Re: meson: avoid PATH bloat from NLS .mo targets in tmp_install test setup

On Wed, Jul 15, 2026 at 12:30 PM Tristan Partin <tristan(at)partin(dot)io> wrote:

> On Tue Jul 14, 2026 at 9:24 PM UTC, Andrew Dunstan wrote:
> > Hi
> >
> > While setting up a Windows/MSVC buildfarm animal with NLS enabled, the
> > meson 'tmp_install' and 'initdb_cache' setup tests failed immediately
> > with exit status 1 and no stdout/stderr at all.
> >
> > With a little help from Claude, I (eventually) found this diagnosis and
> > solution:
> >
> > The root cause turned out to be in meson itself, not in our meson.build.
> > On Windows, determine_windows_extra_paths() in
> > mesonbuild/backend/backends.py builds a test's PATH by walking every
> > target passed via that test's 'depends:' kwarg (extra_bdeps) and adding
> > each one's build directory unconditionally, with no check for whether
> > the target actually produces a DLL. That's fine when 'depends:' lists a
> > handful of real link dependencies, but the 'tmp_install' test depends on
> > installed_targets, which includes nls_mo_targets - one custom_target per
> > locale/domain of compiled .mo catalogs. With NLS enabled that's several
> > hundred targets, none of them DLLs, none of them ever looked up via
> > PATH, and each one still gets its own entry.
> >
> > In our case this inflated PATH to ~39000 characters across 584 entries
> > (448 of them po/*/LC_MESSAGES directories), comfortably past practical
> > Windows environment-variable/command-line length limits, which is why
> > the test failed silently - the failure happens before the child process
> > gets a chance to produce any output.
> >
> > I think this is arguably a meson bug (determine_windows_extra_paths()
> > should filter extra_bdeps the same way it already filters a test
> > executable's own link dependencies), but regardless of whether that
> > gets fixed upstream, we can sidestep it on our end cheaply: depend on a
> > trivial stamp custom_target instead of installed_targets directly. Its
> > own 'depends:' still forces installed_targets to build first, so build
> > ordering is unaffected, but since a custom_target is not a
> > build.BuildTarget, meson doesn't recurse into its dependencies when
> > computing the test PATH - it contributes at most one harmless directory
> > instead of hundreds.
> >
> > I initially tried wrapping installed_targets in an alias_target()
> > instead, which would avoid the recursion the same way, but test()'s
> > 'depends:' kwarg is typechecked to only accept
> > BuildTarget | CustomTarget | CustomTargetIndex and rejects AliasTarget
> > outright:
> >
> > meson.build:NNNN:0: ERROR: test keyword argument 'depends' was of
> > type array[AliasTarget] but should have been type
> > array[BuildTarget | CustomTarget | CustomTargetIndex]
> >
> > The attached patch uses a custom_target instead, which satisfies that
> > type check.
> >
> > Tested on Windows/MSVC (meson 1.11.1), building with -Dnls=enabled:
> >
> > before: tmp_install test PATH = 39193 chars, 584 entries, 448
> > LC_MESSAGES - tmp_install and initdb_cache setup tests FAIL
> > (exit status 1, no output)
> > after: tmp_install test PATH = 2189 chars, 43 entries, 0
> > LC_MESSAGES - all three setup tests (tmp_install,
> > install_test_files, initdb_cache) OK
> >
> > I did not attempt to fix the same class of problem for any other test
> > in the tree - this patch only touches the one setup test that was
> > actually failing for us. If there's interest, the same technique could
> > presumably be applied wherever else a test's 'depends:' pulls in a
> > large target list.
>
> Hey Andrew,
>
> I took some time to raise this issue with the Meson team in their
> Matrix/IRC channel. I'll see if the discussion goes anywhere.
>
> As for the fix and the results, they look good to me. I couldn't
> identify any other problems where this could be an issue, so this seems
> like a one-off. Did you notice any other problems? I only noticed
> installed_targets depended on in tmp_install and install-quiet.
>

I should have mentioned that I raised an issue at <
https://github.com/mesonbuild/meson/issues/16010>

I have not encountered other issues, but I had to disable NLS to make
progress with the animal, so fixing this one might uncover others.

cheers

andrew