Memoizing macro values

[Panu Matilainen]

On 12/21/22 12:17, Florian Weimer wrote:
> In Fedora, we have this:
> 
> /usr/lib/rpm/macros.d/macros.python:%python_sitelib %(RPM_BUILD_ROOT= %{__python} -Esc "import sysconfig; print(sysconfig.get_path('purelib', vars={'platbase': '%{_prefix}', 'base': '%{_prefix}'}))")
> /usr/lib/rpm/macros.d/macros.python3:%python3_sitelib %(RPM_BUILD_ROOT= %{__python3} -Ic "import sysconfig; print(sysconfig.get_path('purelib', vars={'platbase': '%{_prefix}', 'base': '%{_prefix}'}))")
> 
> This is a %define-style macro, so the macro gets re-evaluated for every
> expansion.  It can make spec file operations really, really slow.
> 
> It's possible to work around this in spec files like this:
> 
> %{?python3_sitearch: %global python3_sitearch %{python3_sitearch}}
> 
> But this is quite ugly, and I wonder if there is a nice way to do it
> directly in the macros file instead.  I think I know how do it with Lua,
> but maybe there is a better way?

%python3_sitearch %{global python3_sitearch %(RPM_BUILD_ROOT= 
%{__python3} -Esc "import sysconfig; print(sysconfig.get_path('platlib', 
vars={'platbase': '%{_prefix}', 'base': 
'%{_prefix}'}))")}%{python3_sitearch}

Ie just push the evaluated macro on top of the original one as a global 
on first use, re-evaluation can be forced with %undefine and no helper 
macros needed.

There's also an upstream RFE on this use-case, it's quite common indeed: 
https://github.com/rpm-software-management/rpm/issues/1155

	- Panu -