Mojo: Armed Watchers

mojo/public/cpp/system/simple_watcher.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/public/cpp/system/simple_watcher.h"
+
+#include "base/bind.h"
+#include "base/macros.h"
+#include "base/memory/ptr_util.h"
+#include "base/single_thread_task_runner.h"
+#include "base/synchronization/lock.h"
+#include "base/trace_event/heap_profiler.h"
+#include "mojo/public/c/system/watcher.h"
+
+namespace mojo {
+
+// Thread-safe Context object used to dispatch watch notifications from a
+// arbitrary threads.
+class SimpleWatcher::Context : public base::RefCountedThreadSafe<Context> {
+ public:
+  // Creates a |Context| instance for a new watch on |watcher|, to watch
+  // |handle| for |signals|.
+  static scoped_refptr<Context> Create(
+      base::WeakPtr<SimpleWatcher> watcher,
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner,
+      WatcherHandle watcher_handle,
+      Handle handle,
+      MojoHandleSignals signals,
+      MojoResult* watch_result) {
+    // We use a scoped_refptr<Context> instance as watch context value. This
+    // instance (and thus the Context ref it holds) is effectively owned by the
+    // registered watch. We delete it on cancellation, which is a guaranteed
+    // event.
+    auto context_ref = base::MakeUnique<scoped_refptr<Context>>();
+    *context_ref = new Context(watcher, task_runner,
+                               reinterpret_cast<uintptr_t>(context_ref.get()));
+
+    *watch_result = MojoWatch(watcher_handle.value(), handle.value(), signals,
+                              (*context_ref)->value());
+    if (*watch_result != MOJO_RESULT_OK)
+      return nullptr;
+
+    // Ownership of the ref has been transferred into the successfully
+    // registered watch.
+    return *context_ref.release();
+  }
+
+  static void CallNotify(uintptr_t context_value,
+                         MojoResult result,
+                         MojoHandleSignalsState signals_state,
+                         MojoWatcherNotificationFlags flags) {
+    auto* context_ref =
+        reinterpret_cast<scoped_refptr<Context>*>(context_value);
+    (*context_ref)->Notify(result, signals_state, flags);
+
+    // That was the last notification for the context. We can delete the ref
+    // owned by the watch.
+    if (result == MOJO_RESULT_CANCELLED)
+      delete context_ref;

[yzshen1, 2017/03/13 20:21:18]

Does it make sense to use Context* as the |context_value|, which has one ref
owned by the registered watch, and we release that ref on cancellation?

[Ken Rockot(use gerrit already), 2017/03/13 22:00:10]

Sure, that's effectively equivalent to this, but I guess it does seem a little
cleaner. Done.

+  }
+
+  uintptr_t value() const { return context_value_; }
+
+  void DisableCancellationNotifications() {
+    base::AutoLock lock(lock_);
+    enable_cancellation_notifications_ = false;
+  }
+
+ private:
+  friend class base::RefCountedThreadSafe<Context>;
+
+  Context(base::WeakPtr<SimpleWatcher> weak_watcher,
+          scoped_refptr<base::SingleThreadTaskRunner> task_runner,
+          uintptr_t context_value)
+      : weak_watcher_(weak_watcher),
+        task_runner_(task_runner),
+        context_value_(context_value) {}
+  ~Context() {}
+
+  void Notify(MojoResult result,
+              MojoHandleSignalsState signals_state,
+              MojoWatcherNotificationFlags flags) {
+    if (result == MOJO_RESULT_CANCELLED) {
+      // The SimpleWatcher may have explicitly cancelled this watch, so we don't
+      // bother dispatching the notification - it would be ignored anyway.
+      //
+      // TODO(rockot): This shouldn't really be necessary, but there are already
+      // instances today where bindings object may be bound and subsequently
+      // closed due to pipe error, all before the thread's TaskRunner has been
+      // properly initialized.
+      base::AutoLock lock(lock_);
+      if (!enable_cancellation_notifications_)
+        return;
+    }
+
+    if ((flags & MOJO_WATCHER_NOTIFICATION_FLAG_FROM_SYSTEM) &&
+        task_runner_->RunsTasksOnCurrentThread() && weak_watcher_ &&
+        weak_watcher_->is_default_task_runner_) {
+      // System notifications will trigger from the task runner passed to
+      // mojo::edk::InitIPCSupport(). In Chrome this happens to always be the
+      // default task runner for the IO thread.
+      weak_watcher_->OnHandleReady(make_scoped_refptr(this), result);
+    } else {
+      task_runner_->PostTask(
+          FROM_HERE, base::Bind(&SimpleWatcher::OnHandleReady, weak_watcher_,
+                                make_scoped_refptr(this), result));
+    }
+  }
+
+  const base::WeakPtr<SimpleWatcher> weak_watcher_;
+  const scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+  const uintptr_t context_value_;
+
+  base::Lock lock_;
+  bool enable_cancellation_notifications_ = true;
+
+  DISALLOW_COPY_AND_ASSIGN(Context);
+};
+
+SimpleWatcher::SimpleWatcher(const tracked_objects::Location& from_here,
+                             ArmingPolicy arming_policy,
+                             scoped_refptr<base::SingleThreadTaskRunner> runner)
+    : arming_policy_(arming_policy),
+      task_runner_(std::move(runner)),
+      is_default_task_runner_(task_runner_ ==
+                              base::ThreadTaskRunnerHandle::Get()),
+      heap_profiler_tag_(from_here.file_name()),
+      weak_factory_(this) {
+  MojoResult rv = CreateWatcher(&Context::CallNotify, &watcher_handle_);
+  DCHECK_EQ(MOJO_RESULT_OK, rv);
+  DCHECK(task_runner_->BelongsToCurrentThread());
+}
+
+SimpleWatcher::~SimpleWatcher() {
+  if (IsWatching())
+    Cancel();
+}
+
+bool SimpleWatcher::IsWatching() const {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  return context_ != nullptr;
+}
+
+MojoResult SimpleWatcher::Watch(Handle handle,
+                                MojoHandleSignals signals,
+                                const ReadyCallback& callback) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(!IsWatching());
+  DCHECK(!callback.is_null());
+
+  callback_ = callback;
+  handle_ = handle;
+
+  MojoResult watch_result = MOJO_RESULT_UNKNOWN;
+  context_ =
+      Context::Create(weak_factory_.GetWeakPtr(), task_runner_,
+                      watcher_handle_.get(), handle_, signals, &watch_result);
+  if (!context_) {
+    handle_.set_value(kInvalidHandleValue);
+    callback_.Reset();
+    DCHECK_EQ(MOJO_RESULT_INVALID_ARGUMENT, watch_result);
+    return watch_result;
+  }
+
+  if (arming_policy_ == ArmingPolicy::AUTOMATIC)
+    ArmOrNotify();
+
+  return MOJO_RESULT_OK;
+}
+
+void SimpleWatcher::Cancel() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  // The watcher may have already been cancelled if the handle was closed.
+  if (!context_)
+    return;
+
+  context_->DisableCancellationNotifications();
+
+  handle_.set_value(kInvalidHandleValue);
+  callback_.Reset();
+
+  // Ensure |context_| is unset by the time we call MojoCancelWatch, as may
+  // re-enter the notification callback and we want to ensure |context_| is
+  // unset by then. This prevents the cancellation notification from reaching

[yzshen1, 2017/03/13 20:21:18]

Is the last sentence a comment for line 178 instead?

[Ken Rockot(use gerrit already), 2017/03/13 22:00:10]

Ah good catch. It used to be accurate here, but I added
DisableCancellationNotifications() later after encountering issues described in
the TODO there. Moved the comment.

+  // OnHandleReady() when cancellation is explicit.
+  scoped_refptr<Context> context;
+  std::swap(context, context_);
+  MojoResult rv =
+      MojoCancelWatch(watcher_handle_.get().value(), context->value());
+
+  // It's possible this cancellation could race with a handle closure
+  // notification, in which case the watch may have already been implicitly
+  // cancelled.
+  DCHECK(rv == MOJO_RESULT_OK || rv == MOJO_RESULT_NOT_FOUND);
+}
+
+MojoResult SimpleWatcher::Arm(MojoResult* ready_result) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  uint32_t num_ready_contexts = 1;
+  uintptr_t ready_context;
+  MojoResult local_ready_result;
+  MojoHandleSignalsState ready_state;
+  MojoResult rv =
+      MojoArmWatcher(watcher_handle_.get().value(), &num_ready_contexts,
+                     &ready_context, &local_ready_result, &ready_state);
+  if (rv == MOJO_RESULT_FAILED_PRECONDITION) {
+    DCHECK(context_);
+    DCHECK_EQ(1u, num_ready_contexts);
+    DCHECK_EQ(context_->value(), ready_context);
+    if (ready_result)
+      *ready_result = local_ready_result;
+  }
+
+  return rv;
+}
+
+void SimpleWatcher::ArmOrNotify() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  // Already cancelled, nothing to do.
+  if (!IsWatching())
+    return;
+
+  MojoResult ready_result;
+  MojoResult rv = Arm(&ready_result);
+  if (rv == MOJO_RESULT_OK)
+    return;
+
+  DCHECK_EQ(MOJO_RESULT_FAILED_PRECONDITION, rv);
+  task_runner_->PostTask(FROM_HERE, base::Bind(&SimpleWatcher::OnHandleReady,
+                                               weak_factory_.GetWeakPtr(),
+                                               context_, ready_result));
+}
+
+void SimpleWatcher::OnHandleReady(scoped_refptr<const Context> context,
+                                  MojoResult result) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  // This notification may be for a previously watched context, in which case
+  // we just ignore it.
+  if (context != context_)
+    return;
+
+  ReadyCallback callback = callback_;
+  if (result == MOJO_RESULT_CANCELLED) {
+    // Implicit cancellation due to someone closing the watched handle. We clear
+    // the SimppleWatcher's state before dispatching this.
+    context_ = nullptr;
+    handle_.set_value(kInvalidHandleValue);
+    callback_.Reset();
+  }
+
+  // NOTE: It's legal for |callback| to delete |this|.
+  if (!callback.is_null()) {
+    TRACE_HEAP_PROFILER_API_SCOPED_TASK_EXECUTION event(heap_profiler_tag_);
+
+    base::WeakPtr<SimpleWatcher> weak_self = weak_factory_.GetWeakPtr();
+    callback.Run(result);
+    if (!weak_self)
+      return;
+
+    if (unsatisfiable_)
+      return;
+
+    // Prevent |MOJO_RESULT_FAILED_PRECONDITION| task spam by only notifying
+    // at most once in AUTOMATIC arming mode.
+    if (result == MOJO_RESULT_FAILED_PRECONDITION)
+      unsatisfiable_ = true;
+
+    if (arming_policy_ == ArmingPolicy::AUTOMATIC && IsWatching())
+      ArmOrNotify();
+  }
+}
+
+}  // namespace mojo