1 files changed, 93 insertions, 0 deletions
diff --git a/xpcom/threads/LazyIdleThread.h b/xpcom/threads/LazyIdleThread.h
new file mode 100644
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+++ b/xpcom/threads/LazyIdleThread.h
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+/* -*- 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/. */
+
+#ifndef mozilla_lazyidlethread_h__
+#define mozilla_lazyidlethread_h__
+
+#ifndef MOZILLA_INTERNAL_API
+#  error "This header is only usable from within libxul (MOZILLA_INTERNAL_API)."
+#endif
+
+#include "mozilla/TaskQueue.h"
+#include "nsIObserver.h"
+#include "nsThreadPool.h"
+
+namespace mozilla {
+
+/**
+ * This class provides a basic event target that creates its thread lazily and
+ * destroys its thread after a period of inactivity. It may be created and used
+ * on any thread but it may only be shut down from the thread on which it is
+ * created.
+ */
+class LazyIdleThread final : public nsISerialEventTarget, public nsIObserver {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET_FULL
+  NS_DECL_NSIOBSERVER
+
+  /**
+   * If AutomaticShutdown is specified, and the LazyIdleThread is created on the
+   * main thread, Shutdown() will automatically be called during
+   * xpcom-shutdown-threads.
+   */
+  enum ShutdownMethod { AutomaticShutdown = 0, ManualShutdown };
+
+  /**
+   * Create a new LazyIdleThread that will destroy its thread after the given
+   * number of milliseconds.
+   */
+  LazyIdleThread(uint32_t aIdleTimeoutMS, const char* aName,
+                 ShutdownMethod aShutdownMethod = AutomaticShutdown);
+
+  /**
+   * Shuts down the LazyIdleThread, waiting for any pending work to complete.
+   * Must be called from mOwningEventTarget.
+   */
+  void Shutdown();
+
+  /**
+   * Register a nsIThreadPoolListener on the underlying threadpool to track the
+   * thread as it is created/destroyed.
+   */
+  nsresult SetListener(nsIThreadPoolListener* aListener);
+
+ private:
+  /**
+   * Asynchronously shuts down the LazyIdleThread on mOwningEventTarget.
+   */
+  ~LazyIdleThread();
+
+  /**
+   * The thread which created this LazyIdleThread and is responsible for
+   * shutting it down.
+   */
+  const nsCOMPtr<nsISerialEventTarget> mOwningEventTarget;
+
+  /**
+   * The single-thread backing threadpool which provides the actual threads used
+   * by LazyIdleThread, and implements the timeout.
+   */
+  const RefPtr<nsThreadPool> mThreadPool;
+
+  /**
+   * The serial event target providing a `nsISerialEventTarget` implementation
+   * when on the LazyIdleThread.
+   */
+  const RefPtr<TaskQueue> mTaskQueue;
+
+  /**
+   * Only safe to access on the owning thread or in the destructor (as no other
+   * threads have access then). If `true`, means the LazyIdleThread has already
+   * been shut down, so it does not need to be shut down asynchronously from the
+   * destructor.
+   */
+  bool mShutdown = false;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_lazyidlethread_h__