Mojo: Armed Watchers

mojo/public/cpp/bindings/lib/connector.cc

 // Copyright 2013 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/bindings/connector.h"
 
 #include <stdint.h>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 170 matching lines @@
   return sync_watcher_->SyncWatch(should_stop);
 }
 
 void Connector::SetWatcherHeapProfilerTag(const char* tag) {
   heap_profiler_tag_ = tag;
   if (handle_watcher_) {
     handle_watcher_->set_heap_profiler_tag(tag);
   }
 }
 
+void Connector::EnableNestedDispatch(bool enabled) {
+  nested_dispatch_enabled_ = enabled;
+  CancelWait();

[yzshen1, 2017/03/03 00:03:50]

Will this cause problem while we are in the middle of a sync watch? For example:
1) Endpoint a uses Connector::SyncWatch() to wait for an incoming sync response.
2) The same thread is re-entered by another endpoint b.
3) Endpoint b calls EnableNestedDispatch() on a.
4) It will destroy the sync_watcher_ of a, which will cause the sync watch to
quit.

WDYT?

[Ken Rockot(use gerrit already), 2017/03/03 00:37:05]

Good point. Now I just reset handle_watcher_ before WaitToReadMore.

+  WaitToReadMore();
+}
+
 void Connector::OnWatcherHandleReady(MojoResult result) {
   OnHandleReadyInternal(result);
 }
 
 void Connector::OnSyncHandleWatcherHandleReady(MojoResult result) {
   base::WeakPtr<Connector> weak_self(weak_self_);
 
   sync_handle_watcher_callback_count_++;
   OnHandleReadyInternal(result);
   // At this point, this object might have been deleted.
   if (weak_self) {
     DCHECK_LT(0u, sync_handle_watcher_callback_count_);
     sync_handle_watcher_callback_count_--;
   }
 }
 
 void Connector::OnHandleReadyInternal(MojoResult result) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
   if (result != MOJO_RESULT_OK) {
     HandleError(result != MOJO_RESULT_FAILED_PRECONDITION, false);
     return;
   }
-  ReadAllAvailableMessages();
-  // At this point, this object might have been deleted. Return.
+
+  for (;;) {

[yzshen1, 2017/03/03 00:03:50]

nit: Does it make sense to merge ReadAllAvailableMessages() into this method or
merge this loop into ReadAllAvailableMessages()?

[Ken Rockot(use gerrit already), 2017/03/03 00:37:05]

Done

+    DestructionTracker::Flag was_destroyed(&destruction_tracker_);
+
+    // May delete |this|.
+    ReadAllAvailableMessages();
+
+    if (was_destroyed)
+      return;
+
+    // We also may have been paused by some dispatch, in which case we're done.
+    if (!handle_watcher_)
+      return;
+
+    // Attempt to re-arm the Watcher.
+    result = handle_watcher_->Arm();
+    switch (result) {
+      case MOJO_RESULT_OK:
+        // Everything's cool. No more work to do.
+        return;
+
+      case MOJO_RESULT_ALREADY_EXISTS:
+        // The handle is already readable again. Continue reading messagexs.
+        break;
+
+      case MOJO_RESULT_FAILED_PRECONDITION:
+        // The handle will never be readable again. Notify of error immediately.
+        // May delete |this|.
+        HandleError(false, false);
+        return;
+
+      default:
+        NOTREACHED();
+        break;
+    }
+  }
 }
 
 void Connector::WaitToReadMore() {
   CHECK(!paused_);
   DCHECK(!handle_watcher_);
 
-  handle_watcher_.reset(new Watcher(FROM_HERE, task_runner_));
+  handle_watcher_.reset(
+      new Watcher(FROM_HERE, Watcher::ArmingPolicy::MANUAL, task_runner_));
   if (heap_profiler_tag_)
     handle_watcher_->set_heap_profiler_tag(heap_profiler_tag_);
   MojoResult rv = handle_watcher_->Start(
       message_pipe_.get(), MOJO_HANDLE_SIGNAL_READABLE,
       base::Bind(&Connector::OnWatcherHandleReady, base::Unretained(this)));
 
   if (rv != MOJO_RESULT_OK) {
     // If the watch failed because the handle is invalid or its conditions can
     // no longer be met, we signal the error asynchronously to avoid reentry.
     task_runner_->PostTask(
         FROM_HERE,
         base::Bind(&Connector::OnWatcherHandleReady, weak_self_, rv));
+  } else {
+    handle_watcher_->ArmOrNotify();
   }
 
   if (allow_woken_up_by_others_) {
     EnsureSyncWatcherExists();
     sync_watcher_->AllowWokenUpBySyncWatchOnSameThread();
   }
 }
 
 bool Connector::ReadSingleMessage(MojoResult* read_result) {
   CHECK(!paused_);
 
   bool receiver_result = false;
 
   // Detect if |this| was destroyed or the message pipe was closed/transferred
   // during message dispatch.
   base::WeakPtr<Connector> weak_self = weak_self_;
 
   Message message;
   const MojoResult rv = ReadMessage(message_pipe_.get(), &message);
   *read_result = rv;
 
+  if (nested_dispatch_enabled_) {
+    // When supporting nested dispatch, we have to rearm the Watcher immediately
+    // after reading each message (i.e. before dispatch) to ensure that the next
+    // inbound message can trigger OnHandleReady on the nested loop.
+    handle_watcher_->ArmOrNotify();
+  }
+
   if (rv == MOJO_RESULT_OK) {
     receiver_result =
         incoming_receiver_ && incoming_receiver_->Accept(&message);
   }
 
   if (!weak_self)
     return false;
 
   if (rv == MOJO_RESULT_SHOULD_WAIT)
     return true;
@@ skipping 70 matching lines @@
 void Connector::EnsureSyncWatcherExists() {
   if (sync_watcher_)
     return;
   sync_watcher_.reset(new SyncHandleWatcher(
       message_pipe_.get(), MOJO_HANDLE_SIGNAL_READABLE,
       base::Bind(&Connector::OnSyncHandleWatcherHandleReady,
                  base::Unretained(this))));
 }
 
 }  // namespace mojo