Bug: learn-ocaml build serve --replace doesn't recompile the cmo as expected

[erikmd]

Related user(s):

@AltGr, @erikmd, @yurug

Related issue(s) or PR(s):

• feat(server): add a --replace option #567

Related project scope(s):

build

Bug description:

learn-ocaml build serve --replace never recompiles the cmo's

To reproduce:

learn-ocaml build --repo=demo-repository serve
^C
echo '# Update !' >> demo-repository/exercises/demo2/descr.md
# …more edits…
learn-ocaml build --repo=demo-repository serve --replace

↓

Learnocaml v.0.16.0 running.
Updating app at ./www.temp
demo2                    (no changes)
demo                     (no changes)
Learnocaml server v.0.16.0 starting on port 8080

Expected behavior:

Learnocaml v.0.16.0 running.
Updating app at ./www
demo                     (no changes)
demo2                        [OK]
Learnocaml server v.0.16.0 starting on port 8080

learn-ocaml --version

No response

git describe --long --always --abbrev=40 --tags

No response

What OS do you use?

GNU/Linux

What OS version/distribution do you use?

Debian 11

What browser(s) do you use with learn-ocaml?

Firefox

What browser(s) version did you used to reproduce the issue?

No response

Screenshots (if need be):

No response

Additional context:

No response

[AltGr]

Argh, indeed that's pretty annoying. :(
After looking into it I can diagnose it as follows:

• rebuilding the exercises is based only on timestamp comparisons (à la make)
• when --replace is activated, the build dir is first copied to a temporary place, which updates the timestamps

After a quick test, it seems that adding -p (which is POSIX) at https://github.com/ocaml-sf/learn-ocaml/blob/master/src/utils/lwt_utils.ml#L43 (ie replace "-PR" with "-pPR") is enough to solve the issue.

[AltGr]

Thanks a lot @erikmd for seriously doing the testing! :)