[mojo-edk] Add MojoWatch and MojoCancelWatch APIs

mojo/edk/system/request_context.h

Index: mojo/edk/system/request_context.h
diff --git a/mojo/edk/system/request_context.h b/mojo/edk/system/request_context.h
new file mode 100644
index 0000000000000000000000000000000000000000..14cc1dd7cfa65f2b14b6bf27995c0da770612e98
--- /dev/null
+++ b/mojo/edk/system/request_context.h
@@ -0,0 +1,72 @@
+// 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.
+
+#ifndef MOJO_EDK_SYSTEM_REQUEST_CONTEXT_H_
+#define MOJO_EDK_SYSTEM_REQUEST_CONTEXT_H_
+
+#include "base/containers/stack_container.h"
+#include "mojo/edk/system/watcher.h"
+
+namespace mojo {
+namespace edk {
+
+// A RequestContext is a thread-local object which exists for the duration of
+// a single system API call. It is constructed immediately upon EDK entry and
+// destructed immediately before returning to the caller, after any internal
+// locks have been released.
+//
+// NOTE: It is legal to construct a RequestContext while another one already

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

Are there any specific cases where this happens right now? I'd like to know if
you can remove this and always require that there's only one RequestContext
active on a thread.

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

So if you WriteMessage to a pipe endpoint whose peer is in the same process,
OnPortStatusChanged will be called synchronously on peer's dispatcher. But
OnPortStatusChanged may also be called as a result of EDK entry via IPC arrival
on a NodeChannel, so it needs to ensure it has a RequestContext in that case
too.

I couldn't decide between doing this, or just adding a RequestContext somewhere
in NodeController (basically anywhere Node::AcceptMessage can be called) and
removing them from OnPortStatusChanged. I went with this because it seemed
harder to get wrong. Not sure that is the right call though. WDYT?

[Anand Mistry (off Chromium), 2016/03/02 01:50:35]

This approach is good. The thing that confused me a bit was that you end up in a
situation where there'a RequestContext on the stack that is completely unused.
But I think this is the way to go. In fact, I think it's desirable to unwind the
stack as much as possible before executing any callbacks.

+// exists on the current thread, but it is not safe to use the nested context
+// for any reason. Therefore it is important to always use
+// |RequestContext::current()| rather than referring to any local instance
+// directly.
+class RequestContext {
+ public:
+  RequestContext();
+  ~RequestContext();
+
+  // Returns the current thread-local RequestContext.
+  static RequestContext* current();
+
+  // Adds a finalizer to this RequestContext corresponding to a watch callback
+  // which should be triggered in response to some handle state change. If
+  // the Watcher hasn't been cancelled by the time this RequestContext is
+  // destroyed, its WatchCallback will be invoked with |result| and |state|
+  // arguments.
+  void AddWatchFinalizer(scoped_refptr<Watcher> watcher,
+                         MojoResult result,
+                         MojoHandleSignalsState state);
+
+ private:
+  // Is this request context the current one?
+  bool IsCurrent() const;
+
+  struct WatchFinalizer {
+    WatchFinalizer(scoped_refptr<Watcher> watcher,
+                   MojoResult result,
+                   MojoHandleSignalsState state);
+    ~WatchFinalizer();
+
+    scoped_refptr<Watcher> watcher;
+    MojoResult result;
+    MojoHandleSignalsState state;
+  };
+
+  // Chosen by fair dice roll.
+  //
+  // TODO: We should measure the distribution of # of finalizers typical to
+  // any RequestContext and adjust this number accordingly. It's probably
+  // almost always 1, but 4 seems like a harmless upper bound for now.
+  static const size_t kStaticWatchFinalizersCapacity = 4;
+
+  using WatchFinalizerList =
+      base::StackVector<WatchFinalizer, kStaticWatchFinalizersCapacity>;
+
+  WatchFinalizerList watch_finalizers_;
+};

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

DISALLOW_COPY_AND_ASSIGN

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

Done

+
+}  // namespace edk
+}  // namespace mojo
+
+#endif  // MOJO_EDK_SYSTEM_REQUEST_CONTEXT_H_