dune pkg management fails on optional directories (jsont/bytesrw)

[avsm]

Expected Behavior

With the sample project with:

dune-project:

(lang dune 3.17)
(package
 (allow_empty)
 (name test)
 (depends jsont bytesrw))

dune:

(executable
  (name t)
  (libraries jsont jsont.bytesrw))

and a t.ml to build:

open Jsont
open Bytesrw

The lock works

$ dune pkg lock
Solution for dune.lock:      e
- bytesrw.0.1.0
- jsont.0.1.1
- ocaml.5.3.0
- ocaml-base-compiler.5.3.0
- ocaml-compiler.5.3.0
- ocaml-config.3
- ocamlbuild.0.15.0+dune
- ocamlfind.1.9.6+dune
- topkg.1.0.7

but the build fails:

$ dune build
    Building ocaml-config.3
    Building ocaml.5.3.0
 Downloading ocamlfind.1.9.6+dune
    Building ocamlfind.1.9.6+dune
    Building ocamlbuild.0.15.0+dune
    Building topkg.1.0.7
    Building bytesrw.0.1.0
    Building jsont.0.1.1
Error: This rule defines a directory target "default/.pkg/jsont/target" that
matches the requested path "default/.pkg/jsont/target/lib/jsont/brr" but the
rule's action didn't produce it

Actual Behavior

The build should work

Specifications

"Dune Developer Preview: build 2025-01-28T02:10:32Z, git revision
c0e3bb6"
on macOS

[rgrinberg]

Thanks for the bug report. depopts aren't the cause of this issue but the fact that we have an optional directory inside the META file. Unfortunately, there's a limitation that makes it impossible to check for the existence of a subdirectory inside a directory produced by a rule without triggering this error.

I'll see if I can workaround this somehow.

Repro in #11426

[avsm]

Thanks for the triage! I've edited the title to reflect the diagnosis.

[rgrinberg]

@ElectreAAS could you have a look at this one? Should be quite similar to the other directory targets issue you've fixed.