionPacket");)
+        VERIFY(NtAssociateWaitCompletionPacket);
+        VERIFY(NtCancelWaitCompletionPacket);
+        VERIFY(NtCreateWaitCompletionPacket);
+    }
+} g_system;
+
 #    undef timeval
 #    undef IN
 #    pragma comment(lib, "ws2_32.lib")

Libraries/LibCore/CMakeLists.txt

@@ -136,6 +136,10 @@ if (APPLE)
     target_link_libraries(LibCore PUBLIC "-framework IOSurface")
 endif()
 
+if (WIN32)
+    target_link_libraries(LibCore PRIVATE ntdll.dll)
+endif()
+
 if (ANDROID)
     target_link_libraries(LibCore PRIVATE log)
 endif()

Libraries/LibCore/Event.h

@@ -18,7 +18,7 @@ namespace Core {
 
 class Event {
 public:
-    enum Type {
+    enum Type : u8 {
         Invalid = 0,
         Quit,
         Timer,
@@ -71,7 +71,7 @@ public:
     ~TimerEvent() = default;
 };
 
-enum class NotificationType {
+enum class NotificationType : u8 {
     None = 0,
     Read = 1,
     Write = 2,

Libraries/LibCore/EventLoopImplementationWindows.cpp

@@ -2,59 +2,106 @@
  * Copyright (c) 2023, Andreas Kling <andreas@ladybird.org>
  * Copyright (c) 2024-2025, stasoid <stasoid@yahoo.com>
  * Copyright (c) 2025, ayeteadoe <ayeteadoe@gmail.com>
+ * Copyright (c) 2025, Ryszard Goc <ryszardgoc@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Assertions.h>
+#include <AK/Diagnostics.h>
 #include <AK/HashMap.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/Windows.h>
 #include <LibCore/EventLoopImplementationWindows.h>
 #include <LibCore/Notifier.h>
 #include <LibCore/ThreadEventQueue.h>
 #include <LibCore/Timer.h>
+#include <LibThreading/Mutex.h>
 
-#include <AK/Windows.h>
-
-struct Handle {
+struct OwnHandle {
     HANDLE handle = NULL;
 
-    explicit Handle(HANDLE h = NULL)
+    explicit OwnHandle(HANDLE h = NULL)
         : handle(h)
     {
     }
-    Handle(Handle&& h)
+
+    OwnHandle(OwnHandle&& h)
     {
         handle = h.handle;
         h.handle = NULL;
     }
-    void operator=(Handle&& h)
+
+    // This operation can only be done when handle is NULL
+    OwnHandle& operator=(OwnHandle&& other)
     {
         VERIFY(!handle);
-        handle = h.handle;
-        h.handle = NULL;
+        if (this == &other)
+            return *this;
+        handle = other.handle;
+        other.handle = NULL;
+        return *this;
     }
-    ~Handle()
+
+    ~OwnHandle()
     {
         if (handle)
             CloseHandle(handle);
     }
 
-    bool operator==(Handle const& h) const { return handle == h.handle; }
+    bool operator==(OwnHandle const& h) const { return handle == h.handle; }
     bool operator==(HANDLE h) const { return handle == h; }
 };
 
 template<>
-struct Traits<Handle> : DefaultTraits<Handle> {
-    static unsigned hash(Handle const& h) { return Traits<HANDLE>::hash(h.handle); }
+struct Traits<OwnHandle> : DefaultTraits<OwnHandle> {
+    static unsigned hash(OwnHandle const& h) { return Traits<HANDLE>::hash(h.handle); }
 };
 template<>
-constexpr bool IsHashCompatible<HANDLE, Handle> = true;
+constexpr bool IsHashCompatible<HANDLE, OwnHandle> = true;
 
 namespace Core {
 
-struct EventLoopTimer {
+enum class CompletionType : u8 {
+    Wake,
+    Timer,
+    Notifer,
+};
+
+struct CompletionPacket {
+    CompletionType type;
+};
+
+struct EventLoopTimer final : CompletionPacket {
+
+    ~EventLoopTimer()
+    {
+        CancelWaitableTimer(timer.handle);
+    }
+
+    OwnHandle timer;
+    OwnHandle wait_packet;
+    bool is_periodic;
     WeakPtr<EventReceiver> owner;
 };
 
+struct EventLoopNotifier final : CompletionPacket {
+
+    ~EventLoopNotifier()
+    {
+    }
+
+    Notifier::Type notifier_type() const { return m_notifier_type; }
+    int fd() const { return to_fd(object_handle); }
+
+    // These are a space tradeoff for avoiding a double indirection through the notifier*.
+    Notifier* notifier;
+    Notifier::Type m_notifier_type;
+    HANDLE object_handle;
+    OwnHandle wait_packet;
+    OwnHandle wait_event;
+};
+
 struct ThreadData {
     static ThreadData* the()
     {
@@ -65,21 +112,29 @@ struct ThreadData {
     }
 
     ThreadData()
+        : wake_completion_key(make<CompletionPacket>(CompletionType::Wake))
     {
-        wake_event.handle = CreateEvent(NULL, FALSE, FALSE, NULL);
-        VERIFY(wake_event.handle);
+        // Consider a way for different event loops to have a different number of threads
+        iocp.handle = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 1);
+        VERIFY(iocp.handle);
     }
 
-    // Each thread has its own timers, notifiers and a wake event.
-    HashMap<Handle, EventLoopTimer> timers;
-    HashMap<Handle, Notifier*> notifiers;
+    OwnHandle iocp;
 
-    // The wake event is used to notify another event loop that someone has called wake().
-    Handle wake_event;
+    // These are only used to register and unregister. The event loop doesn't access these.
+    HashMap<intptr_t, NonnullOwnPtr<EventLoopTimer>> timers;
+    HashMap<Notifier*, NonnullOwnPtr<EventLoopNotifier>> notifiers;
+
+    // The wake completion key is posted to the thread's event loop to wake it.
+    NonnullOwnPtr<CompletionPacket> wake_completion_key;
 };
 
 EventLoopImplementationWindows::EventLoopImplementationWindows()
-    : m_wake_event(ThreadData::the()->wake_event.handle)
+    : m_wake_completion_key((void*)ThreadData::the()->wake_completion_key.ptr())
+{
+}
+
+EventLoopImplementationWindows::~EventLoopImplementationWindows()
 {
 }
 
@@ -93,52 +148,60 @@ int EventLoopImplementationWindows::exec()
     VERIFY_NOT_REACHED();
 }
 
+static constexpr bool debug_event_loop = false;
+
 size_t EventLoopImplementationWindows::pump(PumpMode pump_mode)
 {
     auto& event_queue = ThreadEventQueue::current();
     auto* thread_data = ThreadData::the();
-    auto& notifiers = thread_data->notifiers;
-    auto& timers = thread_data->timers;
 
-    size_t event_count = 1 + notifiers.size() + timers.size();
-    // If 64 events limit proves to be insufficient RegisterWaitForSingleObject or other methods
-    // can be used instead as mentioned in https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-waitformultipleobjects
-    // TODO: investigate if event_count can realistically exceed 64
-    VERIFY(event_count <= MAXIMUM_WAIT_OBJECTS);
-
-    Vector<HANDLE, MAXIMUM_WAIT_OBJECTS> event_handles;
-    event_handles.append(thread_data->wake_event.handle);
-
-    for (auto& entry : notifiers)
-        event_handles.append(entry.key.handle);
-    for (auto& entry : timers)
-        event_handles.append(entry.key.handle);
+    // NOTE: The number of entries to dequeue is to be optimized. Ideally we always dequeue all outstanding packets,
+    // but we don't want to increase the cost of each pump unnecessarily. If more than one entry is never dequeued
+    // at once, we could switch to using GetQueuedCompletionStatus which directly returns the values.
+    constexpr ULONG entry_count = 32;
+    OVERLAPPED_ENTRY entries[entry_count];
+    ULONG entries_removed = 0;
 
     bool has_pending_events = event_queue.has_pending_events();
     DWORD timeout = 0;
     if (!has_pending_events && pump_mode == PumpMode::WaitForEvents)
         timeout = INFINITE;
-    DWORD result = WaitForMultipleObjects(event_count, event_handles.data(), FALSE, timeout);
-    if (result == WAIT_TIMEOUT) {
-        // FIXME: This verification sometimes fails with ERROR_INVALID_HANDLE, but when I check
-        //        the handles they all seem to be valid.
-        // VERIFY(GetLastError() == ERROR_SUCCESS || GetLastError() == ERROR_IO_PENDING);
-    } else {
-        size_t const index = result - WAIT_OBJECT_0;
-        VERIFY(index < event_count);
-        // : 1 - skip wake event
-        for (size_t i = index ? index : 1; i < event_count; i++) {
-            // i == index already checked by WaitForMultipleObjects
-            if (i == index || WaitForSingleObject(event_handles[i], 0) == WAIT_OBJECT_0) {
-                if (i <= notifiers.size()) {
-                    Notifier* notifier = *notifiers.get(event_handles[i]);
-                    event_queue.post_event(*notifier, make<NotifierActivationEvent>(notifier->fd(), notifier->type()));
-                } else {
-                    auto& timer = *timers.get(event_handles[i]);
-                    if (auto strong_owner = timer.owner.strong_ref())
-                        event_queue.post_event(*strong_owner, make<TimerEvent>());
-                }
+
+    BOOL success = GetQueuedCompletionStatusEx(thread_data->iocp.handle, entries, entry_count, &entries_removed, timeout, FALSE);
+    dbgln_if(debug_event_loop, "Event loop dequed {} events", entries_removed);
+
+    if (success) {
+        for (ULONG i = 0; i < entries_removed; i++) {
+            auto& entry = entries[i];
+            auto* packet = reinterpret_cast<CompletionPacket*>(entry.lpCompletionKey);
+
+            if (packet == thread_data->wake_completion_key) {
+                continue;
             }
+            if (packet->type == CompletionType::Timer) {
+                auto* timer = static_cast<EventLoopTimer*>(packet);
+                if (auto owner = timer->owner.strong_ref())
+                    event_queue.post_event(*owner, make<TimerEvent>());
+                if (timer->is_periodic)
+                    g_system.NtAssociateWaitCompletionPacket(timer->wait_packet.handle, thread_data->iocp.handle, timer->timer.handle, timer, NULL, 0, 0, NULL);
+                continue;
+            }
+            if (packet->type == CompletionType::Notifer) {
+                auto* notifier_data = reinterpret_cast<EventLoopNotifier*>(packet);
+                event_queue.post_event(*notifier_data->notifier, make<NotifierActivationEvent>(notifier_data->fd(), notifier_data->notifier_type()));
+                g_system.NtAssociateWaitCompletionPacket(notifier_data->wait_packet.handle, thread_data->iocp.handle, notifier_data->wait_event.handle, notifier_data, NULL, 0, 0, NULL);
+                continue;
+            }
+            VERIFY_NOT_REACHED();
+        }
+    } else {
+        DWORD error = GetLastError();
+        switch (error) {
+        case WAIT_TIMEOUT:
+            break;
+        default:
+            dbgln("GetQueuedCompletionStatusEx failed with unexpected error: {}", Error::from_windows_error(error));
+            VERIFY_NOT_REACHED();
         }
     }
 
@@ -160,7 +223,8 @@ void EventLoopImplementationWindows::post_event(EventReceiver& receiver, Nonnull
 
 void EventLoopImplementationWindows::wake()
 {
-    SetEvent(m_wake_event);
+    auto* thread_data = ThreadData::the();
+    PostQueuedCompletionStatus(thread_data->iocp.handle, 0, (ULONG_PTR)m_wake_completion_key, NULL);
 }
 
 static int notifier_type_to_network_event(NotificationType type)
@@ -178,47 +242,86 @@ static int notifier_type_to_network_event(NotificationType type)
 
 void EventLoopManagerWindows::register_notifier(Notifier& notifier)
 {
+    auto* thread_data = ThreadData::the();
+    auto& notifiers = thread_data->notifiers;
+
+    if (notifiers.contains(&notifier))
+        return;
+
     HANDLE event = CreateEvent(NULL, FALSE, FALSE, NULL);
     VERIFY(event);
     int rc = WSAEventSelect(notifier.fd(), event, notifier_type_to_network_event(notifier.type()));
     VERIFY(!rc);
 
-    auto& notifiers = ThreadData::the()->notifiers;
-    VERIFY(!notifiers.get(event).has_value());
-    notifiers.set(Handle(event), &notifier);
+    auto notifier_data = make<EventLoopNotifier>();
+    notifier_data->type = CompletionType::Notifer;
+    notifier_data->notifier = &notifier;
+    notifier_data->m_notifier_type = notifier.type();
+    notifier_data->wait_event.handle = event;
+    NTSTATUS status = NtCreateWaitCompletionPacket(&notifier_data->wait_packet.handle, GENERIC_READ | GENERIC_WRITE, NULL);
+    VERIFY(NT_SUCCESS(status));
+    status = NtAssociateWaitCompletionPacket(notifier_data->wait_packet.handle, thread_data->iocp.handle, event, notifier_data.ptr(), NULL, 0, 0, NULL);
+    VERIFY(NT_SUCCESS(status));
+    notifiers.set(&notifier, move(notifier_data));
 }
 
 void EventLoopManagerWindows::unregister_notifier(Notifier& notifier)
 {
-    // remove_first_matching would be clearer, but currently there is no such method in HashMap
-    if (ThreadData::the())
-        ThreadData::the()->notifiers.remove_all_matching([&](auto&, auto value) { return value == &notifier; });
+    auto* thread_data = ThreadData::the();
+    VERIFY(thread_data);
+
+    auto& notifiers = thread_data->notifiers;
+    auto maybe_notifier_data = notifiers.take(&notifier);
+    if (!maybe_notifier_data.has_value())
+        return;
+    auto notifier_data = move(maybe_notifier_data.value());
+    // We are removing the signalled packets since the caller no longer expects them
+    NTSTATUS status = g_system.NtCancelWaitCompletionPacket(notifier_data->wait_packet.handle, TRUE);
+    VERIFY(NT_SUCCESS(status));
+    // TODO: Reuse the data structure
 }
 
 intptr_t EventLoopManagerWindows::register_timer(EventReceiver& object, int milliseconds, bool should_reload)
 {
     VERIFY(milliseconds >= 0);
-    // FIXME: This is a temporary fix for issue #3641
-    bool manual_reset = static_cast<Timer&>(object).is_single_shot();
-    HANDLE timer = CreateWaitableTimer(NULL, manual_reset, NULL);
-    VERIFY(timer);
+    auto* thread_data = ThreadData::the();
+    VERIFY(thread_data);
+    auto& timers = thread_data->timers;
+
+    auto timer_data = make<EventLoopTimer>();
+    timer_data->type = CompletionType::Timer;
+    timer_data->timer.handle = CreateWaitableTimer(NULL, FALSE, NULL);
+    timer_data->owner = object.make_weak_ptr();
+    timer_data->is_periodic = should_reload;
+    VERIFY(timer_data->timer.handle);
+
+    NTSTATUS status = g_system.NtCreateWaitCompletionPacket(&timer_data->wait_packet.handle, GENERIC_READ | GENERIC_WRITE, NULL);
+    VERIFY(NT_SUCCESS(status));
 
     LARGE_INTEGER first_time = {};
     // Measured in 0.1μs intervals, negative means starting from now
-    first_time.QuadPart = -10'000 * milliseconds;
-    BOOL rc = SetWaitableTimer(timer, &first_time, should_reload ? milliseconds : 0, NULL, NULL, FALSE);
-    VERIFY(rc);
+    first_time.QuadPart = -10'000LL * milliseconds;
+    BOOL succeeded = SetWaitableTimer(timer_data->timer.handle, &first_time, should_reload ? milliseconds : 0, NULL, NULL, FALSE);
+    VERIFY(succeeded);
 
-    auto& timers = ThreadData::the()->timers;
-    VERIFY(!timers.get(timer).has_value());
-    timers.set(Handle(timer), { object });
-    return reinterpret_cast<intptr_t>(timer);
+    status = g_system.NtAssociateWaitCompletionPacket(timer_data->wait_packet.handle, thread_data->iocp.handle, timer_data->timer.handle, timer_data.ptr(), NULL, 0, 0, NULL);
+    VERIFY(NT_SUCCESS(status));
+
+    auto timer_id = reinterpret_cast<intptr_t>(timer_data.ptr());
+    VERIFY(!timers.get(timer_id).has_value());
+    timers.set(timer_id, move(timer_data));
+    return timer_id;
 }
 
 void EventLoopManagerWindows::unregister_timer(intptr_t timer_id)
 {
-    if (ThreadData::the())
-        ThreadData::the()->timers.remove(reinterpret_cast<HANDLE>(timer_id));
+    if (auto* thread_data = ThreadData::the()) {
+        auto maybe_timer = thread_data->timers.take(timer_id);
+        if (!maybe_timer.has_value())
+            return;
+        auto timer = move(maybe_timer.value());
+        g_system.NtCancelWaitCompletionPacket(timer->wait_packet.handle, TRUE);
+    }
 }
 
 int EventLoopManagerWindows::register_signal([[maybe_unused]] int signal_number, [[maybe_unused]] Function<void(int)> handler)

Libraries/LibCore/EventLoopImplementationWindows.h

@@ -34,7 +34,7 @@ public:
     static NonnullOwnPtr<EventLoopImplementationWindows> create() { return make<EventLoopImplementationWindows>(); }
 
     EventLoopImplementationWindows();
-    virtual ~EventLoopImplementationWindows() override = default;
+    virtual ~EventLoopImplementationWindows() override;
 
     virtual int exec() override;
     virtual size_t pump(PumpMode) override;
@@ -48,8 +48,7 @@ private:
     bool m_exit_requested { false };
     int m_exit_code { 0 };
 
-    // The wake event handle of this event loop needs to be accessible from other threads.
-    void*& m_wake_event;
+    void const* m_wake_completion_key;
 };
 
 using EventLoopManagerPlatform = EventLoopManagerWindows;

Libraries/LibCore/Notifier.h

@@ -39,10 +39,10 @@ public:
 private:
     Notifier(int fd, Type type);
 
-    int m_fd { -1 };
-    bool m_is_enabled { false };
     pthread_t m_owner_thread {};
+    int m_fd { -1 };
     Type m_type { Type::None };
+    bool m_is_enabled { false };
 };
 
 }