Test linking against camlp4 (fix #39399)

[Blaisorblade]

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Bump revision to rebuild against 4.07.1, and add test to detect that linking against camlp4 fails. I verified that brew test succeeds after rebuilding from source and fails with the current bottle.

Fix #39399.

[Blaisorblade]

For a standalone version of the testcase, see https://github.com/Blaisorblade/ulex/tree/test-camlp4.
/cc @SMillerDev @fxcoudert from #39399.

EDIT: beware I am still no OCaml expert — I can tell this testcase makes sense, but an expert might have better ideas. I'd be comfortable with this being committed — a review from an OCaml might help. I'll try to find one. EDIT2: I asked on #ocaml.

[Blaisorblade]

And sorry, I added revision after audit. I'll fix. EDIT: done, and hopefully I redid all the checks I should have. Sorry for the rush.

[SMillerDev]

I have no idea about ocaml or camlp but I do have one homebrew remark.

[Blaisorblade]

Only remark on #ocaml was "if you wanted to test it I think it would make sense to attempt to use a macro, not just open the module".
I could try, but I'm not sure how (and them neither), and formula tests seem to be pretty minimal, so I'm not sure I have time soon... Plus, it did work for the actual issue we had, so maybe YAGNI.

[SMillerDev]

Thanks @Blaisorblade! Without contributions like yours it'd be impossible to keep homebrew going! 👍 🎉

[avsm]

Thanks, I appreciate you submitting this fix, as it's been causing problems for OCaml users (camlp4/camlp4#150).

As a general rule, whenever the OCaml compiler package is upgraded, all dependent OCaml libraries should be recompiled and their Homebrew package revision bumped. The OCaml compiler doesnt guarantee ABI compat across releases, and has CRC checks to ensure consistent linking.

So the test case in this PR will be most useful in spotting errors, but if there's some process the Homebrew team could use to ensure that the tests are run whenever the OCaml package is updated, that would be most appreciated.

[Blaisorblade]

So the test case in this PR will be most useful in spotting errors, but if there's some process the Homebrew team could use to ensure that the tests are run whenever the OCaml package is updated, that would be most appreciated.

Thanks! IIUC the maintainers implied that this test is run when OCaml is upgraded, so next time the problem should be detected. @fxcoudert could you please confirm?

But this test is just for camlp4 and will only rebuild camlp4. Do any other packages need this setup? E.g. homebrew packages camlp5, but opam builds won't use the system camlp5, so maybe they avoid this problem? Executables are statically linked, hence fine.

[avsm]

It would be good to have the test for camlp5 -- it's still safer to recompile it with the host OCaml even if it's an executable. It might, for example, use Marshal to transmit data across a pipe which would fail.

Camlp4 is the only system package in opam due to its historically special status -- it was not possible to have a system OCaml compiler and a local camlp4 in the past due to some layout issues in the filesystem.

[fxcoudert]

Whenever a formula is updated, our CI testing will run the tests for all the formulas that depend on it (including recursive dependences). So when a PR for ocaml is submitted, the following tests are run:

$ brew uses --recursive ocaml
bibtex2html     camlp5          coq             hevea           ledit           menhir          ocaml-num       ocamlsdl
camlp4          coccinelle      fstar           lablgtk         math-comp       ocaml-findlib   ocamlbuild

with the rebuilt ocaml. We consider it the responsibility of those formulas' test blocks to accurately exercise it enough that we know nothing is broken. Now the camlp4 test has been improved, and it appears the camlp5 test is checking its sync with ocaml.

Other formulas' tests need to be audited by someone with knowledge of how ocaml works.