Make promoted files read-only

[MisterDA]

Files promoted to the source directory should be read-only because they are generated. If they're edited, they're going to be eventually overwritten by a following build. Setting them read-only would prevent user confusion.

In this case, read-only would only be useful for humans, and Dune should make the file writable again if it needs to overwrite it.

If the user need to overwrite a promoted file, for instance before distributing a source archive, then she needs to manually set the write permission.

Fixes #12465.

[Leonidas-from-XIV]

Thanks for the PR, I've got a few comments, mostly about the readability of the code.

[MisterDA]

Thanks for the review.

Catching the return code and the error message like this is not portable (even between macOS and Linux).

  $ echo plop > file
  file: Permission denied
  [1]

I'm tempted to do, if there is no better alternative,

if ! echo plop >file 2>/dev/null; then >&2 echo "file: Permission denied"; exit 1; fi

[Leonidas-from-XIV]

I think the test could also be changed to check whether the flags are set, so something like dune_cmd stat permissions (which I assume mostly exists to deal with ls output being nonportable).

[Leonidas-from-XIV]

Ping @nojb on this, as Nicolás expressed interest in this on #12465.

I'm a bit worried about the test-suite errors actually. I will re-review when those are fixed.

[Leonidas-from-XIV]

One of the things I suggested in the original issue was to make this change gated on dune-lang 3.21, to not break users that depend on potentially writing to the file (however I don't know if that would actually affect anyone, so maybe ok to include it in the alpha-release and see if the revdeps have failures).

[MisterDA]

Thanks all for the pointers. The patch is nicer now, and the CI is passing.

[ejgallego]

One of the things I suggested in the original issue was to make this change gated on dune-lang 3.21, to not break users that depend on potentially writing to the file (however I don't know if that would actually affect anyone, so maybe ok to include it in the alpha-release and see if the revdeps have failures).

I came to this issue to ask for the same, this seems like a user impacting change, so indeed gating it under dune-lang 3.21 seems like a good idea.

I am positive that there are lot of projects in the Rocq land (jsRocq, industrial users) that are not in opam, so the impact of these kind of changes are hard to evaluate until dune is fully released.

[MisterDA]

so the impact of these kind of changes are hard to evaluate until dune is fully released.

(🙈🙉🙊 is this why we have patch releases)