Connecting an extension with its main app

[olebole]

Output of brew config

==> brew config
HOMEBREW_VERSION: 5.0.2-56-g07a983c
ORIGIN: https://github.com/Homebrew/brew
HEAD: 07a983c338fe0df4409d75c04318c81f654805a4
Last commit: 14 hours ago
Branch: main
Core tap HEAD: a71014b2991dd202ad175296547874cfc92cd83f
Core tap last commit: 2 weeks ago
Core tap JSON: 19 Nov 07:57 UTC
Core cask tap HEAD: 23b5a246c9d086316a440cac584981c565faeb03
Core cask tap last commit: 2 weeks ago
Core cask tap JSON: 19 Nov 07:57 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CACHE: /Users/runner/Library/Caches/Homebrew
HOMEBREW_CASK_OPTS: ["--no-quarantine"]
HOMEBREW_COLOR: set
HOMEBREW_CURL_PATH: /usr/bin/curl
HOMEBREW_DEVELOPER: set
HOMEBREW_DOWNLOAD_CONCURRENCY: 6
HOMEBREW_FAIL_LOG_LINES: 150
HOMEBREW_GITHUB_API_TOKEN: set
HOMEBREW_GIT_EMAIL: [email protected]
HOMEBREW_GIT_NAME: BrewTestBot
HOMEBREW_GIT_PATH: /usr/bin/git
HOMEBREW_LOGS: /Users/runner/work/homebrew-tap/homebrew-tap/logs
HOMEBREW_MAKE_JOBS: 3
HOMEBREW_NO_AUTO_UPDATE: set
HOMEBREW_NO_EMOJI: set
HOMEBREW_NO_ENV_HINTS: set
HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK: set
HOMEBREW_NO_INSTALL_CLEANUP: set
HOMEBREW_SORBET_RUNTIME: set
Homebrew Ruby: 3.4.7 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.7/bin/ruby
CPU: 3-core 64-bit dunno
Clang: 17.0.0 build 1700
Git: 2.50.1 => /usr/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 26.0.1-arm64
CLT: 26.0.0.0.1.1757719676
Xcode: 26.0.1 => /Applications/Xcode_26.0.1.app/Contents/Developer
Rosetta 2: false

Output of brew doctor

==> brew doctor
All steps passed!

Description of issue

I am developing a local tap https://github.com/iraf-community/homebrew-tap containing IRAF and related packages.
IRAF can be extended by external modules that originally would go as subdirectories into an specific extern directory in the IRAF directory structure. This however does not work directly in homebrew, because a formula shall not write into a different keg.

So I made the following structure:

1. iraf creates a shared directory in lib and links its extern path there in its post-install step:

   def post_install
     iraf_extern = HOMEBREW_PREFIX/"lib/iraf-extern"
     mkdir_p iraf_extern
     libexec.install_symlink iraf_extern => "extern"
   end

2. The extensions (f.e. iraf-fitsutil) create a link from their own directory into this shared directory:

   def post_install
     iraf_extern = HOMEBREW_PREFIX/"lib/iraf-extern"
     mkdir_p iraf_extern
     iraf_extern.install_symlink libexec => "fitsutil"
   end

My question: is this the recommended way to do so? And (how) shall one remove the links that were made by the extensions when they get uninstalled or updated?

p.s. I am a newcomer, and don't have permanent access to macOS, so most of the development is done directly with the Github actions. The outputs above are therefore posted from the Github Actions log, not from a local computer.

[gromgit]

If I'm not mistaken, the simplest method is basically:

1. Configure main_formula's software to look for plugins and stuff in ${HOMEBREW_PREFIX}/lib/<main_formula>.
2. Have plugin_formula install its stuff in lib/<main_formula>, and depends_on "<main_formula>".

Upon installation of both formulae, brew should then symlink the plugin files into ${HOMEBREW_PREFIX}/lib/<main_formula>.

[olebole]

I tried this in iraf-community/homebrew-tap#10, iraf-fitsutil.rb:

class IrafFitsutil < Formula
  […]
  depends_on "iraf"

  def install
    […]
    iraf_extern = HOMEBREW_PREFIX/"lib/iraf/extern"
    (iraf_extern/"fitsutil").install Dir["*"]
  end
end

but this failed with

  Error: ::error::An exception occurred within a child process:
  Errno::EPERM: Operation not permitted @ dir_s_mkdir - /opt/homebrew/lib/iraf

I'd guess that one can't write outside of the own keg in the install step, can one?

[gromgit]

Yes. The problem is this line:

    iraf_extern = HOMEBREW_PREFIX/"lib/iraf/extern"

It should instead be:

    iraf_extern = lib/"iraf/extern"

as I mentioned above, then when you install iraf-fitsutil, brew will symlink /opt/homebrew/Cellar/iraf-fitsutil/<ver>/lib/iraf/* into /opt/homebrew/lib/iraf.

See the php-intl extension formula to php if it still isn't clear.

[olebole]

This worked (at least the test shows that IRAF finds the extension).

Thank you for your help!