dune-build-info only works when public_name is specified, even if only mode is JS

[phated]

Expected Behavior

I was expecting to be able to include dune-build-info (specifically the project version) in an executable target that only supports the js mode.

For example:

(executable
 (name Example)
 (modes js)
 (modules Example)
 (libraries dune-build-info))

Actual Behavior

Build_info.V1.version () is always None because the executable doesn't specify a public_name, but adding a public_name if I'm only specifying (modes js) results in this error:

Error: No installable mode found for this executable.
When public_name is set, one of the following modes is required:
- exe
- native
- byte

Reproduction

1. Clone https://github.com/phated/dune-build-info-jsoo
2. Run esy inside the repo
3. Run esy test

Adding an exe mode and public_name to that executable allows the JS to include the project version.

Specifications

• Version of dune (output of dune --version): 2.8.5
• Version of ocaml (output of ocamlc --version): 4.11.0
• Operating system (distribution and version): MacOS 10.15 (Catalina)

Have you tried with an explicit install stanza?

(install
  (section lib)
  (files foo.js))

Dune doesn't know where .js files should be installed, so you need to tell it.

BTW, the build info will always be None from inside the _build directory. This is to preserve good incrementality. It's only when the files are copied out of the build directory that the build info are set. There are two ways for files to be copied out (and rewritten):

• installation
• promotion

You can ask Dune to automatically promote your .js files with:

(executable
 ...
 (promote (until-clean)))

[rgrinberg]

I second what Jeremie said. Though you probably want:

(install
  (section bin)
  (files foo.js))

If your js file is meant to be executed.

[phated]

Thanks! I wonder if I'm still doing something wrong because when I add this to my dune file:

(install
 (section bin)
 (files Example.bc.js))

And then run dune install --prefix _install, I still get None for the version in the resulting file.

I added a commit at phated/dune-build-info-jsoo@ae3fdd7

Can you grep the .js file for DUNE_PLACEHOLDER? I'd like to see if the placeholder whether the placeholder is preserved or whether it is scrambled for some reason.

[rgrinberg]

I'd like to see if the placeholder whether the placeholder is preserved or whether it is scrambled for some reason.

I checked and couldn't find the place holder. Here's the JS:

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aE=d("unknown");function
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That's the issue then. Something must be scrambling the placeholder somehow.

[phated]

That's the issue then. Something must be scrambling the placeholder somehow.

How should I determine if dune is doing it or something else?

I can't think of anything in Dune that would cause that. I expect this is done by some "js minimiser" stage.

We have two options here:

• try to understand what's going on, and see if we can find a way to make the placeholder method work with javascript targets. For instance, maybe there is a way in javascript to say "don't touch this string constant". I can help you here as I don't know much about javascript
• accept that this method is fragile and doesn't work with javascript, and adapt Dune accordingly. For instance:
  • when the js mode is selected, don't use the placeholder method at the cost of more frequent rebuilds
  • add a general option to not use the placeholder method at all

Might be worth pining esy developers about this BTW.

[phated]

@jeremiedimino While digging into the artifacts, it seems that the build_info_data.ml-gen has this inside it:

let p0 = eval (Sys.opaque_identity "%%DUNE_PLACEHOLDER:64:vcs-describe:1:.%%%%%%%%%%%%%%%%%%%%%%%%%%")

Instead of the let p0 = "0.2.0" when there's also an exe target.

[rgrinberg]

Perhaps @hhugo knows why jsoo removed the marker?

@phated that's expected. The rational is explained here.

However, if you write an explicit version field in the dune-project file then the version will be hardcoded.

[phated]

However, if you write an explicit version field in the dune-project file then the version will be hardcoded.

Yeah, that's what I am doing in the reproduction, so why is it still being set as opaque?

To prevent the compiler from being too clever if eval gets inlined, since this could cause the string constant to be processed at compilation time. Maybe that's what happening with jsoo here?

[hhugo]

I'm pretty sure dune-build-info is broken with js_of_ocaml separate compilation. link_time_code_gen should be used in the jsoo codepath.