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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.

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