Re: [gentoo-dev] reworking python-wrapper and wrapper scripts.

[Brian Harring]

On Thu, Oct 18, 2012 at 11:54:21PM +0200, Micha?? G??rny wrote:
> On Thu, 18 Oct 2012 02:15:43 -0700
> Brian Harring <ferringb@×××××.com> wrote:
>
> > There's a trick to this; currently, those generated scripts hardcode
> > the allowed/known python versions for that package.  We obviously have
> > to preserve that; I propose we shove it into the symlink path.
> >
> > Basically, we add a /usr/libexec/python directory; within it, we have
> > a wrapper binary (explained below), and a set of symlinks pointing at
> > the root of that directory.  To cover our current python versions, the
> > following would suffice:
> >
> > for x in {2.{4,5,6,7},3.{0,1,2,3,4}}-cpy 2.5-jython 2.7-pypy-1.{7,8}
> > \2.7-pypy-1.9; do
> >   ln -s ./ /usr/libexec/python/$x
> > done
> >
> > While that seems insane, there is a reason; via that, we can encode
> > the allowed versions into the symlink.  Using pkgcore's pquery for
> > example (which should support cpy: 2.5, 2.6, 2.7, 3.1, 3.2, 3.3)
> > instead of a wrapper script at /usr/bin/pquery, we'd have thus:
> >
> > targets=( 2.{5,6,7}-cpy 3.{1,2,3}-cpy )
> > targets=$(IFS=/;echo -n "${targets[*]}")
> > # This results in
> > # targets=2.5-cpy/2.6-cpy/2.7-cpy/3.1-cpy/3.2-cpy/3.3-cpy
> > ln -s "/usr/libexec/python/${targets}/wrapper" \
> >   /usr/bin/pquery
> >
> > /usr/libexec/python/wrapper upon invocation, takes a look at argv[0];
> > sees how it was invoked basically.  This will be the /usr/bin/whatever
> > pathway.  It reads the symlink, in the process getting the allowed
> > versions and preferred order of the versions.
> >
> > Few notes; vast majority of filesystems will store the symlink target
> > into the core inode if at all possible- in doing so, this avoids
> > wasting an inode and is only limited by the length of the target.
> > That length is capped by PATH_MAX- which can range from 256 to 4k (or
> > higher).
> >
> > For the pquery example above, that comes out to ~73 bytes for the
> > symlink pathway; well under PATH_MAX.
> >
> > For the scenarios where PATH_MAX caps the symlink pathway, or for
> > whatever reason we don't want to use that trick, a tree of files
> > contained within /usr/libexec/python/ holding the allowed versions for
> > the matching pathway would suffice.
>
> While I agree that it's a clever trick, I doubt it's worth the effort.
> Did you got any numbers proving it being superior over, say, trying to
> exec() scripts like I do in python-exec?
>
> While I can imagine, that in an worst case that bunch of exec()s is
> going to be definitely slower than storing the list anyway, I doubt
> such a bad case is often.

The difference in performance there is going to be negligable; I'm not 
particularly concerned about that, and it shouldn't be a debate point 
between my notion and yours (it's only a debate point when one is 
talking about a c binary vs a python script).

One thing you're ignoring here is that the route(s) I mentioned all 
allow for control/specifying what the order of preference is for 
lookup/fallback (each link/shebang can encode that order how ever the 
hell it wants).


> Considering that the most common Python version used now is Python 2,
> how often doesn't the script support that Python version? That's a very
> rare case, and often just executing "foo-${EPYTHON}" works. In your
> case, that common case involves readlink() + parsing + exec().
>
> Even in case of Python 3 being selected, I doubt the overhead
> of multiple exec()s on the scripts not supporting Python 3 is really
> relevant. Please measure it if you believe so.
>
> To be honest, I don't see any real advantage in this solution. It is
> complex; understanding it requires explanation or some thinking.
> The code will be fragile, and I'm not even sure if I'm not missing
> something important here.

Not particularly sure how you claim this is fragile, but whatever, 
your view.

I suggest you read my response to floppym.  There's differing ways to 
tackle this while getting the required speed, and covering what I want 
(python3.2 sphinx-build working).


> Thus, unless you've got a good arguments how this solution is superior
> to the straightforward one done in python-exec, or numbers proving that
> it is more efficient in a way noticeable to our users, -1. Smart hack,
> yes, but not really beneficial.

Your -1 is duly noted.


> > Either proposal here would be far faster than what we've got now; also
> > will use less space (ancillary benefit).
> >
> > One subtle aspect here is that if we did this, it makes it possible to
> > avoid losing the invocation information- currently if you did
> > `/usr/bin/python3.2 $(which sphinx-build) blah`, because of how things
> > are implemented now (specifically the two layers of wrappers)- you'll
> > get python2.7 running that sphinx-build invocation.
> >
> > This is wrong (it's directly discarding what the invocation
> > requested), although you're only going to see it for scripts that
> > do python introspection.
> >
> > Via doing the restructuring I'm mentioning above, that issue can be
> > fixed, while making things faster/saner.
>
> I don't see how this is relevant to the wrapper. As Mike pointed out,
> python3.2 is the actual Python executable, and the wrapper you're
> suggesting is a C executable -- something that does not really work
> together like that.
>
> So please elaborate on how you are actually going to solve this. Hope
> it's not through patching Python...

Same thing, read my responses to floppym; not going to reiterate those 
points over here.

I will note your python-exec bit still doesn't fly for python3.2 
sphinx-build- which frankly, is the wrong direction to go in my views.

~harring