Memoizing macro values

[Miro Hrončok]

On 21. 12. 22 11:17, 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?

This works (using random to demonstarte the behavior:

%_python3_random %(RPM_BUILD_ROOT= %{__python3} -Ic "import random; 
print(random.randint(0,100))")
%python3_random %{?!_python3_random_cached:%{global _python3_random_cached 
%_python3_random}}%_python3_random_cached

$ rpm --eval '%python3_random %python3_random %python3_random %python3_random 
%python3_random'
42 42 42 42 42

Note that %python3_sitlib must be evaluated after Python is installed so maybe 
there must be some "non-empty" conditional added as well.

-- 
Miro Hrončok
-- 
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IRC: mhroncok