Re: [gentoo-dev] [PATCH 2/2] Support wrapping headers for multilib ABIs.

[Michał Górny]

On Sun, 24 Mar 2013 16:14:52 +0100
Alexis Ballier <aballier@g.o> wrote:

> On Sat, 23 Mar 2013 17:26:38 +0100
> Michał Górny <mgorny@g.o> wrote:
>
> > ---
> >  gx86/eclass/autotools-multilib.eclass | 82
> > +++++++++++++++++++++++++++++++++++ 1 file changed, 82 insertions(+)
> >
> > diff --git a/gx86/eclass/autotools-multilib.eclass
> > b/gx86/eclass/autotools-multilib.eclass index d7372b0..c65c777 100644
> > --- a/gx86/eclass/autotools-multilib.eclass
> > +++ b/gx86/eclass/autotools-multilib.eclass
>
>
> why not multilib-build ?

Because it has two parts which have to be called at the right time.
It doesn't have a public API yet, so I'm putting it where I can test it
sanely.

> > @@ -33,6 +33,28 @@ inherit autotools-utils multilib-build
> >
> >  EXPORT_FUNCTIONS src_prepare src_configure src_compile src_test
> > src_install
> > +# @ECLASS-VARIABLE: MULTILIB_WRAPPED_HEADERS
> > +# @DESCRIPTION:
> > +# A list of headers to wrap for multilib support. The listed headers
> > +# will be moved to a non-standard location and replace with a file
> > +# including them conditionally to current ABI.
> > +#
> > +# This variable has to be a bash array. Paths shall be relative to
> > +# installation root (${D}), and name regular files. Recursive
> > wrapping +# is not supported.
> > +#
> > +# Please note that header wrapping is *discouraged*. It is preferred
> > to +# install all headers in a subdirectory of libdir and use
> > pkg-config to +# locate the headers. Some C preprocessors will not
> > work with wrapped +# headers.
> > +#
> > +# Example:
> > +# @CODE
> > +# MULTILIB_WRAPPED_HEADERS=(
> > +#	/usr/include/foobar/config.h
> > +# )
> > +# @CODE
> > +
> >  autotools-multilib_src_prepare() {
> >  	autotools-utils_src_prepare "${@}"
> >  }
> > @@ -49,13 +71,73 @@ autotools-multilib_src_test() {
> >  	multilib_foreach_abi autotools-utils_src_test "${@}"
> >  }
> >
> > +_autotools-multilib_wrap_headers() {
> > +	debug-print-function ${FUNCNAME} "$@"
> > +	local f
> > +
> > +	for f in "${MULTILIB_WRAPPED_HEADERS[@]}"; do
> > +		# drop leading slash if it's there
> > +		f=${f#/}
> > +
> > +		if [[ ${f} != usr/include/* ]]; then
> > +			die "Wrapping headers outside
> > of /usr/include is not supported at the moment."
> > +		fi
>
> Do you really want to support this at some point? Why?

Honestly? No. But if people need it, I don't see much of a problem to
add the support in the future.

> I'd just go for paths relative to $EPREFIX/usr/include (or
> $ED/usr/include) for MULTILIB_WRAPPED_HEADERS. That would simplify the
> code.

That's true. But on the other hand, that would introduce yet another
root for paths which would probably end up being confusing.
Using /usr/include/... feels more natural IMO.

And of course, it leaves the possibility of supporting other locations
in the future.

> > +		# and then usr/include
> > +		f=${f#usr/include/}
> > +
> > +		local dir=${f%/*}
> > +
> > +		# $CHOST shall be set by multilib_toolchain_setup
> > +		dodir "/tmp/multilib-include/${CHOST}/${dir}"
> > +		mv "${ED}/usr/include/${f}"
> > "${ED}/tmp/multilib-include/${CHOST}/${dir}/" || die +
>
> why not use $T rather than $ED/tmp ?

I prefer using $D rather than $T for files which are intended to be
installed. Purely theoretically, $T could be on another filesystem than
$D. Then, moving the file back and forth could cause it to lose some
of the metadata -- and will be slower, of course.

> > +		if [[ ! -f ${ED}/tmp/multilib-include/${f} ]]; then
> > +			dodir "/tmp/multilib-include/${dir}"
> > +			cat > "${ED}/tmp/multilib-include/${f}"
> > <<_EOF_ || die +/* This file is auto-generated by
> > autotools-multilib.eclass
> > + * as a multilib-friendly wrapper to /${f}. For the original content,
> > + * please see the files that are #included below.
> > + */
> > +_EOF_
> > +		fi
> > +
> > +		local defs
> > +		case "${ABI}" in
>
> didn't you say $ABI may have name collisions?
> considering the code below, it seems much safer to match on $CHOST

Yes, that's why I've put the whole matching inline instead of
introducing a function to obtain the values. The current design
of the eclass -- which was quite stupid of me -- doesn't provide
a way to access that ABI_* thing. I am going to change that
in the future but don't want to step into two different issues
at a time.

At a first glance, $CHOST sounds like a neat idea. But I'm afraid it's
not a really safe choice. From a quick glance, CHOST values for x86
were:

i*86-pc-linux-gnu
x86_64-pc-linux-gnu
x86_64-pc-linux-gnux32

I feel like there's a lot of variables here, ends up in a bit ugly
pattern matching IMO.

> [...]
>
> It'd be nice to have an attempt to support all the ABIs gentoo supports
> in that file: I'd prefer to spot possible problems with this solution
> vs. sedding a template for example to be seen before having to rewrite
> it.
> For example, IIRC, ppc64 defines both __powerpc__ and __powerpc64__,
> the natural way would be:
> #if defined(__powerpc64__)
> ppc64 stuff
> #elif defined(__powerpc__)
> ppc stuff
> #endif
>
> with your approach that'd be:
> #if defined(__powerpc__) && !defined(__powerpc64__)
> ppc stuff
> #elif defined(__powerpc64__)
> ppc64 stuff
> #endif

I had the same problem with x32. I've chosen the solution which worked
and was easy to implement.

> doing with a template has its disadvantages but allows more flexibility
> in how the #ifery is written; I don't want to realize your approach
> cannot deal with some weird arches after it has been deployed.

To be honest, I can't really imagine how we could work with a template
here. It would at least require the function to be aware of other ABIs
which I really wanted to avoid. Of course, then there's the whole code
to handle it, and honestly I don't have a vision how it would work.

-- 
Best regards,
Michał Górny