Mojo: Armed Watchers

third_party/WebKit/Source/core/mojo/MojoWatcher.cpp

 // 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 "core/mojo/MojoWatcher.h"
 
 #include "bindings/core/v8/MojoWatchCallback.h"
 #include "bindings/core/v8/ScriptState.h"
 #include "core/dom/ExecutionContext.h"
 #include "core/dom/TaskRunnerHelper.h"
 #include "core/mojo/MojoHandleSignals.h"
 #include "platform/CrossThreadFunctional.h"
 #include "platform/WebTaskRunner.h"
 
 namespace blink {
 
 static void runWatchCallback(MojoWatchCallback* callback,
                              ScriptWrappable* wrappable,
                              MojoResult result) {
   callback->call(wrappable, result);
 }
 
+// static
 MojoWatcher* MojoWatcher::create(mojo::Handle handle,
                                  const MojoHandleSignals& signalsDict,
                                  MojoWatchCallback* callback,
                                  ExecutionContext* context) {
   MojoWatcher* watcher = new MojoWatcher(context, callback);
   MojoResult result = watcher->watch(handle, signalsDict);
   // TODO(alokp): Consider raising an exception.
   // Current clients expect to recieve the initial error returned by MojoWatch
   // via watch callback.
   //
   // Note that the usage of wrapPersistent is intentional so that the intial
   // error is guaranteed to be reported to the client in case where the given
   // handle is invalid and garbage collection happens before the callback
   // is scheduled.
   if (result != MOJO_RESULT_OK) {
     watcher->m_taskRunner->postTask(
         BLINK_FROM_HERE, WTF::bind(&runWatchCallback, wrapPersistent(callback),
                                    wrapPersistent(watcher), result));
   }
   return watcher;
 }
 
-MojoWatcher::MojoWatcher(ExecutionContext* context, MojoWatchCallback* callback)
-    : ContextLifecycleObserver(context),
-      m_taskRunner(TaskRunnerHelper::get(TaskType::UnspecedTimer, context)),
-      m_callback(this, callback) {}
-
 MojoWatcher::~MojoWatcher() {
   DCHECK(!m_handle.is_valid());
 }
 
-MojoResult MojoWatcher::watch(mojo::Handle handle,
-                              const MojoHandleSignals& signalsDict) {
-  ::MojoHandleSignals signals = MOJO_HANDLE_SIGNAL_NONE;
-  if (signalsDict.readable())
-    signals |= MOJO_HANDLE_SIGNAL_READABLE;
-  if (signalsDict.writable())
-    signals |= MOJO_HANDLE_SIGNAL_WRITABLE;
-  if (signalsDict.peerClosed())
-    signals |= MOJO_HANDLE_SIGNAL_PEER_CLOSED;
+MojoResult MojoWatcher::cancel() {
+  if (!m_watcherHandle.is_valid())
+    return MOJO_RESULT_INVALID_ARGUMENT;
 
-  MojoResult result =
-      MojoWatch(handle.value(), signals, &MojoWatcher::onHandleReady,
-                reinterpret_cast<uintptr_t>(this));
-  if (result == MOJO_RESULT_OK) {
-    m_handle = handle;
-  }
-  return result;
-}
-
-MojoResult MojoWatcher::cancel() {
-  if (!m_handle.is_valid())
-    return MOJO_RESULT_OK;
-
-  MojoResult result =
-      MojoCancelWatch(m_handle.value(), reinterpret_cast<uintptr_t>(this));
-  m_handle = mojo::Handle();
-  return result;
+  m_watcherHandle.reset();
+  return MOJO_RESULT_OK;
 }
 
 DEFINE_TRACE(MojoWatcher) {
   visitor->trace(m_callback);
   ContextLifecycleObserver::trace(visitor);
 }
 
 DEFINE_TRACE_WRAPPERS(MojoWatcher) {
   visitor->traceWrappers(m_callback);
 }
 
 bool MojoWatcher::hasPendingActivity() const {
   return m_handle.is_valid();
 }
 
 void MojoWatcher::contextDestroyed(ExecutionContext*) {
   cancel();
 }
 
+MojoWatcher::MojoWatcher(ExecutionContext* context, MojoWatchCallback* callback)
+    : ContextLifecycleObserver(context),
+      m_taskRunner(TaskRunnerHelper::get(TaskType::UnspecedTimer, context)),
+      m_callback(this, callback) {}
+
+MojoResult MojoWatcher::watch(mojo::Handle handle,
+                              const MojoHandleSignals& signalsDict) {
+  ::MojoHandleSignals signals = MOJO_HANDLE_SIGNAL_NONE;
+  if (signalsDict.readable())
+    signals |= MOJO_HANDLE_SIGNAL_READABLE;
+  if (signalsDict.writable())
+    signals |= MOJO_HANDLE_SIGNAL_WRITABLE;
+  if (signalsDict.peerClosed())
+    signals |= MOJO_HANDLE_SIGNAL_PEER_CLOSED;
+
+  MojoResult result =
+      mojo::CreateWatcher(&MojoWatcher::onHandleReady, &m_watcherHandle);
+  DCHECK_EQ(MOJO_RESULT_OK, result);
+
+  result = MojoWatch(m_watcherHandle.get().value(), handle.value(), signals,
+                     reinterpret_cast<uintptr_t>(this));
+  if (result != MOJO_RESULT_OK)
+    return result;
+
+  m_handle = handle;
+
+  MojoResult readyResult;
+  result = arm(&readyResult);
+  if (result == MOJO_RESULT_OK)
+    return result;
+
+  // We couldn't arm the watcher because the handle is already ready to
+  // trigger a success notification. Post a notification manually.
+  DCHECK_EQ(MOJO_RESULT_FAILED_PRECONDITION, result);
+  m_taskRunner->postTask(BLINK_FROM_HERE,
+                         WTF::bind(&MojoWatcher::runReadyCallback,
+                                   wrapPersistent(this), readyResult));
+  return MOJO_RESULT_OK;
+}
+
+MojoResult MojoWatcher::arm(MojoResult* readyResult) {
+  // Nothing to do if the watcher is inactive.
+  if (!m_handle.is_valid())
+    return MOJO_RESULT_OK;
+
+  uint32_t numReadyContexts = 1;
+  uintptr_t readyContext;
+  MojoResult localReadyResult;
+  MojoHandleSignalsState readySignals;
+  MojoResult result =
+      MojoArmWatcher(m_watcherHandle.get().value(), &numReadyContexts,
+                     &readyContext, &localReadyResult, &readySignals);
+  if (result == MOJO_RESULT_OK)
+    return MOJO_RESULT_OK;
+
+  DCHECK_EQ(MOJO_RESULT_FAILED_PRECONDITION, result);
+  DCHECK_EQ(1u, numReadyContexts);
+  DCHECK_EQ(reinterpret_cast<uintptr_t>(this), readyContext);
+  *readyResult = localReadyResult;
+  return result;
+}
+
 void MojoWatcher::onHandleReady(uintptr_t context,
                                 MojoResult result,
                                 MojoHandleSignalsState,
-                                MojoWatchNotificationFlags) {
-  // It is safe to assume the MojoWatcher still exists because this
-  // callback will never be run after MojoWatcher destructor,
-  // which cancels the watch.
+                                MojoWatcherNotificationFlags) {
+  // It is safe to assume the MojoWatcher still exists. It stays alive at least
+  // as long as |m_handle| is valid, and |m_handle| is only reset after we
+  // dispatch a |MOJO_RESULT_CANCELLED| notification. That is always the last
+  // notification received by this callback.
   MojoWatcher* watcher = reinterpret_cast<MojoWatcher*>(context);
   watcher->m_taskRunner->postTask(
       BLINK_FROM_HERE,
       crossThreadBind(&MojoWatcher::runReadyCallback,
                       wrapCrossThreadWeakPersistent(watcher), result));
 }
 
 void MojoWatcher::runReadyCallback(MojoResult result) {
+  if (result == MOJO_RESULT_CANCELLED) {
+    // Last notification.
+    m_handle = mojo::Handle();
+
+    // Only dispatch to the callback if this cancellation was implicit due to
+    // |m_handle| closure. If it was explicit, |m_watcherHandle| has already
+    // been reset.
+    if (m_watcherHandle.is_valid()) {
+      m_watcherHandle.reset();
+      runWatchCallback(m_callback, this, result);
+    }
+    return;
+  }
+
   // Ignore callbacks if not watching.
-  if (!m_handle.is_valid())
+  if (!m_watcherHandle.is_valid())
     return;
 
-  // MOJO_RESULT_CANCELLED indicates that the handle has been closed, in which
-  // case watch has been implicitly cancelled. There is no need to explicitly
-  // cancel the watch.
-  if (result == MOJO_RESULT_CANCELLED)
-    m_handle = mojo::Handle();
+  runWatchCallback(m_callback, this, result);
 
-  runWatchCallback(m_callback, this, result);
+  // Rearm the watcher so another notification can fire.
+  //
+  // TODO(rockot): MojoWatcher should expose some better approximation of the
+  // new watcher API, including explicit add and removal of handles from the
+  // watcher, as well as explicit arming.
+  MojoResult readyResult;
+  MojoResult armResult = arm(&readyResult);
+  if (armResult == MOJO_RESULT_OK)
+    return;
+
+  DCHECK_EQ(MOJO_RESULT_FAILED_PRECONDITION, armResult);
+
+  m_taskRunner->postTask(BLINK_FROM_HERE,
+                         WTF::bind(&MojoWatcher::runReadyCallback,
+                                   wrapWeakPersistent(this), readyResult));
 }
 
 }  // namespace blink