Re: [Aptitude-devel] Acquire considered non-threadsafe
On Wed, Nov 18, 2009 at 04:03:05PM +0100, Julian Andres Klode <firstname.lastname@example.org> was heard to say:
> On Sun, Nov 15, 2009 at 12:42:38PM -0800, Daniel Burrows wrote:
> > There are some other very nice options involving rewriting or
> > reimplementing the "download queue" logic (i.e., just pkgAcquire), but
> > getting threadsafety does require changing the behavior of
> > pkgAcquire::Item in any event. I'm assuming that we don't want to have
> > two parallel implementations of this functionality, and that
> > non-backwards-compatible changes are considered unacceptable, in which
> > case the opt-in approach is the only solution I can think of.
> We should really start talking about APT 0.8 sometime soon, and break
> some APIs. The problem I have with Acquire is its low speed when queuing
> items which is probably caused by the use of a linked list (20000 items
> take multiple minutes to be queued, whereas APT2 queues them in less than
> a second).
Acquire is also really painful to develop against, although that's
more just the general pain of dealing with apt.
Also, apt2? When did that happen?
> (a) a common abstract base class for the *Summation classes
> (b) a new cache format which can be resized in reality
I wonder if apt really needs an mmap'd cache here. In fact, I wonder
about using sqlite -- I did a little experimentation with it and got
decent performance. The big problem (from my point of view) would be
teaching client code to treat the database transactionally rather than
as a linked set of in-memory objects, but it way more cleaner and more
flexible than what we have now.
(I know I came down on this last time it was mentioned here, but I've
done a bit of work with sqlite since then and I have a lot of respect
If an mmap'd cache really is desired, I suggest looking at
Boost.Interprocess, which provides all sorts of routines for dealing
with mmap'd memory segments.
> (c) new dependency resolver [porting aptitude's one?]
> (d) external dependency resolvers
> (e) new buildsystem (e.g. raw autotools) and reworked
Those sound very good.
> (f) storing errno inside the error handling objects,
> for bindings (nice to have errno).
Please, please, kill the global error queue. This is a disaster from
a threadsafety point of view, but also just from a general design
perspective. There's way too much code that does
when the error could have been caused by something entirely unrelated
to what this code is interested in.
Put error-handling oun either a return-code basis or an exception
basis, but not this wird intermediate thing where random errors cause
failure. If you want to know about inner errors, use a logging
framework (I prefer log4cxx).
I would add one more item:
(g) use modern C++ coding conventions (at least for new code)
apt was designed in 1996 or so, and it shows. Modern C++ is a lot
cleaner, easier to maintain and easier to write than the weird mishmash
of C and "object-oriented programming" that apt is written in. Also,
a lot of modern C++ programs link against Boost, which provides a ton
of well-designed utility libraries for stuff that you would otherwise
have to write yourself, and that you'd end up writing badly.
(apt-pkg/contrib/ is full of that sort of thing)
> But I don't think anyone wants to do this.
Personally, I would totally love to clean up the apt code and fix
some of my chronic annoyances with it. I just don't have the spare
time, and apparently neither does anyone else (except some guy who
thinks it should be written in Perl (!?!?)). :-(