[ronaldtse]

What

tfs_spawn_prepare never translated the chdir: option: posix_spawn's
chdir is a file ACTION (invisible to the preload interpose) and the fork
path's chdir(2) runs where the mount points do not exist on the host, so
a spawn with chdir: into the VFS died ENOENT before the materialized
exec ran.

Reproduced minimally on v0.17.8 (macOS arm64): a payload image carrying a
native binary + a probe —
Open3.capture3("/pkg/native-true") passes;
Open3.capture3("/pkg/native-true", chdir: "/pkg") dies
Errno::ENOENT - /pkg. Full evidence in tebako-runtime-ruby#333.

The production bite: metanorma 1.18.5's html-xml check — isodoc
compiles SCSS through sass-embedded's
Open3.popen3(*CLI::COMMAND, "--embedded", chdir: __dir__), __dir__
being the in-image gem dir. tebako-packages/metanorma#124's CI fails the
check on all four macOS legs and both windows legs (Linux passes).

The fix

Both tfs_spawn_prepare variants (POSIX + windows) gain the arm, first
in the hook so shell and array forms both get it: a chdir_dir naming an
embedded path becomes its host twin through
tebako_fs_exec_materialize_dir — the link unit's export since v2.8.74
(the pinned v2.8.76 unit carries it: the header declares it and both
archives match). Parent-side, before fork — the hook's own
async-signal-safety discipline.

Completes #152: the subtree verb shipped when that issue
closed, but the hook never called it.

Verification

• All five series regenerated mechanically (pristine upstream + the
  previous patch + the arm, diffed fresh — no hand-edited hunks) and
  apply through tools/apply on 3.1.7 / 3.2.11 / 3.3.9 / 3.4.9 / 4.0.7.
  (4.0.9 is not yet in versions.yml — the 4.0 series check rode 4.0.7.)
• The patched tree carries both arms (exec_materialize_dir ×2).
• The compile gate runs in CI (the factory's own rule); the runtime
  rebuild is the v0.17.9 line, then payload 1.18.5 + tools v2.8.75: the team's retarget on the current line tebako-packages/metanorma#124
  re-pins.

Draft until CI is green end to end.

[ronaldtse]

Superseded: #153 already landed the chdir translation on main (my local checkout was stale when I branched). The remaining gap is upstream consumption — tracking which source release carries it and what the runtime pins.