[mojo-edk] Add MojoWatch and MojoCancelWatch APIs

mojo/edk/system/watcher_set.cc

+// Copyright 2016 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/edk/system/watcher_set.h"
+
+#include "mojo/public/c/system/types.h"
+
+namespace mojo {
+namespace edk {
+
+WatcherSet::WatcherSet() {}
+
+WatcherSet::~WatcherSet() {}
+
+void WatcherSet::NotifyForStateChange(const HandleSignalsState& state) {
+  for (const auto& entry : watchers_) {
+    entry.second->NotifyForStateChange(
+        static_cast<MojoHandleSignalsState>(state));
+  }
+}
+
+void WatcherSet::NotifyClosed() {
+  for (const auto& entry : watchers_)
+    entry.second->NotifyClosed();
+}
+
+MojoResult WatcherSet::Add(MojoHandleSignals signals,
+                           const Watcher::WatchCallback& callback,
+                           uintptr_t context,
+                           const HandleSignalsState& current_state) {
+  auto it = watchers_.find(context);
+  if (it != watchers_.end())
+    return MOJO_RESULT_ALREADY_EXISTS;
+
+  if (!current_state.can_satisfy(signals))
+    return MOJO_RESULT_FAILED_PRECONDITION;
+
+  scoped_refptr<Watcher> watcher(new Watcher(signals, callback));
+  watchers_.insert(std::make_pair(context, watcher));
+
+  watcher->NotifyForStateChange(current_state);
+
+  return MOJO_RESULT_OK;
+}
+
+MojoResult WatcherSet::Remove(uintptr_t context) {
+  auto it = watchers_.find(context);
+  if (it == watchers_.end())
+    return MOJO_RESULT_INVALID_ARGUMENT;
+
+  it->second->Cancel();

[Anand Mistry (off Chromium), 2016/03/01 07:30:40]

Hmm. There's a potential deadlock here. If on thread |A| you try and remove a
watcher, and on thread |B|, the callback is running and tries to add a watcher,
you have a deadlock.
Specifically:
A - has acquired dispatcher lock, tries to acquire Watcher::lock_
B - has Watcher::lock_, tries to acquire dispatcher lock

At the very least, TSAN will complain about lock inversion if you try it.

[Ken Rockot(use gerrit already), 2016/03/01 08:17:44]

Good catch. I've changed this so that cancellation also uses a finalizer.

+  watchers_.erase(it);
+  return MOJO_RESULT_OK;
+}
+
+}  // namespace edk
+}  // namespace mojo