Re: [Boost-users] asio: cancelling a named pipe client

Subject: Re: [Boost-users] asio: cancelling a named pipe client
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2018-01-31 22:14:15

On 1/02/2018 05:39, Stian Zeljko Vrba wrote:
> So is the following possible:
> 
>  1. Another read operation is started, but that one completes
>     immediately because data is ready on the pipe. I.e., async_read_some
>     immediately posts the completion handler.
>  2. close() has nothing to cancel, but closes the socket.
>  3. The handler posted in 1 is executed, it goes to initiate another
>     read, and that read finds the pipe closed, returning
>     errc::invalid_handle.

No, because you don't call close() directly, you post() a method that 
calls close(), which cannot execute before #3 unless either the 
operation is still pending or you have already started a new operation 
that is now pending.

> Even if async_read_some does *not* post the handler immediately but just 
> initiates the I/O, this I/O can complete immediately and cannot be 
> canceled on the OS-level. Thus attempted cancellation by close() will be 
> a noop, the read handler will get an OK status and proceed to use the 
> closed handle...

No, because the close hasn't actually happened yet, it's still sitting 
in the queue.

The queue itself is OS-managed, so the moment the operation completes it 
will push the completion handler to the queue.  It doesn't require 
intervention from ASIO code.