1 files changed, 383 insertions, 0 deletions
diff --git a/netwerk/ipc/ChannelEventQueue.h b/netwerk/ipc/ChannelEventQueue.h
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+++ b/netwerk/ipc/ChannelEventQueue.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set sw=2 ts=8 et 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_net_ChannelEventQueue_h
+#define mozilla_net_ChannelEventQueue_h
+
+#include "nsTArray.h"
+#include "nsIEventTarget.h"
+#include "nsThreadUtils.h"
+#include "nsXULAppAPI.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/RecursiveMutex.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
+
+class nsISupports;
+
+namespace mozilla {
+namespace net {
+
+class ChannelEvent {
+ public:
+  MOZ_COUNTED_DEFAULT_CTOR(ChannelEvent)
+  MOZ_COUNTED_DTOR_VIRTUAL(ChannelEvent) virtual void Run() = 0;
+  virtual already_AddRefed<nsIEventTarget> GetEventTarget() = 0;
+};
+
+// Note that MainThreadChannelEvent should not be used in child process since
+// GetEventTarget() directly returns an unlabeled event target.
+class MainThreadChannelEvent : public ChannelEvent {
+ public:
+  MOZ_COUNTED_DEFAULT_CTOR(MainThreadChannelEvent)
+  MOZ_COUNTED_DTOR_OVERRIDE(MainThreadChannelEvent)
+
+  already_AddRefed<nsIEventTarget> GetEventTarget() override {
+    MOZ_ASSERT(XRE_IsParentProcess());
+
+    return do_AddRef(GetMainThreadSerialEventTarget());
+  }
+};
+
+class ChannelFunctionEvent : public ChannelEvent {
+ public:
+  ChannelFunctionEvent(
+      std::function<already_AddRefed<nsIEventTarget>()>&& aGetEventTarget,
+      std::function<void()>&& aCallback)
+      : mGetEventTarget(std::move(aGetEventTarget)),
+        mCallback(std::move(aCallback)) {}
+
+  void Run() override { mCallback(); }
+  already_AddRefed<nsIEventTarget> GetEventTarget() override {
+    return mGetEventTarget();
+  }
+
+ private:
+  const std::function<already_AddRefed<nsIEventTarget>()> mGetEventTarget;
+  const std::function<void()> mCallback;
+};
+
+// UnsafePtr is a work-around our static analyzer that requires all
+// ref-counted objects to be captured in lambda via a RefPtr
+// The ChannelEventQueue makes it safe to capture "this" by pointer only.
+// This is required as work-around to prevent cycles until bug 1596295
+// is resolved.
+template <typename T>
+class UnsafePtr {
+ public:
+  explicit UnsafePtr(T* aPtr) : mPtr(aPtr) {}
+
+  T& operator*() const { return *mPtr; }
+  T* operator->() const {
+    MOZ_ASSERT(mPtr, "dereferencing a null pointer");
+    return mPtr;
+  }
+  operator T*() const& { return mPtr; }
+  explicit operator bool() const { return mPtr != nullptr; }
+
+ private:
+  T* const mPtr;
+};
+
+class NeckoTargetChannelFunctionEvent : public ChannelFunctionEvent {
+ public:
+  template <typename T>
+  NeckoTargetChannelFunctionEvent(T* aChild, std::function<void()>&& aCallback)
+      : ChannelFunctionEvent(
+            [child = UnsafePtr<T>(aChild)]() {
+              MOZ_ASSERT(child);
+              return child->GetNeckoTarget();
+            },
+            std::move(aCallback)) {}
+};
+
+// Workaround for Necko re-entrancy dangers. We buffer IPDL messages in a
+// queue if still dispatching previous one(s) to listeners/observers.
+// Otherwise synchronous XMLHttpRequests and/or other code that spins the
+// event loop (ex: IPDL rpc) could cause listener->OnDataAvailable (for
+// instance) to be dispatched and called before mListener->OnStartRequest has
+// completed.
+// The ChannelEventQueue implementation ensures strict ordering of
+// event execution across target threads.
+
+class ChannelEventQueue final {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ChannelEventQueue)
+
+ public:
+  explicit ChannelEventQueue(nsISupports* owner)
+      : mSuspendCount(0),
+        mSuspended(false),
+        mForcedCount(0),
+        mFlushing(false),
+        mHasCheckedForXMLHttpRequest(false),
+        mForXMLHttpRequest(false),
+        mOwner(owner),
+        mMutex("ChannelEventQueue::mMutex"),
+        mRunningMutex("ChannelEventQueue::mRunningMutex") {}
+
+  // Puts IPDL-generated channel event into queue, to be run later
+  // automatically when EndForcedQueueing and/or Resume is called.
+  //
+  // @param aCallback - the ChannelEvent
+  // @param aAssertionWhenNotQueued - this optional param will be used in an
+  //   assertion when the event is executed directly.
+  inline void RunOrEnqueue(ChannelEvent* aCallback,
+                           bool aAssertionWhenNotQueued = false);
+
+  // Append ChannelEvent in front of the event queue.
+  inline void PrependEvent(UniquePtr<ChannelEvent>&& aEvent);
+  inline void PrependEvents(nsTArray<UniquePtr<ChannelEvent>>& aEvents);
+
+  // After StartForcedQueueing is called, RunOrEnqueue() will start enqueuing
+  // events that will be run/flushed when EndForcedQueueing is called.
+  // - Note: queueing may still be required after EndForcedQueueing() (if the
+  //   queue is suspended, etc):  always call RunOrEnqueue() to avoid race
+  //   conditions.
+  inline void StartForcedQueueing();
+  inline void EndForcedQueueing();
+
+  // Suspend/resume event queue.  RunOrEnqueue() will start enqueuing
+  // events and they will be run/flushed when resume is called.  These should be
+  // called when the channel owning the event queue is suspended/resumed.
+  void Suspend();
+  // Resume flushes the queue asynchronously, i.e. items in queue will be
+  // dispatched in a new event on the current thread.
+  void Resume();
+
+  void NotifyReleasingOwner() {
+    MutexAutoLock lock(mMutex);
+    mOwner = nullptr;
+  }
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  bool IsEmpty() {
+    MutexAutoLock lock(mMutex);
+    return mEventQueue.IsEmpty();
+  }
+#endif
+
+ private:
+  // Private destructor, to discourage deletion outside of Release():
+  ~ChannelEventQueue() = default;
+
+  void SuspendInternal();
+  void ResumeInternal();
+
+  bool MaybeSuspendIfEventsAreSuppressed() MOZ_REQUIRES(mMutex);
+
+  inline void MaybeFlushQueue();
+  void FlushQueue();
+  inline void CompleteResume();
+
+  ChannelEvent* TakeEvent();
+
+  nsTArray<UniquePtr<ChannelEvent>> mEventQueue MOZ_GUARDED_BY(mMutex);
+
+  uint32_t mSuspendCount MOZ_GUARDED_BY(mMutex);
+  bool mSuspended MOZ_GUARDED_BY(mMutex);
+  uint32_t mForcedCount  // Support ForcedQueueing on multiple thread.
+      MOZ_GUARDED_BY(mMutex);
+  bool mFlushing MOZ_GUARDED_BY(mMutex);
+
+  // Whether the queue is associated with an XHR. This is lazily instantiated
+  // the first time it is needed. These are MainThread-only.
+  bool mHasCheckedForXMLHttpRequest;
+  bool mForXMLHttpRequest;
+
+  // Keep ptr to avoid refcount cycle: only grab ref during flushing.
+  nsISupports* mOwner MOZ_GUARDED_BY(mMutex);
+
+  // For atomic mEventQueue operation and state update
+  Mutex mMutex;
+
+  // To guarantee event execution order among threads
+  RecursiveMutex mRunningMutex MOZ_ACQUIRED_BEFORE(mMutex);
+
+  friend class AutoEventEnqueuer;
+};
+
+inline void ChannelEventQueue::RunOrEnqueue(ChannelEvent* aCallback,
+                                            bool aAssertionWhenNotQueued) {
+  MOZ_ASSERT(aCallback);
+  // Events execution could be a destruction of the channel (and our own
+  // destructor) unless we make sure its refcount doesn't drop to 0 while this
+  // method is running.
+  nsCOMPtr<nsISupports> kungFuDeathGrip;
+
+  // To avoid leaks.
+  UniquePtr<ChannelEvent> event(aCallback);
+
+  // To guarantee that the running event and all the events generated within
+  // it will be finished before events on other threads.
+  RecursiveMutexAutoLock lock(mRunningMutex);
+  {
+    MutexAutoLock lock(mMutex);
+    kungFuDeathGrip = mOwner;  // must be under the lock
+
+    bool enqueue = !!mForcedCount || mSuspended || mFlushing ||
+                   !mEventQueue.IsEmpty() ||
+                   MaybeSuspendIfEventsAreSuppressed();
+    // To ensure strict ordering of events across multiple threads we buffer the
+    // events for the below cases:
+    // a. event queuing is forced by AutoEventEnqueuer
+    // b. event queue is suspended
+    // c. an event is currently flushed/executed from the queue
+    // d. queue is non-empty (pending events on remote thread targets)
+    if (enqueue) {
+      mEventQueue.AppendElement(std::move(event));
+      return;
+    }
+
+    nsCOMPtr<nsIEventTarget> target = event->GetEventTarget();
+    MOZ_ASSERT(target);
+
+    bool isCurrentThread = false;
+    DebugOnly<nsresult> rv = target->IsOnCurrentThread(&isCurrentThread);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+    if (!isCurrentThread) {
+      // Leverage Suspend/Resume mechanism to trigger flush procedure without
+      // creating a new one.
+      // The execution of further events in the queue is blocked until the
+      // target thread completes the execution of this event.
+      // A callback is dispatched to the target thread to flush events from the
+      // queue. This is done
+      // by ResumeInternal which dispatches a runnable
+      // (CompleteResumeRunnable) to the target thread. The target thread will
+      // call CompleteResume to flush the queue. All the events are run
+      // synchronously in their respective target threads.
+      SuspendInternal();
+      mEventQueue.AppendElement(std::move(event));
+      ResumeInternal();
+      return;
+    }
+  }
+
+  MOZ_RELEASE_ASSERT(!aAssertionWhenNotQueued);
+  // execute the event synchronously if we are not queuing it and
+  // the target thread is the current thread
+  event->Run();
+}
+
+inline void ChannelEventQueue::StartForcedQueueing() {
+  MutexAutoLock lock(mMutex);
+  ++mForcedCount;
+}
+
+inline void ChannelEventQueue::EndForcedQueueing() {
+  bool tryFlush = false;
+  {
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mForcedCount > 0);
+    if (!--mForcedCount) {
+      tryFlush = true;
+    }
+  }
+
+  if (tryFlush) {
+    MaybeFlushQueue();
+  }
+}
+
+inline void ChannelEventQueue::PrependEvent(UniquePtr<ChannelEvent>&& aEvent) {
+  MutexAutoLock lock(mMutex);
+
+  // Prepending event while no queue flush foreseen might cause the following
+  // channel events not run. This assertion here guarantee there must be a
+  // queue flush, either triggered by Resume or EndForcedQueueing, to execute
+  // the added event.
+  MOZ_ASSERT(mSuspended || !!mForcedCount);
+
+  mEventQueue.InsertElementAt(0, std::move(aEvent));
+}
+
+inline void ChannelEventQueue::PrependEvents(
+    nsTArray<UniquePtr<ChannelEvent>>& aEvents) {
+  MutexAutoLock lock(mMutex);
+
+  // Prepending event while no queue flush foreseen might cause the following
+  // channel events not run. This assertion here guarantee there must be a
+  // queue flush, either triggered by Resume or EndForcedQueueing, to execute
+  // the added events.
+  MOZ_ASSERT(mSuspended || !!mForcedCount);
+
+  mEventQueue.InsertElementsAt(0, aEvents.Length());
+
+  for (uint32_t i = 0; i < aEvents.Length(); i++) {
+    mEventQueue[i] = std::move(aEvents[i]);
+  }
+}
+
+inline void ChannelEventQueue::CompleteResume() {
+  bool tryFlush = false;
+  {
+    MutexAutoLock lock(mMutex);
+
+    // channel may have been suspended again since Resume fired event to call
+    // this.
+    if (!mSuspendCount) {
+      // we need to remain logically suspended (for purposes of queuing incoming
+      // messages) until this point, else new incoming messages could run before
+      // queued ones.
+      mSuspended = false;
+      tryFlush = true;
+    }
+  }
+
+  if (tryFlush) {
+    MaybeFlushQueue();
+  }
+}
+
+inline void ChannelEventQueue::MaybeFlushQueue() {
+  // Don't flush if forced queuing on, we're already being flushed, or
+  // suspended, or there's nothing to flush
+  bool flushQueue = false;
+
+  {
+    MutexAutoLock lock(mMutex);
+    flushQueue = !mForcedCount && !mFlushing && !mSuspended &&
+                 !mEventQueue.IsEmpty() && !MaybeSuspendIfEventsAreSuppressed();
+
+    // Only one thread is allowed to run FlushQueue at a time.
+    if (flushQueue) {
+      mFlushing = true;
+    }
+  }
+
+  if (flushQueue) {
+    FlushQueue();
+  }
+}
+
+// Ensures that RunOrEnqueue() will be collecting events during its lifetime
+// (letting caller know incoming IPDL msgs should be queued). Flushes the queue
+// when it goes out of scope.
+class MOZ_STACK_CLASS AutoEventEnqueuer {
+ public:
+  explicit AutoEventEnqueuer(ChannelEventQueue* queue) : mEventQueue(queue) {
+    {
+      // Probably not actually needed, since NotifyReleasingOwner should
+      // only happen after this, but safer to take it in case things change
+      MutexAutoLock lock(queue->mMutex);
+      mOwner = queue->mOwner;
+    }
+    mEventQueue->StartForcedQueueing();
+  }
+  ~AutoEventEnqueuer() { mEventQueue->EndForcedQueueing(); }
+
+ private:
+  RefPtr<ChannelEventQueue> mEventQueue;
+  // Ensure channel object lives longer than ChannelEventQueue.
+  nsCOMPtr<nsISupports> mOwner;
+};
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif