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

mojo/edk/system/waiter.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_WAITER_H_
 #define MOJO_EDK_SYSTEM_WAITER_H_
 
 #include <stdint.h>
 
 #include "mojo/edk/system/awakable.h"
 #include "mojo/edk/util/cond_var.h"
 #include "mojo/edk/util/mutex.h"
 #include "mojo/edk/util/thread_annotations.h"
 #include "mojo/public/c/system/result.h"
 #include "mojo/public/c/system/time.h"
 #include "mojo/public/cpp/system/macros.h"
 
 namespace mojo {
 namespace system {
 
-// IMPORTANT (all-caps gets your attention, right?): |Waiter| methods are called
-// under other locks, in particular, |Dispatcher::lock_|s, so |Waiter| methods
-// must never call out to other objects (in particular, |Dispatcher|s). This
-// class is thread-safe.
+// An implementation of |Awakable| that is used for blocking waits. This should
+// be used in a non-persistent way (i.e., |Awake()| should be called at most
+// once by each source, and only for "leading edges").
+//
+// IMPORTANT: |Waiter| methods are called under other locks, in particular,
+// |Dispatcher::lock_|s, so |Waiter| methods must never call out to other
+// objects (in particular, |Dispatcher|s).
+//
+// This class is thread-safe.
 class Waiter final : public Awakable {
  public:
   Waiter();
   ~Waiter() override;
 
   // A |Waiter| can be used multiple times; |Init()| should be called before
   // each time it's used.
   void Init() MOJO_NOT_THREAD_SAFE;
 
   // Waits until a suitable |Awake()| is called. (|context| may be null, in
   // which case, obviously no context is ever returned.)
   // Returns:
   //   - |MOJO_RESULT_OK| if |Awake()| was called with |AwakeReason::SATISFIED|;
   //   - |MOJO_RESULT_CANCELLED| if |Awake()| was called with
   //     |AwakeReason::CANCELLED|;
   //   - |MOJO_RESULT_FAILED_PRECONDITION| if |Awake()| was called with
   //     |AwakeReason::UNSATISFIABLE|; or
   //   - |MOJO_RESULT_DEADLINE_EXCEEDED| if the deadline was exceeded.
   //
   // In all the cases except |MOJO_RESULT_DEADLINE_EXCEEDED|, the context and
   // signals state passed to |Awake()| be made available via |*context| and
   // |*signals_state|, respectively (unless the respective pointer is null).
   MojoResult Wait(MojoDeadline deadline,
                   uint64_t* context,
                   HandleSignalsState* signals_state);
 
   // |Awakable| implementation:
-  bool Awake(uint64_t context,
+  void Awake(uint64_t context,
              AwakeReason reason,
              const HandleSignalsState& signals_state) override;
 
  private:
   util::CondVar cv_;  // Associated to |mutex_|.
   util::Mutex mutex_;
 #ifndef NDEBUG
   bool initialized_ MOJO_GUARDED_BY(mutex_);
 #endif
   bool awoken_ MOJO_GUARDED_BY(mutex_);
   AwakeReason awake_reason_ MOJO_GUARDED_BY(mutex_);
   HandleSignalsState signals_state_ MOJO_GUARDED_BY(mutex_);
   uint64_t awake_context_ MOJO_GUARDED_BY(mutex_);
 
   MOJO_DISALLOW_COPY_AND_ASSIGN(Waiter);
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_WAITER_H_