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diff --git a/mojo/edk/system/request_context.h b/mojo/edk/system/request_context.h
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+++ b/mojo/edk/system/request_context.h
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+// 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/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
+// 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 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:
+  // 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>;
+
+  WatchNotifyFinalizerList watch_notify_finalizers_;
+  WatchCancelFinalizerList watch_cancel_finalizers_;
+
+  DISALLOW_COPY_AND_ASSIGN(RequestContext);
+};
+
+}  // namespace edk
+}  // namespace mojo
+
+#endif  // MOJO_EDK_SYSTEM_REQUEST_CONTEXT_H_