I noticed that the capnp-rpc-unix package's tests now hang with the latest dune, whereas they used to pass (ocaml/opam-repository#29035 (comment)).

Reproduction

I simplified it, and trying to test this file results in a hang:

<!-- $MDX dir=example -->
```sh
$ dune exec -- ./main.exe run-server &
$ dune exec -- ./main.exe run-client
```

My dummy example/main.ml is:

let () =
  if Sys.argv.(1) = "run-server" then Unix.sleep 10
  else print_endline "hi"

dune test hangs and strace shows a dune exec -- ./main.exe run-client process doing this endlessly:

newfstatat(AT_FDCWD, "_build/.rpc/dune", 0x7ffc014ef1f0, 0) = -1 ENOENT (No such file or directory)
futex(0x562c57544710, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 258, NULL, FUTEX_BITSET_MATCH_ANY) = 0
futex(0x562c5751f110, FUTEX_WAKE_PRIVATE, 1) = 0
newfstatat(AT_FDCWD, "_build/.rpc/dune", 0x7ffc014ef1f0, 0) = -1 ENOENT (No such file or directory)
futex(0x562c57544714, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 259, NULL, FUTEX_BITSET_MATCH_ANY) = 0
futex(0x562c5751f110, FUTEX_WAKE_PRIVATE, 1) = 0

I suppose running the first command in the background is confusing things somehow. Is this expected to work?

Specifications

• Version of dune (output of dune --version): 3.20.2
• Version of ocaml (output of ocamlc --version): 5.4.0
• Operating system (distribution and version): Debian/13.2

Additional information

• Link to gist with verbose output (run dune with the --verbose flag): https://gist.github.com/talex5/184df52eb3ed127ef425b761a5a962c5

I also noticed that the nested log file has some mysterious null bytes:

# Workspace root: /home/user/work/mirage/capnp-rpc/_build/default
^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@# Dune context: