1 files changed, 303 insertions, 0 deletions
diff --git a/xpcom/threads/WinHandleWatcher.cpp b/xpcom/threads/WinHandleWatcher.cpp
new file mode 100644
index 0000000000..07d49e3730
--- /dev/null
+++ b/xpcom/threads/WinHandleWatcher.cpp
@@ -0,0 +1,303 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <windows.h>
+#include <threadpoolapiset.h>
+
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ThreadSafety.h"
+#include "mozilla/WinHandleWatcher.h"
+
+#include "nsCOMPtr.h"
+#include "nsIRunnable.h"
+#include "nsISerialEventTarget.h"
+#include "nsISupportsImpl.h"
+#include "nsITargetShutdownTask.h"
+#include "nsIWeakReferenceUtils.h"
+#include "nsThreadUtils.h"
+
+mozilla::LazyLogModule sHWLog("HandleWatcher");
+
+namespace mozilla {
+namespace details {
+struct WaitHandleDeleter {
+  void operator()(PTP_WAIT waitHandle) {
+    MOZ_LOG(sHWLog, LogLevel::Debug, ("Closing PTP_WAIT %p", waitHandle));
+    ::CloseThreadpoolWait(waitHandle);
+  }
+};
+}  // namespace details
+using WaitHandlePtr = UniquePtr<TP_WAIT, details::WaitHandleDeleter>;
+
+// HandleWatcher::Impl
+//
+// The backing implementation of HandleWatcher is a PTP_WAIT, an OS-threadpool
+// wait-object. Windows doesn't actually create a new thread per wait-object;
+// OS-threadpool threads are assigned to wait-objects only when their associated
+// handle become signaled -- although explicit documentation of this fact is
+// somewhat obscurely placed. [1]
+//
+// Throughout this class, we use manual locking and unlocking guarded by Clang's
+// thread-safety warnings, rather than scope-based lock-guards. See `Replace()`
+// for an explanation and justification.
+//
+// [1]https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-waitformultipleobjects#remarks
+class HandleWatcher::Impl final : public nsITargetShutdownTask {
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+ public:
+  Impl() = default;
+
+ private:
+  ~Impl() { MOZ_ASSERT(IsStopped()); }
+
+  struct Data {
+    // The watched handle and its callback.
+    HANDLE handle;
+    RefPtr<nsIEventTarget> target;
+    nsCOMPtr<nsIRunnable> runnable;
+
+    // Handle to the threadpool wait-object.
+    WaitHandlePtr waitHandle;
+    // A pointer to ourselves, notionally owned by the wait-object.
+    RefPtr<Impl> self;
+
+    // (We can't actually do this because a) it has annoying consequences in
+    // C++20 thanks to P1008R1, and b) Clang just ignores it anyway.)
+    //
+    // ~Data() MOZ_EXCLUDES(mMutex) = default;
+  };
+
+  mozilla::Mutex mMutex{"HandleWatcher::Impl"};
+  Data mData MOZ_GUARDED_BY(mMutex) = {};
+
+  // Callback from OS threadpool wait-object.
+  static void CALLBACK WaitCallback(PTP_CALLBACK_INSTANCE, void* ctx,
+                                    PTP_WAIT aWaitHandle,
+                                    TP_WAIT_RESULT aResult) {
+    static_cast<Impl*>(ctx)->OnWaitCompleted(aWaitHandle, aResult);
+  }
+
+  void OnWaitCompleted(PTP_WAIT aWaitHandle, TP_WAIT_RESULT aResult)
+      MOZ_EXCLUDES(mMutex) {
+    MOZ_ASSERT(aResult == WAIT_OBJECT_0);
+
+    mMutex.Lock();
+    // If this callback is no longer the active callback, skip out.
+    // All cleanup is someone else's problem.
+    if (aWaitHandle != mData.waitHandle.get()) {
+      MOZ_LOG(sHWLog, LogLevel::Debug,
+              ("Recv'd already-stopped callback: HW %p | PTP_WAIT %p", this,
+               aWaitHandle));
+      mMutex.Unlock();
+      return;
+    }
+
+    // Take our self-pointer so that we release it on exit.
+    RefPtr<Impl> self = std::move(mData.self);
+
+    MOZ_LOG(sHWLog, LogLevel::Info,
+            ("Recv'd callback: HW %p | handle %p | target %p | PTP_WAIT %p",
+             this, mData.handle, mData.target.get(), aWaitHandle));
+
+    // This may fail if (for example) `mData.target` is being shut down, but we
+    // have not yet received the shutdown callback.
+    mData.target->Dispatch(mData.runnable.forget());
+    Replace(Data{});
+  }
+
+ public:
+  static RefPtr<Impl> Create(HANDLE aHandle, nsIEventTarget* aTarget,
+                             already_AddRefed<nsIRunnable> aRunnable) {
+    auto impl = MakeRefPtr<Impl>();
+    bool const ok [[maybe_unused]] =
+        impl->Watch(aHandle, aTarget, std::move(aRunnable));
+    MOZ_ASSERT(ok);
+    return impl;
+  }
+
+ private:
+  bool Watch(HANDLE aHandle, nsIEventTarget* aTarget,
+             already_AddRefed<nsIRunnable> aRunnable) MOZ_EXCLUDES(mMutex) {
+    MOZ_ASSERT(aHandle);
+    MOZ_ASSERT(aTarget);
+
+    RefPtr<nsIEventTarget> target(aTarget);
+
+    WaitHandlePtr waitHandle{
+        ::CreateThreadpoolWait(&WaitCallback, this, nullptr)};
+    if (!waitHandle) {
+      return false;
+    }
+
+    {
+      mMutex.Lock();
+
+      nsresult const ret = aTarget->RegisterShutdownTask(this);
+      if (NS_FAILED(ret)) {
+        mMutex.Unlock();
+        return false;
+      }
+
+      MOZ_LOG(sHWLog, LogLevel::Info,
+              ("Setting callback: HW %p | handle %p | target %p | PTP_WAIT %p",
+               this, aHandle, aTarget, waitHandle.get()));
+
+      // returns `void`; presumably always succeeds given a successful
+      // `::CreateThreadpoolWait()`
+      ::SetThreadpoolWait(waitHandle.get(), aHandle, nullptr);
+      // After this point, you must call `FlushWaitHandle(waitHandle.get())`
+      // before destroying the wait handle. (Note that this must be done while
+      // *not* holding `mMutex`!)
+
+      Replace(Data{.handle = aHandle,
+                   .target = std::move(target),
+                   .runnable = aRunnable,
+                   .waitHandle = std::move(waitHandle),
+                   .self = this});
+    }
+
+    return true;
+  }
+
+  void TargetShutdown() MOZ_EXCLUDES(mMutex) override final {
+    mMutex.Lock();
+
+    MOZ_LOG(sHWLog, LogLevel::Debug,
+            ("Target shutdown: HW %p | handle %p | target %p | PTP_WAIT %p",
+             this, mData.handle, mData.target.get(), mData.waitHandle.get()));
+
+    // Clear mData.target, since there's no need to unregister the shutdown task
+    // anymore. Hold onto it until we release the mutex, though, to avoid any
+    // reentrancy issues.
+    //
+    // This is more for internal consistency than safety: someone has to be
+    // shutting `target` down, and that someone isn't us, so there's necessarily
+    // another reference out there. (Although decrementing the refcount might
+    // still have arbitrary effects if someone's been excessively clever with
+    // nsISupports::Release...)
+    auto const oldTarget = std::move(mData.target);
+    Replace(Data{});
+    // (Static-assert that the mutex has indeed been released.)
+    ([&]() MOZ_EXCLUDES(mMutex) {})();
+  }
+
+ public:
+  void Stop() MOZ_EXCLUDES(mMutex) {
+    mMutex.Lock();
+    Replace(Data{});
+  }
+
+  bool IsStopped() MOZ_EXCLUDES(mMutex) {
+    mozilla::MutexAutoLock lock(mMutex);
+    return !mData.handle;
+  }
+
+ private:
+  // Throughout this class, we use manual locking and unlocking guarded by
+  // Clang's thread-safety warnings, rather than scope-based lock-guards. This
+  // is largely driven by `Replace()`, below, which performs both operations
+  // which require the mutex to be held and operations which require it to not
+  // be held, and therefore must explicitly sequence the mutex release.
+  //
+  // These explicit locks, unlocks, and annotations are both alien to C++ and
+  // offensively tedious; but they _are_ still checked for state consistency at
+  // scope boundaries. (The concerned reader is invited to test this by
+  // deliberately removing an `mMutex.Unlock()` call from anywhere in the class
+  // and viewing the resultant compiler diagnostics.)
+  //
+  // A more principled, or at least differently-principled, implementation might
+  // create a scope-based lock-guard and pass it to `Replace()` to dispose of at
+  // the proper time. Alas, it cannot be communicated to Clang's thread-safety
+  // checker that such a guard is associated with `mMutex`.
+  //
+  void Replace(Data&& aData) MOZ_CAPABILITY_RELEASE(mMutex) {
+    // either both handles are NULL, or neither is
+    MOZ_ASSERT(!!aData.handle == !!aData.waitHandle);
+
+    if (mData.handle) {
+      MOZ_LOG(sHWLog, LogLevel::Info,
+              ("Stop callback: HW %p | handle %p | target %p | PTP_WAIT %p",
+               this, mData.handle, mData.target.get(), mData.waitHandle.get()));
+    }
+
+    if (mData.target) {
+      mData.target->UnregisterShutdownTask(this);
+    }
+
+    // Extract the old data and insert the new -- but hold onto the old data for
+    // now. (See [1] and [2], below.)
+    Data oldData = std::exchange(mData, std::move(aData));
+
+    ////////////////////////////////////////////////////////////////////////////
+    // Release the mutex.
+    mMutex.Unlock();
+    ////////////////////////////////////////////////////////////////////////////
+
+    // [1] `oldData.self` will be unset if the old callback already ran (or if
+    // there was no old callback in the first place). If it's set, though, we
+    // need to explicitly clear out the wait-object first.
+    if (oldData.self) {
+      MOZ_ASSERT(oldData.waitHandle);
+      FlushWaitHandle(oldData.waitHandle.get());
+    }
+
+    // [2] oldData also includes several other reference-counted pointers. It's
+    // possible that these may be the last pointer to something, so releasing
+    // them may have arbitrary side-effects -- like calling this->Stop(), which
+    // will try to reacquire the mutex.
+    //
+    // Now that we've released the mutex, we can (implicitly) release them all
+    // here.
+  }
+
+  // Either confirm as complete or cancel any callbacks on aWaitHandle. Block
+  // until this is done. (See documentation for ::CloseThreadpoolWait().)
+  void FlushWaitHandle(PTP_WAIT aWaitHandle) MOZ_EXCLUDES(mMutex) {
+    ::SetThreadpoolWait(aWaitHandle, nullptr, nullptr);
+    // This might block on `OnWaitCompleted()`, so we can't hold `mMutex` here.
+    ::WaitForThreadpoolWaitCallbacks(aWaitHandle, TRUE);
+    // ::CloseThreadpoolWait() itself is the caller's responsibility.
+  }
+};
+
+NS_IMPL_ISUPPORTS(HandleWatcher::Impl, nsITargetShutdownTask)
+
+//////
+// HandleWatcher member function implementations
+
+HandleWatcher::HandleWatcher() : mImpl{} {}
+HandleWatcher::~HandleWatcher() {
+  if (mImpl) {
+    MOZ_ASSERT(mImpl->IsStopped());
+    mImpl->Stop();  // just in case, in release
+  }
+}
+
+void HandleWatcher::Watch(HANDLE aHandle, nsIEventTarget* aTarget,
+                          already_AddRefed<nsIRunnable> aRunnable) {
+  auto impl = Impl::Create(aHandle, aTarget, std::move(aRunnable));
+  MOZ_ASSERT(impl);
+
+  if (mImpl) {
+    mImpl->Stop();
+  }
+  mImpl = std::move(impl);
+}
+
+void HandleWatcher::Stop() {
+  if (mImpl) {
+    mImpl->Stop();
+  }
+}
+
+bool HandleWatcher::IsStopped() { return !mImpl || mImpl->IsStopped(); }
+
+}  // namespace mozilla