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

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.

--
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)