Fix for #356 is incomplete, resulting in link errors on macos on certain configurations.

[wz1000]

#363 attempted to fix #356

posix_spawn_file_actions_addchdir was introduced in macOS 26.

When MACOSX_DEPLOYMENT_TARGET=11.0 (as it is on GHC CI) is set, clang will emit a weak reference to this symbol. However, weak references on MachO objects don't behave like weak references on ELF (see iains/gcc-darwin-arm64#133). A final link with undefined weak references still fails.

We see this on GHC CI, where a process library built against macOS 26 is attempted to be linked against a homebrew/clang toolchain not exposing posix_spawn_file_actions_addchdir, resulting in a link failure:
https://gitlab.haskell.org/ghc/ghcup-ci/-/jobs/2539703

Undefined symbols for architecture arm64:
  "_posix_spawn_file_actions_addchdir", referenced from:
      _do_spawn_posix in libHSprocess-1.6.29.0-d296.a[4](posix_spawn.o)
      _do_spawn_posix in libHSprocess-1.6.29.0-d296.a[4](posix_spawn.o)

I think that the correct fix is to pass -Wl,-U,_posix_spawn_file_actions_addchdir in ld-options: in the cabal file, which will allow the link to succeed when this symbol is missing (setting it to null), which will then allow the fix in #363 to take effect.

[tomjaguarpaw]

Thanks, can you comment on whether there is any connection to #375

[wz1000]

Yes #375 does not work because of the MachO linking behaviour I describe.

An alternative fix to ld-options is to use dlsym to dynamically look up the symbol at runtime, but that can be slow (especially the first time the function is called) and seems rather unclean, which is why i prefer the ld-options fix.

[tomjaguarpaw]

Released as https://hackage.haskell.org/package/process-1.6.30.0