Compilation of some packages hanging!

[AltGr]

Thanks @avsm for finding this, it seems it is a very recent regression.

$ cd /tmp
$ git clone https://github.com/avsm/debug-opam-dune-hang
$ opam switch create debug-opam-dune-hang/ -v --debug --yes
[...]
02:25.329  FILE(opam)             Wrote /tmp/debug-opam-dune-hang/_opam/.opam-switch/packages/dune-configurator.2.8.4/opam in 0.000s
02:25.329  PARALLEL               Job 44612439 finished
Processing 46/51: [bigstringaf: dune build]
02:25.329  PARALLEL               Starting job 268152395 (worker 2/24 -/1 -/5): λ mirage-crypto.0.8.10
02:25.329  SYSTEM                 rmdir /tmp/debug-opam-dune-hang/_opam/.opam-switch/build/mirage-crypto.0.8.10
02:25.335  ACTION                 prepare_package_source: mirage-crypto.0.8.10 at /tmp/debug-opam-dune-hang/_opam/.opam-switch/build/mirage-crypto.0.8.10
02:25.335  PARALLEL               Next task in job 268152395: $HOME/.opam/opam-init/hooks/sandbox.sh build dune build -p mirage-crypto -j 24
Processing 47/51: [bigstringaf: dune build] [mirage-crypto: dune build]
<hangs here>

Disabling the sandbox doesn't seem to help.

[AltGr]

It seems that mirage-crypto builds with dune that calls make in freestanding that calls opam config var prefix which hangs ; killing the latter releases the build.

[dra27]

Simpler repro: opam switch create hangs ocaml-system mirage-crypto bigstringaf. The issue only occurs during switch creation (i.e. opam switch create works ocaml-system && opam install mirage-crypto bigstringaf works).

The fault's been present since the first beta - it was introduced by c6aaa60 in #4314.

[AltGr]

Thanks for the bisecting!