1 files changed, 129 insertions, 0 deletions
diff --git a/xpcom/threads/SharedThreadPool.h b/xpcom/threads/SharedThreadPool.h
new file mode 100644
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+++ b/xpcom/threads/SharedThreadPool.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/. */
+
+#ifndef SharedThreadPool_h_
+#define SharedThreadPool_h_
+
+#include <queue>
+#include "mozilla/RefPtr.h"
+#include "nsThreadUtils.h"
+#include "nsIThreadManager.h"
+#include "nsIThreadPool.h"
+#include "nsISupports.h"
+#include "nsISupportsImpl.h"
+#include "nsCOMPtr.h"
+
+namespace mozilla {
+
+// Wrapper that makes an nsIThreadPool a singleton, and provides a
+// consistent threadsafe interface to get instances. Callers simply get a
+// SharedThreadPool by the name of its nsIThreadPool. All get requests of
+// the same name get the same SharedThreadPool. Users must store a reference
+// to the pool, and when the last reference to a SharedThreadPool is dropped
+// the pool is shutdown and deleted. Users aren't required to manually
+// shutdown the pool, and can release references on any thread. This can make
+// it significantly easier to use thread pools, because the caller doesn't need
+// to worry about joining and tearing it down.
+//
+// On Windows all threads in the pool have MSCOM initialized with
+// COINIT_MULTITHREADED. Note that not all users of MSCOM use this mode see [1],
+// and mixing MSCOM objects between the two is terrible for performance, and can
+// cause some functions to fail. So be careful when using Win32 APIs on a
+// SharedThreadPool, and avoid sharing objects if at all possible.
+//
+// [1] https://dxr.mozilla.org/mozilla-central/search?q=coinitialize&redirect=false
+class SharedThreadPool : public nsIThreadPool
+{
+public:
+
+  // Gets (possibly creating) the shared thread pool singleton instance with
+  // thread pool named aName.
+  static already_AddRefed<SharedThreadPool> Get(const nsCString& aName,
+                                            uint32_t aThreadLimit = 4);
+
+  // We implement custom threadsafe AddRef/Release pair, that destroys the
+  // the shared pool singleton when the refcount drops to 0. The addref/release
+  // are implemented using locking, so it's not recommended that you use them
+  // in a tight loop.
+  NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
+  NS_IMETHOD_(MozExternalRefCountType) AddRef(void) override;
+  NS_IMETHOD_(MozExternalRefCountType) Release(void) override;
+
+  // Forward behaviour to wrapped thread pool implementation.
+  NS_FORWARD_SAFE_NSITHREADPOOL(mPool);
+
+  // Call this when dispatching from an event on the same
+  // threadpool that is about to complete. We should not create a new thread
+  // in that case since a thread is about to become idle.
+  nsresult DispatchFromEndOfTaskInThisPool(nsIRunnable *event)
+  {
+    return Dispatch(event, NS_DISPATCH_AT_END);
+  }
+
+  NS_IMETHOD DispatchFromScript(nsIRunnable *event, uint32_t flags) override {
+      return Dispatch(event, flags);
+  }
+
+  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> event, uint32_t flags) override
+    { return !mEventTarget ? NS_ERROR_NULL_POINTER : mEventTarget->Dispatch(Move(event), flags); }
+
+  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t) override
+    { return NS_ERROR_NOT_IMPLEMENTED; }
+
+  using nsIEventTarget::Dispatch;
+
+  NS_IMETHOD IsOnCurrentThread(bool *_retval) override { return !mEventTarget ? NS_ERROR_NULL_POINTER : mEventTarget->IsOnCurrentThread(_retval); }
+
+  // Creates necessary statics. Called once at startup.
+  static void InitStatics();
+
+  // Spins the event loop until all thread pools are shutdown.
+  // *Must* be called on the main thread.
+  static void SpinUntilEmpty();
+
+#if defined(MOZ_ASAN)
+  // Use the system default in ASAN builds, because the default is assumed to be
+  // larger than the size we want to use and is hopefully sufficient for ASAN.
+  static const uint32_t kStackSize = nsIThreadManager::DEFAULT_STACK_SIZE;
+#elif defined(XP_WIN) || defined(XP_MACOSX) || defined(LINUX)
+  static const uint32_t kStackSize = (256 * 1024);
+#else
+  // All other platforms use their system defaults.
+  static const uint32_t kStackSize = nsIThreadManager::DEFAULT_STACK_SIZE;
+#endif
+
+private:
+
+  // Returns whether there are no pools in existence at the moment.
+  static bool IsEmpty();
+
+  // Creates a singleton SharedThreadPool wrapper around aPool.
+  // aName is the name of the aPool, and is used to lookup the
+  // SharedThreadPool in the hash table of all created pools.
+  SharedThreadPool(const nsCString& aName,
+                   nsIThreadPool* aPool);
+  virtual ~SharedThreadPool();
+
+  nsresult EnsureThreadLimitIsAtLeast(uint32_t aThreadLimit);
+
+  // Name of mPool.
+  const nsCString mName;
+
+  // Thread pool being wrapped.
+  nsCOMPtr<nsIThreadPool> mPool;
+
+  // Refcount. We implement custom ref counting so that the thread pool is
+  // shutdown in a threadsafe manner and singletonness is preserved.
+  nsrefcnt mRefCnt;
+
+  // mPool QI'd to nsIEventTarget. We cache this, so that we can use
+  // NS_FORWARD_SAFE_NSIEVENTTARGET above.
+  nsCOMPtr<nsIEventTarget> mEventTarget;
+};
+
+} // namespace mozilla
+
+#endif // SharedThreadPool_h_