1 files changed, 390 insertions, 0 deletions
diff --git a/xpcom/threads/nsProxyRelease.h b/xpcom/threads/nsProxyRelease.h
new file mode 100644
index 0000000000..00f69e3287
--- /dev/null
+++ b/xpcom/threads/nsProxyRelease.h
@@ -0,0 +1,390 @@
+/* -*- 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 nsProxyRelease_h__
+#define nsProxyRelease_h__
+
+#include <utility>
+
+#include "MainThreadUtils.h"
+#include "mozilla/Likely.h"
+#include "mozilla/Unused.h"
+#include "nsCOMPtr.h"
+#include "nsIEventTarget.h"
+#include "nsISerialEventTarget.h"
+#include "nsIThread.h"
+#include "nsPrintfCString.h"
+#include "nsThreadUtils.h"
+
+#ifdef XPCOM_GLUE_AVOID_NSPR
+#  error NS_ProxyRelease implementation depends on NSPR.
+#endif
+
+class nsIRunnable;
+
+namespace detail {
+
+template <typename T>
+class ProxyReleaseEvent : public mozilla::CancelableRunnable {
+ public:
+  ProxyReleaseEvent(const char* aName, already_AddRefed<T> aDoomed)
+      : CancelableRunnable(aName), mDoomed(aDoomed.take()) {}
+
+  NS_IMETHOD Run() override {
+    NS_IF_RELEASE(mDoomed);
+    return NS_OK;
+  }
+
+  nsresult Cancel() override { return Run(); }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  NS_IMETHOD GetName(nsACString& aName) override {
+    if (mName) {
+      aName.Append(nsPrintfCString("ProxyReleaseEvent for %s", mName));
+    } else {
+      aName.AssignLiteral("ProxyReleaseEvent");
+    }
+    return NS_OK;
+  }
+#endif
+
+ private:
+  T* MOZ_OWNING_REF mDoomed;
+};
+
+template <typename T>
+nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                      already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
+  // Auto-managing release of the pointer.
+  RefPtr<T> doomed = aDoomed;
+  nsresult rv;
+
+  if (!doomed || !aTarget) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  if (!aAlwaysProxy) {
+    bool onCurrentThread = false;
+    rv = aTarget->IsOnCurrentThread(&onCurrentThread);
+    if (NS_SUCCEEDED(rv) && onCurrentThread) {
+      return NS_OK;
+    }
+  }
+
+  nsCOMPtr<nsIRunnable> ev = new ProxyReleaseEvent<T>(aName, doomed.forget());
+
+  rv = aTarget->Dispatch(ev, NS_DISPATCH_NORMAL);
+  if (NS_FAILED(rv)) {
+    NS_WARNING(nsPrintfCString(
+                   "failed to post proxy release event for %s, leaking!", aName)
+                   .get());
+    // It is better to leak the aDoomed object than risk crashing as
+    // a result of deleting it on the wrong thread.
+  }
+  return rv;
+}
+
+template <bool nsISupportsBased>
+struct ProxyReleaseChooser {
+  template <typename T>
+  static nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                               already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
+    return ::detail::ProxyRelease(aName, aTarget, std::move(aDoomed),
+                                  aAlwaysProxy);
+  }
+};
+
+template <>
+struct ProxyReleaseChooser<true> {
+  // We need an intermediate step for handling classes with ambiguous
+  // inheritance to nsISupports.
+  template <typename T>
+  static nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                               already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
+    return ProxyReleaseISupports(aName, aTarget, ToSupports(aDoomed.take()),
+                                 aAlwaysProxy);
+  }
+
+  static nsresult ProxyReleaseISupports(const char* aName,
+                                        nsIEventTarget* aTarget,
+                                        nsISupports* aDoomed,
+                                        bool aAlwaysProxy);
+};
+
+}  // namespace detail
+
+/**
+ * Ensures that the delete of a smart pointer occurs on the target thread.
+ * Note: The doomed object will be leaked if dispatch to the target thread
+ * fails, as releasing it on the current thread may be unsafe
+ *
+ * @param aName
+ *        the labelling name of the runnable involved in the releasing.
+ * @param aTarget
+ *        the target thread where the doomed object should be released.
+ * @param aDoomed
+ *        the doomed object; the object to be released on the target thread.
+ * @param aAlwaysProxy
+ *        normally, if NS_ProxyRelease is called on the target thread, then the
+ *        doomed object will be released directly. However, if this parameter is
+ *        true, then an event will always be posted to the target thread for
+ *        asynchronous release.
+ * @return result of the task which is dispatched to delete the smart pointer
+ *        on the target thread.
+ *        Note: The caller should not attempt to recover from an
+ *        error code returned by trying to perform the final ->Release()
+ *        manually.
+ */
+template <class T>
+inline NS_HIDDEN_(nsresult)
+    NS_ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                    already_AddRefed<T> aDoomed, bool aAlwaysProxy = false) {
+  return ::detail::ProxyReleaseChooser<
+      std::is_base_of<nsISupports, T>::value>::ProxyRelease(aName, aTarget,
+                                                            std::move(aDoomed),
+                                                            aAlwaysProxy);
+}
+
+/**
+ * Ensures that the delete of a smart pointer occurs on the main thread.
+ *
+ * @param aName
+ *        the labelling name of the runnable involved in the releasing
+ * @param aDoomed
+ *        the doomed object; the object to be released on the main thread.
+ * @param aAlwaysProxy
+ *        normally, if NS_ReleaseOnMainThread is called on the main
+ *        thread, then the doomed object will be released directly. However, if
+ *        this parameter is true, then an event will always be posted to the
+ *        main thread for asynchronous release.
+ */
+template <class T>
+inline NS_HIDDEN_(void)
+    NS_ReleaseOnMainThread(const char* aName, already_AddRefed<T> aDoomed,
+                           bool aAlwaysProxy = false) {
+  RefPtr<T> doomed = aDoomed;
+  if (!doomed) {
+    return;  // Nothing to do.
+  }
+
+  // NS_ProxyRelease treats a null event target as "the current thread".  So a
+  // handle on the main thread is only necessary when we're not already on the
+  // main thread or the release must happen asynchronously.
+  nsCOMPtr<nsIEventTarget> target;
+  if (!NS_IsMainThread() || aAlwaysProxy) {
+    target = mozilla::GetMainThreadSerialEventTarget();
+
+    if (!target) {
+      MOZ_ASSERT_UNREACHABLE("Could not get main thread; leaking an object!");
+      mozilla::Unused << doomed.forget().take();
+      return;
+    }
+  }
+
+  NS_ProxyRelease(aName, target, doomed.forget(), aAlwaysProxy);
+}
+
+/**
+ * Class to safely handle main-thread-only pointers off the main thread.
+ *
+ * Classes like XPCWrappedJS are main-thread-only, which means that it is
+ * forbidden to call methods on instances of these classes off the main thread.
+ * For various reasons (see bug 771074), this restriction applies to
+ * AddRef/Release as well.
+ *
+ * This presents a problem for consumers that wish to hold a callback alive
+ * on non-main-thread code. A common example of this is the proxy callback
+ * pattern, where non-main-thread code holds a strong-reference to the callback
+ * object, and dispatches new Runnables (also with a strong reference) to the
+ * main thread in order to execute the callback. This involves several AddRef
+ * and Release calls on the other thread, which is verboten.
+ *
+ * The basic idea of this class is to introduce a layer of indirection.
+ * nsMainThreadPtrHolder is a threadsafe reference-counted class that internally
+ * maintains one strong reference to the main-thread-only object. It must be
+ * instantiated on the main thread (so that the AddRef of the underlying object
+ * happens on the main thread), but consumers may subsequently pass references
+ * to the holder anywhere they please. These references are meant to be opaque
+ * when accessed off-main-thread (assertions enforce this).
+ *
+ * The semantics of RefPtr<nsMainThreadPtrHolder<T>> would be cumbersome, so we
+ * also introduce nsMainThreadPtrHandle<T>, which is conceptually identical to
+ * the above (though it includes various convenience methods). The basic pattern
+ * is as follows.
+ *
+ * // On the main thread:
+ * nsCOMPtr<nsIFooCallback> callback = ...;
+ * nsMainThreadPtrHandle<nsIFooCallback> callbackHandle =
+ *   new nsMainThreadPtrHolder<nsIFooCallback>(callback);
+ * // Pass callbackHandle to structs/classes that might be accessed on other
+ * // threads.
+ *
+ * All structs and classes that might be accessed on other threads should store
+ * an nsMainThreadPtrHandle<T> rather than an nsCOMPtr<T>.
+ */
+template <class T>
+class MOZ_IS_SMARTPTR_TO_REFCOUNTED nsMainThreadPtrHolder final {
+ public:
+  // We can only acquire a pointer on the main thread. We want to fail fast for
+  // threading bugs, so by default we assert if our pointer is used or acquired
+  // off-main-thread. But some consumers need to use the same pointer for
+  // multiple classes, some of which are main-thread-only and some of which
+  // aren't. So we allow them to explicitly disable this strict checking.
+  nsMainThreadPtrHolder(const char* aName, T* aPtr, bool aStrict = true,
+                        nsIEventTarget* aMainThreadEventTarget = nullptr)
+      : mRawPtr(aPtr),
+        mStrict(aStrict),
+        mMainThreadEventTarget(aMainThreadEventTarget)
+#ifndef RELEASE_OR_BETA
+        ,
+        mName(aName)
+#endif
+  {
+    // We can only AddRef our pointer on the main thread, which means that the
+    // holder must be constructed on the main thread.
+    MOZ_ASSERT(!mStrict || NS_IsMainThread());
+    NS_IF_ADDREF(mRawPtr);
+  }
+  nsMainThreadPtrHolder(const char* aName, already_AddRefed<T> aPtr,
+                        bool aStrict = true,
+                        nsIEventTarget* aMainThreadEventTarget = nullptr)
+      : mRawPtr(aPtr.take()),
+        mStrict(aStrict),
+        mMainThreadEventTarget(aMainThreadEventTarget)
+#ifndef RELEASE_OR_BETA
+        ,
+        mName(aName)
+#endif
+  {
+    // Since we don't need to AddRef the pointer, this constructor is safe to
+    // call on any thread.
+  }
+
+  // Copy constructor and operator= deleted. Once constructed, the holder is
+  // immutable.
+  T& operator=(nsMainThreadPtrHolder& aOther) = delete;
+  nsMainThreadPtrHolder(const nsMainThreadPtrHolder& aOther) = delete;
+
+ private:
+  // We can be released on any thread.
+  ~nsMainThreadPtrHolder() {
+    if (NS_IsMainThread()) {
+      NS_IF_RELEASE(mRawPtr);
+    } else if (mRawPtr) {
+      if (!mMainThreadEventTarget) {
+        mMainThreadEventTarget = do_GetMainThread();
+      }
+      MOZ_ASSERT(mMainThreadEventTarget);
+      NS_ProxyRelease(
+#ifdef RELEASE_OR_BETA
+          nullptr,
+#else
+          mName,
+#endif
+          mMainThreadEventTarget, dont_AddRef(mRawPtr));
+    }
+  }
+
+ public:
+  T* get() const {
+    // Nobody should be touching the raw pointer off-main-thread.
+    if (mStrict && MOZ_UNLIKELY(!NS_IsMainThread())) {
+      NS_ERROR("Can't dereference nsMainThreadPtrHolder off main thread");
+      MOZ_CRASH();
+    }
+    return mRawPtr;
+  }
+
+  bool operator==(const nsMainThreadPtrHolder<T>& aOther) const {
+    return mRawPtr == aOther.mRawPtr;
+  }
+  bool operator!() const { return !mRawPtr; }
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsMainThreadPtrHolder<T>)
+
+ private:
+  // Our wrapped pointer.
+  T* mRawPtr = nullptr;
+
+  // Whether to strictly enforce thread invariants in this class.
+  bool mStrict = true;
+
+  nsCOMPtr<nsIEventTarget> mMainThreadEventTarget;
+
+#ifndef RELEASE_OR_BETA
+  const char* mName = nullptr;
+#endif
+};
+
+template <class T>
+class MOZ_IS_SMARTPTR_TO_REFCOUNTED nsMainThreadPtrHandle {
+ public:
+  nsMainThreadPtrHandle() : mPtr(nullptr) {}
+  MOZ_IMPLICIT nsMainThreadPtrHandle(decltype(nullptr)) : mPtr(nullptr) {}
+  explicit nsMainThreadPtrHandle(nsMainThreadPtrHolder<T>* aHolder)
+      : mPtr(aHolder) {}
+  explicit nsMainThreadPtrHandle(
+      already_AddRefed<nsMainThreadPtrHolder<T>> aHolder)
+      : mPtr(aHolder) {}
+  nsMainThreadPtrHandle(const nsMainThreadPtrHandle& aOther) = default;
+  nsMainThreadPtrHandle(nsMainThreadPtrHandle&& aOther) = default;
+  nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther) =
+      default;
+  nsMainThreadPtrHandle& operator=(nsMainThreadPtrHandle&& aOther) = default;
+  nsMainThreadPtrHandle& operator=(nsMainThreadPtrHolder<T>* aHolder) {
+    mPtr = aHolder;
+    return *this;
+  }
+
+  // These all call through to nsMainThreadPtrHolder, and thus implicitly
+  // assert that we're on the main thread (if strict). Off-main-thread consumers
+  // must treat these handles as opaque.
+  T* get() const {
+    if (mPtr) {
+      return mPtr.get()->get();
+    }
+    return nullptr;
+  }
+
+  operator T*() const { return get(); }
+  T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return get(); }
+
+  // These are safe to call on other threads with appropriate external locking.
+  bool operator==(const nsMainThreadPtrHandle<T>& aOther) const {
+    if (!mPtr || !aOther.mPtr) {
+      return mPtr == aOther.mPtr;
+    }
+    return *mPtr == *aOther.mPtr;
+  }
+  bool operator!=(const nsMainThreadPtrHandle<T>& aOther) const {
+    return !operator==(aOther);
+  }
+  bool operator==(decltype(nullptr)) const { return mPtr == nullptr; }
+  bool operator!=(decltype(nullptr)) const { return mPtr != nullptr; }
+  bool operator!() const { return !mPtr || !*mPtr; }
+
+ private:
+  RefPtr<nsMainThreadPtrHolder<T>> mPtr;
+};
+
+class nsCycleCollectionTraversalCallback;
+template <typename T>
+void CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
+                              T* aChild, const char* aName, uint32_t aFlags);
+
+template <typename T>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    nsMainThreadPtrHandle<T>& aField, const char* aName, uint32_t aFlags = 0) {
+  CycleCollectionNoteChild(aCallback, aField.get(), aName, aFlags);
+}
+
+template <typename T>
+inline void ImplCycleCollectionUnlink(nsMainThreadPtrHandle<T>& aField) {
+  aField = nullptr;
+}
+
+#endif