[mojo-edk] Expose notification source to MojoWatch callbacks

mojo/edk/system/request_context.h

 // 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 "base/macros.h"
 #include "mojo/edk/system/handle_signals_state.h"
+#include "mojo/edk/system/system_impl_export.h"
 #include "mojo/edk/system/watcher.h"
 
 namespace base {
 template<typename T> class ThreadLocalPointer;
 }
 
 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
 // 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 {
+class MOJO_SYSTEM_IMPL_EXPORT RequestContext {
  public:
-  RequestContext();
+  // Identifies the source of the current stack frame's RequestContext.
+  enum class Source {
+    LOCAL_API_CALL,
+    EXTERNAL_PROCESS,
+  };
+
+  explicit RequestContext(Source source);

[Anand Mistry (off Chromium), 2016/03/16 23:04:14]

If local is the default and "external process" is the exceptional case, have a
default constructor that uses local. It avoids all the dispatcher changes. If
you do, make sure you add a comment that local is the default.

And we can use delegated constructors now, so this should be trivial.

[Ken Rockot(use gerrit already), 2016/03/17 00:15:34]

Done

   ~RequestContext();
 
   // Returns the current thread-local RequestContext.
   static RequestContext* current();
 
+  Source source() const { return source_; }
+
   // 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 AddWatchNotifyFinalizer(scoped_refptr<Watcher> watcher,
                                MojoResult result,
                                const HandleSignalsState& state);
 
   // Adds a finalizer to this RequestContext which cancels a watch.
   void AddWatchCancelFinalizer(scoped_refptr<Watcher> watcher);
 
  private:
+  //

[Anand Mistry (off Chromium), 2016/03/16 23:04:14]

nit: blank comment line.

[Ken Rockot(use gerrit already), 2016/03/17 00:15:34]

done

   // Is this request context the current one?
   bool IsCurrent() const;
 
   struct WatchNotifyFinalizer {
     WatchNotifyFinalizer(scoped_refptr<Watcher> watcher,
                          MojoResult result,
                          const HandleSignalsState& state);
     ~WatchNotifyFinalizer();
 
     scoped_refptr<Watcher> watcher;
     MojoResult result;
     HandleSignalsState 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 WatchNotifyFinalizerList =
       base::StackVector<WatchNotifyFinalizer, kStaticWatchFinalizersCapacity>;
   using WatchCancelFinalizerList =
       base::StackVector<scoped_refptr<Watcher>, kStaticWatchFinalizersCapacity>;
 
+  Source source_;
+
+  // A nested RequestContext may override the Source of the thread-local
+  // RequestContext for the duration of the nested context's lifetime.
+  Source outer_source_;
+
   WatchNotifyFinalizerList watch_notify_finalizers_;
   WatchCancelFinalizerList watch_cancel_finalizers_;
 
   // Pointer to the TLS context. Although this can easily be accessed via the
   // global LazyInstance, accessing a LazyInstance has a large cost relative to
   // the rest of this class and its usages.
   base::ThreadLocalPointer<RequestContext>* tls_context_;
 
   DISALLOW_COPY_AND_ASSIGN(RequestContext);
 };
 
 }  // namespace edk
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_REQUEST_CONTEXT_H_