1 files changed, 226 insertions, 0 deletions
diff --git a/xpcom/threads/LazyIdleThread.h b/xpcom/threads/LazyIdleThread.h
new file mode 100644
index 000000000..6bf8e8e81
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+++ b/xpcom/threads/LazyIdleThread.h
@@ -0,0 +1,226 @@
+/* -*- 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 "nsIObserver.h"
+#include "nsIThreadInternal.h"
+#include "nsITimer.h"
+
+#include "mozilla/Mutex.h"
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+#include "nsString.h"
+#include "mozilla/Attributes.h"
+
+#define IDLE_THREAD_TOPIC "thread-shutting-down"
+
+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 on any
+ * thread but it may only be used from the thread on which it is created. If it
+ * is created on the main thread then it will automatically join its thread on
+ * XPCOM shutdown using the Observer Service.
+ */
+class LazyIdleThread final
+  : public nsIThread
+  , public nsITimerCallback
+  , public nsIThreadObserver
+  , public nsIObserver
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET
+  NS_DECL_NSITHREAD
+  NS_DECL_NSITIMERCALLBACK
+  NS_DECL_NSITHREADOBSERVER
+  NS_DECL_NSIOBSERVER
+  using nsIEventTarget::Dispatch;
+
+  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 nsCSubstring& aName,
+                 ShutdownMethod aShutdownMethod = AutomaticShutdown,
+                 nsIObserver* aIdleObserver = nullptr);
+
+  /**
+   * Add an observer that will be notified when the thread is idle and about to
+   * be shut down. The aSubject argument can be QueryInterface'd to an nsIThread
+   * that can be used to post cleanup events. The aTopic argument will be
+   * IDLE_THREAD_TOPIC, and aData will be null. The LazyIdleThread does not add
+   * a reference to the observer to avoid circular references as it is assumed
+   * to be the owner. It is the caller's responsibility to clear this observer
+   * if the pointer becomes invalid.
+   */
+  void SetWeakIdleObserver(nsIObserver* aObserver);
+
+  /**
+   * Disable the idle timeout for this thread. No effect if the timeout is
+   * already disabled.
+   */
+  void DisableIdleTimeout();
+
+  /**
+   * Enable the idle timeout. No effect if the timeout is already enabled.
+   */
+  void EnableIdleTimeout();
+
+private:
+  /**
+   * Calls Shutdown().
+   */
+  ~LazyIdleThread();
+
+  /**
+   * Called just before dispatching to mThread.
+   */
+  void PreDispatch();
+
+  /**
+   * Makes sure a valid thread lives in mThread.
+   */
+  nsresult EnsureThread();
+
+  /**
+   * Called on mThread to set up the thread observer.
+   */
+  void InitThread();
+
+  /**
+   * Called on mThread to clean up the thread observer.
+   */
+  void CleanupThread();
+
+  /**
+   * Called on the main thread when mThread believes itself to be idle. Sets up
+   * the idle timer.
+   */
+  void ScheduleTimer();
+
+  /**
+   * Called when we are shutting down mThread.
+   */
+  nsresult ShutdownThread();
+
+  /**
+   * Deletes this object. Used to delay calling mThread->Shutdown() during the
+   * final release (during a GC, for instance).
+   */
+  void SelfDestruct();
+
+  /**
+   * Returns true if events should be queued rather than immediately dispatched
+   * to mThread. Currently only happens when the thread is shutting down.
+   */
+  bool UseRunnableQueue()
+  {
+    return !!mQueuedRunnables;
+  }
+
+  /**
+   * Protects data that is accessed on both threads.
+   */
+  mozilla::Mutex mMutex;
+
+  /**
+   * Touched on both threads but set before mThread is created. Used to direct
+   * timer events to the owning thread.
+   */
+  nsCOMPtr<nsIThread> mOwningThread;
+
+  /**
+   * Only accessed on the owning thread. Set by EnsureThread().
+   */
+  nsCOMPtr<nsIThread> mThread;
+
+  /**
+   * Protected by mMutex. Created when mThread has no pending events and fired
+   * at mOwningThread. Any thread that dispatches to mThread will take ownership
+   * of the timer and fire a separate cancel event to the owning thread.
+   */
+  nsCOMPtr<nsITimer> mIdleTimer;
+
+  /**
+   * Idle observer. Called when the thread is about to be shut down. Released
+   * only when Shutdown() is called.
+   */
+  nsIObserver* MOZ_UNSAFE_REF("See the documentation for SetWeakIdleObserver for "
+                              "how the owner of LazyIdleThread should manage the "
+                              "lifetime information of this field") mIdleObserver;
+
+  /**
+   * Temporary storage for events that happen to be dispatched while we're in
+   * the process of shutting down our real thread.
+   */
+  nsTArray<nsCOMPtr<nsIRunnable>>* mQueuedRunnables;
+
+  /**
+   * The number of milliseconds a thread should be idle before dying.
+   */
+  const uint32_t mIdleTimeoutMS;
+
+  /**
+   * The number of events that are pending on mThread. A nonzero value means
+   * that the thread cannot be cleaned up.
+   */
+  uint32_t mPendingEventCount;
+
+  /**
+   * The number of times that mThread has dispatched an idle notification. Any
+   * timer that fires while this count is nonzero can safely be ignored as
+   * another timer will be on the way.
+   */
+  uint32_t mIdleNotificationCount;
+
+  /**
+   * Whether or not the thread should automatically shutdown. If the owner
+   * specified ManualShutdown at construction time then the owner should take
+   * care to call Shutdown() manually when appropriate.
+   */
+  ShutdownMethod mShutdownMethod;
+
+  /**
+   * Only accessed on the owning thread. Set to true when Shutdown() has been
+   * called and prevents EnsureThread() from recreating mThread.
+   */
+  bool mShutdown;
+
+  /**
+   * Set from CleanupThread and lasting until the thread has shut down. Prevents
+   * further idle notifications during the shutdown process.
+   */
+  bool mThreadIsShuttingDown;
+
+  /**
+   * Whether or not the idle timeout is enabled.
+   */
+  bool mIdleTimeoutEnabled;
+
+  /**
+   * Name of the thread, set on the actual thread after it gets created.
+   */
+  nsCString mName;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_lazyidlethread_h__