How to install custom sites when cross-compiling

[toots]

Hi,

It's not clear to me how to install packages using custom dune-site when cross-compiling.

Here's a gist to reproduce: https://gist.github.com/toots/1859a37c264f3c1f75b75e21011ed17e

Normal build:

$ dune build -p repro-main --promote-install-files=false @install
$ dune install -p repro-main --create-install-files repro-main
Copying to _destdir/home/opam/.opam/4.12.0/lib/repro-main/META
Copying to _destdir/home/opam/.opam/4.12.0/lib/repro-main/Sites.ml
Copying to _destdir/home/opam/.opam/4.12.0/lib/repro-main/dune-package
Copying to _destdir/home/opam/.opam/4.12.0/lib/repro-main/repro_main.a
Copying to _destdir/home/opam/.opam/4.12.0/lib/repro-main/repro_main.cma
Copying to _destdir/home/opam/.opam/4.12.0/lib/repro-main/repro_main__Sites.cmt
Copying to _destdir/home/opam/.opam/4.12.0/lib/repro-main/repro_main.cmxs

Cross-compiled build:

$ dune build -p repro-main --promote-install-files=false @install -x windows
$ dune install -p repro-main --create-install-files repro-main -x windows
Error: The following <package>.install are missing:
- _build/default.windows/repro-main.install
Hint: try running: dune build [-p <pkg>] @install

[toots]

If I define the packages the traditional way, i.e.:

• repro-main-windows.opam:

opam-version: "2.0"
synopsis: "Repro main"
depends: [
  "dune" {>= "3.2"}
  "ocaml-windows" {>= "4.12.0"}
  "dune-site-windows"
  "odoc" {with-doc}
]
build: [
  ["dune" "subst"] {dev}
  [
    "dune"
    "build"
    "-p"
    "repro-main"
    "-x"
    "windows"
    "-j"
    jobs
    "@install"
  ]
]

• repro-libs-windows.opam:

opam-version: "2.0"
synopsis: "Repro libs"
depends: [
  "dune" {>= "3.2"}
  "repro-main-windows"
  "odoc" {with-doc}
]
build: [
  ["dune" "subst"] {dev}
  [
    "dune"
    "build"
    "-p"
    "repro-libs"
    "-x"
    "windows"
    "-j"
    jobs
    "@install"
  ]
]

Then I get this error when installing repro-libs-windows:

#=== ERROR while compiling repro-libs-windows.~dev ============================#
# context     2.1.2 | linux/x86_64 | ocaml-base-compiler.4.14.0 | pinned(file:///tmp/repro)
# path        ~/.opam/4.12.0/.opam-switch/build/repro-libs-windows.~dev
# command     ~/.opam/4.12.0/bin/dune build -p repro-libs -x windows -j 4 @install
# exit-code   1
# env-file    ~/.opam/log/repro-libs-windows-4994-668c31.env
# output-file ~/.opam/log/repro-libs-windows-4994-668c31.out
### output ###
# File "dune", line 14, characters 3-20:
# 14 |    (repro-main libs)))
#         ^^^^^^^^^^^^^^^^^
# Error: The package repro-main is not found