1 files changed, 129 insertions, 0 deletions
diff --git a/xpcom/threads/AbstractThread.h b/xpcom/threads/AbstractThread.h
new file mode 100644
index 0000000000..b53bcf8ca3
--- /dev/null
+++ b/xpcom/threads/AbstractThread.h
@@ -0,0 +1,129 @@
+/* -*- 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/. */
+
+#if !defined(AbstractThread_h_)
+#  define AbstractThread_h_
+
+#  include "mozilla/AlreadyAddRefed.h"
+#  include "mozilla/ThreadLocal.h"
+#  include "nscore.h"
+#  include "nsISerialEventTarget.h"
+#  include "nsISupports.h"
+
+class nsIEventTarget;
+class nsIRunnable;
+class nsIThread;
+
+namespace mozilla {
+
+class TaskDispatcher;
+
+/*
+ * We often want to run tasks on a target that guarantees that events will never
+ * run in parallel. There are various target types that achieve this - namely
+ * nsIThread and TaskQueue. Note that nsIThreadPool (which implements
+ * nsIEventTarget) does not have this property, so we do not want to use
+ * nsIEventTarget for this purpose. This class encapsulates the specifics of
+ * the structures we might use here and provides a consistent interface.
+ *
+ * At present, the supported AbstractThread implementations are TaskQueue,
+ * AbstractThread::MainThread() and XPCOMThreadWrapper which can wrap any
+ * nsThread.
+ *
+ * The primary use of XPCOMThreadWrapper is to allow any threads to provide
+ * Direct Task dispatching which is similar (but not identical to) the microtask
+ * semantics of JS promises. Instantiating a XPCOMThreadWrapper on the current
+ * nsThread is sufficient to enable direct task dispatching.
+ *
+ * You shouldn't use pointers when comparing AbstractThread or nsIThread to
+ * determine if you are currently on the thread, but instead use the
+ * nsISerialEventTarget::IsOnCurrentThread() method.
+ */
+class AbstractThread : public nsISerialEventTarget {
+ public:
+  // Returns the AbstractThread that the caller is currently running in, or null
+  // if the caller is not running in an AbstractThread.
+  static AbstractThread* GetCurrent() { return sCurrentThreadTLS.get(); }
+
+  AbstractThread(bool aSupportsTailDispatch)
+      : mSupportsTailDispatch(aSupportsTailDispatch) {}
+
+  // We don't use NS_DECL_NSIEVENTTARGET so that we can remove the default
+  // |flags| parameter from Dispatch. Otherwise, a single-argument Dispatch call
+  // would be ambiguous.
+  using nsISerialEventTarget::IsOnCurrentThread;
+  NS_IMETHOD_(bool) IsOnCurrentThreadInfallible(void) override;
+  NS_IMETHOD IsOnCurrentThread(bool* _retval) override;
+  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> event,
+                      uint32_t flags) override;
+  NS_IMETHOD DispatchFromScript(nsIRunnable* event, uint32_t flags) override;
+  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable> event,
+                             uint32_t delay) override;
+
+  enum DispatchReason { NormalDispatch, TailDispatch };
+  virtual nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                            DispatchReason aReason = NormalDispatch) = 0;
+
+  virtual bool IsCurrentThreadIn() const = 0;
+
+  // Returns a TaskDispatcher that will dispatch its tasks when the currently-
+  // running tasks pops off the stack.
+  //
+  // May only be called when running within the it is invoked up, and only on
+  // threads which support it.
+  virtual TaskDispatcher& TailDispatcher() = 0;
+
+  // Returns true if we have tail tasks scheduled, or if this isn't known.
+  // Returns false if we definitely don't have any tail tasks.
+  virtual bool MightHaveTailTasks() { return true; }
+
+  // Returns true if the tail dispatcher is available. In certain edge cases
+  // like shutdown, it might not be.
+  virtual bool IsTailDispatcherAvailable() { return true; }
+
+  // Helper functions for methods on the tail TasklDispatcher. These check
+  // HasTailTasks to avoid allocating a TailDispatcher if it isn't
+  // needed.
+  nsresult TailDispatchTasksFor(AbstractThread* aThread);
+  bool HasTailTasksFor(AbstractThread* aThread);
+
+  // Returns true if this supports the tail dispatcher.
+  bool SupportsTailDispatch() const { return mSupportsTailDispatch; }
+
+  // Returns true if this thread requires all dispatches originating from
+  // aThread go through the tail dispatcher.
+  bool RequiresTailDispatch(AbstractThread* aThread) const;
+  bool RequiresTailDispatchFromCurrentThread() const;
+
+  virtual nsIEventTarget* AsEventTarget() { MOZ_CRASH("Not an event target!"); }
+
+  // Returns the non-DocGroup version of AbstractThread on the main thread.
+  // A DocGroup-versioned one is available in
+  // DispatcherTrait::AbstractThreadFor(). Note:
+  // DispatcherTrait::AbstractThreadFor() SHALL be used when possible.
+  static AbstractThread* MainThread();
+
+  // Must be called exactly once during startup.
+  static void InitTLS();
+  static void InitMainThread();
+  static void ShutdownMainThread();
+
+  void DispatchStateChange(already_AddRefed<nsIRunnable> aRunnable);
+
+  static void DispatchDirectTask(already_AddRefed<nsIRunnable> aRunnable);
+
+ protected:
+  virtual ~AbstractThread() = default;
+  static MOZ_THREAD_LOCAL(AbstractThread*) sCurrentThreadTLS;
+
+  // True if we want to require that every task dispatched from tasks running in
+  // this queue go through our queue's tail dispatcher.
+  const bool mSupportsTailDispatch;
+};
+
+}  // namespace mozilla
+
+#endif