Add different behavior to AwakableList for "persistent" vs "one-shot" awakables.

mojo/edk/system/awakable.h

 // 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.
 
 #ifndef MOJO_EDK_SYSTEM_AWAKABLE_H_
 #define MOJO_EDK_SYSTEM_AWAKABLE_H_
 
 #include <stdint.h>
 
 #include "mojo/edk/system/handle_signals_state.h"
 #include "mojo/public/c/system/result.h"
 
 namespace mojo {
 namespace system {
 
 // An interface for things that may be awoken. E.g., |Waiter| is an
 // implementation that blocks while waiting to be awoken.
 class Awakable {
  public:
-  enum class AwakeReason { SATISFIED, UNSATISFIABLE, CANCELLED };
+  // See |AwakableList| (in particular its |Add()| method).
+  enum class AwakeReason { SATISFIED, UNSATISFIABLE, CANCELLED, CHANGED };
 
   // Helper function that translates:
   //   - |AwakeReason::SATISFIED| -> |MOJO_RESULT_OK|,
   //   - |AwakeReason::UNSATISFIABLE| -> |MOJO_RESULT_FAILED_PRECONDITION|, and
   //   - |AwakeReason::CANCELLED| -> |MOJO_RESULT_CANCELLED|.
+  //   - |AwakeReason::CHANGED| -> |MOJO_RESULT_INTERNAL| (this function never
+  //     be called with this reason).
   static MojoResult MojoResultForAwakeReason(AwakeReason reason);
 
   // |Awake()| must satisfy the following contract:
   //   - It must be thread-safe.
   //   - Since it is called with a mutex held, it must not call anything that
   //     takes "non-terminal" locks, i.e., those which are always safe to take.
-  //   - It should return false if it must not be called again for the same
-  //     reason (e.g., for the same call to |AwakableList::Add()|).
-  virtual bool Awake(uint64_t context,
+  // If |reason| is |AwakeReason::CANCELLED|, |Awake()| will not be called
+  // again (by the same source).
+  virtual void Awake(uint64_t context,
                      AwakeReason reason,
                      const HandleSignalsState& signals_state) = 0;
 
  protected:
   Awakable() {}
   virtual ~Awakable() {}
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_AWAKABLE_H_