Revert making HandleWatcher block until no longer waiting on pipe (r285266).

mojo/public/c/environment/async_waiter.h

 // Copyright 2014 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_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_
 #define MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_
 
 #include "mojo/public/c/system/types.h"
 
 typedef uintptr_t MojoAsyncWaitID;
 
 typedef void (*MojoAsyncWaitCallback)(void* closure, MojoResult result);
 
-// Functions for asynchronously waiting (and cancelling asynchronous waits) on a
-// handle.
-//
-// Thread-safety:
-//   - |CancelWait(wait_id)| may only be called on the same thread as the
-//     |AsyncWait()| that provided |wait_id| was called on.
-//   - A given |MojoAsyncWaiter|'s functions may only be called on the thread(s)
-//     that it is defined to be valid on (typically including the thread on
-//     which the |MojoAsyncWaiter| was provided). E.g., a library may require
-//     initialization with a single |MojoAsyncWaiter| and stipulate that it only
-//     be used on threads on which that |MojoAsyncWaiter| is valid.
-//   - If a |MojoAsyncWaiter| is valid on multiple threads, then its functions
-//     must be thread-safe (subject to the first restriction above).
 struct MojoAsyncWaiter {
-  // Arranges for |callback| to be called on the current thread at some future
-  // when |handle| satisfies |signals| or it is known that it will never satisfy
-  // |signals| (with the same behavior as |MojoWait()|).
-  //
-  // |callback| will not be called in the nested context of |AsyncWait()|, but
-  // only, e.g., from some run loop. |callback| is provided with the |closure|
-  // argument as well as the result of the wait. For each call to |AsyncWait()|,
-  // |callback| will be called at most once.
-  //
-  // |handle| must not be closed or transferred (via |MojoWriteMessage()|; this
-  // is equivalent to closing the handle) until either the callback has been
-  // executed or the async wait has been cancelled using the returned (nonzero)
-  // |MojoAsyncWaitID| (see |CancelWait()|). Otherwise, an invalid (or, worse,
-  // re-used) handle may be waited on by the implementation of this
-  // |MojoAsyncWaiter|.
-  //
-  // Note that once the callback has been called, the returned |MojoAsyncWaitID|
-  // becomes invalid.
+  // Asynchronously call MojoWait on a background thread, and pass the result
+  // of MojoWait to the given MojoAsyncWaitCallback on the current thread.
+  // Returns a non-zero MojoAsyncWaitID that can be used with CancelWait to
+  // stop waiting. This identifier becomes invalid once the callback runs.
   MojoAsyncWaitID (*AsyncWait)(MojoHandle handle,
                                MojoHandleSignals signals,
                                MojoDeadline deadline,
                                MojoAsyncWaitCallback callback,
                                void* closure);
 
-  // Cancels an outstanding async wait (specified by |wait_id|) initiated by
-  // |AsyncWait()|. This may only be called from the same thread on which the
-  // corresponding |AsyncWait()| was called, and may only be called if the
-  // callback to |AsyncWait()| has not been called.
-  //
-  // Once this has been called, the callback provided to |AsyncWait()| will not
-  // be called. Moreover, it is then immediately safe to close or transfer the
-  // handle provided to |AsyncWait()|. (I.e., the implementation of this
-  // |MojoAsyncWaiter| will no longer wait on, or do anything else with, the
-  // handle.)
+  // Cancel an existing call to AsyncWait with the given MojoAsyncWaitID. The
+  // corresponding MojoAsyncWaitCallback will not be called in this case.
   void (*CancelWait)(MojoAsyncWaitID wait_id);
 };
 
 #endif  // MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_