While working on reproducible builds for openSUSE, I found that our ocaml-dune 3.12.1 package varies depending on the amount of parallelism.

Expected Behavior

Build output should be deterministic - as it still was in the previous version 3.11.1

Actual Behavior

Binaries produced by -j 1 and -j 4 vary even in length.
A diff of strings output looks thus:

 camlDune_rules__Package_db__fun_2964                                           
-camlOpamSystem__303
 camlDune_engine__Action_exec__fun_5160
 camlDune_engine__Action_builder0__code_begin
+camlStdune__Signal__3410
 camlDune_re__Core__witness_2586
 camlDune_rules__Modules__fun_4126
+camlNotty_uucp_data__16639
 camlDune_rpc_impl__fun_970
[...snipped...]

one created a _boot/compiled_ml_files and the other a _boot/mods_list. Build logs also differ a lot.

Reproduction

• PR with a reproducing test:

1. on openSUSE or Debian

#!/bin/sh
osc co openSUSE:Factory/ocaml-dune && cd $_ || exit 1
for N in 1 2 ; do
    osc build -j $N --vm-type=kvm --noservice --no-preinstallimage --keep-pkg=RPMS.$N
    unrpm RPMS.$N/ocaml-dune-*.x86_64.rpm
    strings usr/bin/dune > $N.strings
done
diff -u {1,2}.strings

Specifications

• Version of dune (output of dune --version): 3.12.1
• Version of ocaml (output of ocamlc --version): 4.14.1
• Operating system (distribution and version): openSUSE Tumbleweed 20231213

Additional information

• Link to gist with verbose output (run dune with the --verbose flag):
  https://gist.github.com/bmwiedemann/c05ab97a162d966ae40b7a39065efd6e