1 files changed, 130 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 +1,130 @@
+/* -*- 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 <utility>
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/RefCountType.h"
+#include "nsCOMPtr.h"
+#include "nsID.h"
+#include "nsIThreadPool.h"
+#include "nsString.h"
+#include "nscore.h"
+
+class nsIRunnable;
+
+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://searchfox.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 = NS_DISPATCH_NORMAL) override {
+    return !mPool ? NS_ERROR_NULL_POINTER
+                  : mPool->Dispatch(std::move(event), flags);
+  }
+
+  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  using nsIEventTarget::Dispatch;
+
+  NS_IMETHOD RegisterShutdownTask(nsITargetShutdownTask* task) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD UnregisterShutdownTask(nsITargetShutdownTask* task) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD IsOnCurrentThread(bool* _retval) override {
+    return !mPool ? NS_ERROR_NULL_POINTER : mPool->IsOnCurrentThread(_retval);
+  }
+
+  NS_IMETHOD_(bool) IsOnCurrentThreadInfallible() override {
+    return mPool && mPool->IsOnCurrentThread();
+  }
+
+  // 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();
+
+ 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;
+};
+
+}  // namespace mozilla
+
+#endif  // SharedThreadPool_h_