Dependency between generated dune-build-info file and clients

[jberdine]

I'm trying to hunt down why I am getting a

File "_none_", line 1:
Warning 58: no cmx file was found in path for module Build_info__Build_info_data, and its interface was not compiled with -opaque

warning, which has led me to what looks like a missing dependency from a client of Build_info to the generated module.

Expected Behavior

For a module e.g. in cli.ml that uses Build_info, build_info_data.ml-gen is always compiled before cli.ml, so that build_info__Build_info_data.cmx is available when compiling cli.ml

Actual Behavior

I see build logs where sometimes e.g.

$ (cd _build/default && /Users/jjb/.opam/sledge/bin/ocamlopt.opt -I .cli.eobjs/byte -I .cli.eobjs/native -I /Users/jjb/.opam/sledge/lib/dune-build-info -intf-suffix .ml-gen -no-alias-deps -opaque -o .cli.eobjs/native/build_info__Build_info_data.cmx -c -impl .cli.eobjs/build_info_data.ml-gen)

appears after the compilation line for the user.

Reproduction

This seems to be sensitive to parallelism, and I have not managed to create an isolated test that reproduces the warning. Attached is a tarball of a small project that I think is representative of the larger case where this occurs, but the small project does not appear to trigger the warning. The _build/log file is included in the archive as build_log for reference.

The larger repro is running dune build _build/dbg/bin/sledge.exe from infer/sledge. I mention this in case browsing the dune files there is helpful, but actually setting up the deps for that build is tough (fork of llvm and whatnot).

Specifications

• Version of dune (output of dune --version): 2.3.1
• Version of ocaml (output of ocamlc --version): 4.08.1
• Operating system (distribution and version): macos 10.14.6

[jberdine]

dune-build-info-test.tar.gz

[ghost]

Hmm, so I was pretty sure that build_info_data.mli was compiled with -opaque to avoid this issue. I'm having a look.

[jberdine]

If it helps, in my _build/log files, there is no mention of an mli for build_info_data.

[ghost]

Indeed, it is because the mli is part of dune-build-info directly. I haven't been able to reproduce the issue so far.

[jberdine]

In case it is useful, it seems that in watch mode, this leads to IO errors such as:

Error: I/O error: bin/.sledge_cli.eobjs/native/build_info__Build_info_data.cmx: No such file or directory

I don't know that this is the same underlying issue, just a guess.

[ghost]

Ah, I just observed a similar error in watch mode recently. More precisely, if I modify a file while dune is building, I'm seeing such errors. If I modify a file while dune is not running, everything is fine.

There might a cleanup that we don't do when we interrupt a build because of file changes.

[jberdine]

I'm not sure, but ocaml/ocaml#9790 (comment) notes a potential difference in flambda mode which might be related to this issue.

[ghost]

Given what we discovered in #3599, this might be due to a bug in the compiler :/

[rgrinberg]

There's not much more we can do, as we already compile the mli only module with -opaque.