new aio engine

[Stefan (metze) Metzmacher]

Hi Volker,

> Under
> 
> https://git.samba.org/?p=vl/samba.git/.git;a=shortlog;h=refs/heads/aio
> 
> find a new async I/O engine for Samba. The old one was based
> around the posix aio API, the branch converts it to
> tevent_req.
> 
> It uses a pthread based implementation in by default via
> lib/asys and lib/pthreadpool.
> 
> pthreadpool has a sync fake implementation that works
> without threads.
> 
> The posix api is used in the vfs_aio_posix module for
> in-kernel implementations like FreeBSD and others that might
> be more efficient than a user-space pthreads one.
> 
> The AIO engine is always compiled, HAVE_AIO is reduced to
> the vfs_aio_posix module.
> 
> asys.h has more functions mentioned than actually
> implemented. asys_open is not implemented yet, this will
> require a fork model. I nevertheless want to show the easy
> piece for review early.

It's really good to see this finally in master.

> It has one (at the moment fatal) flaw: If a file is closed
> while async requests are still on the fly, the pread/pwrite
> might end up on the wrong file. Two ways out: Hold back the
> close SMB until all async requests are finished, or put the
> os-level fd on the pending close queue until all the async
> requests are finished. I tend to implement the latter.

I think we should implement asys_cancel() more correct,
which requires a pthreadpool_job_cancel(), which might use
pthread_cancel(). I think that would fix this problem
and it would also be possible to return the correct status
to the smb level read and write.

metze

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