Re: [iostreams] Interest in multithreaded lzma filter?

From: James E. King III (jking_at_[hidden])
Date: 2019-05-23 13:19:37

On Thu, May 23, 2019 at 8:39 AM Mike via Boost <boost_at_[hidden]> wrote:
>
> > Gesendet: Mittwoch, 22. Mai 2019 um 23:40 Uhr
> > Von: "Jeff Bonyun via Boost" <boost_at_[hidden]>
> >
> > > ...  How will we detect whether
> > > liblzma is capable?  If it is not capable but specified, will we allow
> > > the compression anyway?
> >
> > I've been investigating this. liblzma defines a version macro,
> > LZMA_VERSION. It looks like liblzma obtained its multithreaded function
> > in 5.1.1 (in 2011). So the idea would be to branch on this macro, and
> > fall back to the currently-used single-threaded function for an older
> > liblzma version. In that case, requesting any number of threads will
> > carry out the operation, but will only use a single thread. This makes
> > it tolerant of different versions, but does mark up the code with a few
> > #if branches. I'm assuming boost goes for tolerance in this case.
>
> Does a 2019 Boost release really have to be compatible with a Pre 2011
> release of liblzma? Is this combination even tested?
> I.e. are we sure this would be the only incompatibility?
>
> How about just adding this functionality unconditionally and see if
> someone complains?
>
> Mike
>
> _____________

So basically if someone has an older lzma they are limited to boost
1.70.0 or earlier.  That's perhaps reasonable as long as it is
documented, but it hasn't been the typical way I have seen things
added to existing code.

Another way to handle it is to keep the old path in a macro like
BOOST_IOSTREAMS_NO_MULTITHREADED_LZMA.  This is common through boost.
Ideally, detecting the liblzma version from the header could then set
BOOST_IOSTREAMS_NO_MULTITHREADED_LZMA and it would always do the right
thing, but the simplest way to do it is to let the consumer of the
library set this if they need it.

- Jim