The paths of Lwt's installed cmi files

[aantron]

@diml in ocaml/opam-repository#9228 (comment):

qcow-tool fails to build in one the datakit build. I check, it's because mirage-block-unix uses Lwt_unix but lwt.unix is missing from the jbuild. It's not detected at build time since the cmi files for lwt.unix are installed in the same directory as the cmi files for lwt.

/cc @aantron, it'd be better to install the cmi for the various sub-libraries in sub-directories, to avoid problems like these. jbuilder installs the files for a library with findlib name a.b.c in /a/b/c.

This seems like a problem with jbuilder being incompatible with non-jbuilder installs; on the other hand, maybe we can just move these .cmi files in the next release, and avoid friction.

cc @djs55 @rgrinberg

EDIT: instructions for testing are below.

[Drup]

The recent oasis feature called "FindlibDirectory" should allow to do that quite easily.

[aantron]

So I guess this is a matter of

1. Adding FindlibDirectory to each Findlib library package in _oasis that has a FindlibParent (example).
2. Finding out what version of OASIS implements FindlibParent, either by reading its changelog, or somehow through blame in that repo.
3. Making sure Lwt's opam file requires at least that version of OASIS.

[ghost]

FTR, jbuilder is compatible with non-jbuilder installs. It simply reads META files like ocamlfind. The issue I was talking about in the PR is the following:

1. create a file referencing Lwt_unix
2. compile it with ocamlfind ocamlc -package lwt -c ...
3. link an exectuable with ocamlfind ocamlc -package lwt -o ...

Only step (3) will fail. Which is unfortunate because if the user forgets to state a dependency on lwt.unix, then they won't get an error until much later when someone tries to link an executable with their libraries. The right way to deal with include paths is to have one library per directory, and that's what jbuilder enforces for libraries it installs itself.

[aantron]

Thanks for clarifying, @diml.

So, I guess to work on this, one would:

1. Pin Lwt master with opam source --dev-repo --pin lwt.
2. Follow the instructions from @diml's post above to reproduce the issue.
3. Make a git branch and make the change to _oasis described above.
4. opam reinstall lwt (or opam upgrade lwt?) to install a version of Lwt from your branch, with the _oasis change included.
5. Follow the same steps as in (2) to reproduce, but make sure they trigger an error at step 2 this time.
6. PR the branch to the Lwt repo.

[aantron]

This was resolved by #374.