On 1/02/2018 19:36, Stian Zeljko Vrba wrote:
> That's what I meant. I'll try to be more precise. QC below stands for
> the io_service's queue contents.
>
>  1.
>     io_service dequeues  and executes a completed read handler. This
>     handler starts a new async read operation... QC: [] (empty).
>     Pending: async_read.
>  2.
>     The program enqueues to io_service a lambda that calls close. QC: [
>     {close} ] Pending: async_read.
>  3.
>     In the mean-time (say, during the enqueue), the started async_read
>     operation completed successfully in parallel because it didn't block
>     at all. QC: [ {close} {ReadHandler} ] Pending: none.
>  4.
>     io_service dequeues {close}, nothing to cancel, the handle is
>     closed. QC: [ {ReadHandler} ]
>  5.
>     io_service dequeues {ReadHandler}, which initiates a new read from
>     the closed handle. QC: []
>
>
> This assumes that asynchronous operations and notifications are, well,
> asynchronous. IOW, there is a time window (en-/dequeueing and execution
> of {close}) during which async_read can complete and end up
> non-cancellable, so {ReadHandler} is enqueued with success status.
>
> What part of the puzzle am I missing?

In #5, before ReadHandler is executed the socket is already closed.
Thus after processing the successfully read data (or discarding it, if
you prefer), you can check socket.is_open() before starting a fresh read.

This will of course be false in the sequence above, at which point you
just return instead of starting the read, and then once the handler
exits the objects will fall out of existence (if you're using the
shared_ptr lifetime pattern).  If there's no other work to do at that
point then the io_service will also exit naturally.

If the close ended up queued after ReadHandler, then is_open() will
still be true, you will start a new read operation, and then either the
above occurs (if the read actually completes before it starts executing
the close), or the close does find something to abort and this will
enqueue ReadHandler with operation_aborted.

There is no race between checking is_open() and starting the next read
because both the ReadHandler and the close are occurring in the same
strand, so can't happen concurrently.

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