v3 regression over v2: opam exec now fails with "Exec format error" on shell scripts on Windows

[JasonGross]

opam exec -- ./test.sh
Fatal error: exception C:\hostedtoolcache\windows\opam\2.2.1\x86_64\opam.exe: "create_process" failed on ./test.sh: Exec format error

https://github.com/JasonGross/test/actions/runs/10953316111/job/30413386004

logs_28625164728.zip

Full yml file

name: Example Workflow

on: [push, pull_request]

jobs:
  build:

    runs-on: windows-latest
    name: build
    steps:
    - uses: actions/checkout@v4
      with:
        submodules: recursive
    - uses: ocaml/setup-ocaml@v3
      with:
        ocaml-compiler: 4.13.1
    - run: opam exec -- chmod +x test.sh
    - run: opam exec -- cat test.sh
    - run: opam exec -- ./test.sh


  buildv2:

    runs-on: windows-latest
    name: buildv2
    steps:
    - uses: actions/checkout@v2
      with:
        submodules: recursive
    - uses: ocaml/setup-ocaml@v2
      with:
        ocaml-compiler: 4.13.1
    - run: opam exec -- chmod +x test.sh
    - run: opam exec -- cat test.sh
    - run: opam exec -- ./test.sh

[smorimoto]

Is it related to this? #815

[JasonGross]

You are suggesting that it might be related to #815 (comment) in particular, where perhaps indirecting through ocaml-env exec -- opam.exe exec instead of jumping straight to opam exec might change something? Or that opam exec might sometimes recognize bash scripts depending on what shell its run from? (The problem is that opam is trying to run the bash script as a binary executable, but I don't know what's causing that issue.) Is there a way to disentangle opam version issues from setup-ocaml version issues by changing something at https://github.com/JasonGross/test/blob/d70236be3e6d76e9bcac5a65b68298784b33f44d/.github/workflows/main.yml ?

[smorimoto]

What happens if you run this with the latest v3?

[JasonGross]

Still failing at https://github.com/JasonGross/test/actions/runs/13686640123/job/38271477628

[smorimoto]

If I understand correctly, even when using Cygwin, you still need to invoke the shell explicitly. This is because opam is compiled as a native Windows binary (opam.exe) and utilises the Windows CreateProcess API to launch processes. This API does not recognise shell scripts as directly executable files, regardless of whether you’re operating within a Cygwin environment. So, to run your shell script, you must explicitly call an interpreter like bash.

[smorimoto]

Can you execute it by doing the following?

opam exec -- bash ./test.sh

[JasonGross]

Trying that out at https://github.com/JasonGross/test/actions/runs/13693808963/job/38291718192

[JasonGross]

Can you execute it by doing the following?

opam exec -- bash ./test.sh

Yes, this variant works fine

[smorimoto]

Nice. This is not strictly an issue with this action, as I believe it comes from opam now native rather than regression. I'm going to close this issue.