Using `findlib.dynload` inside a PPX

Inside a PPX transformer, I do not manage to use Fl_dynload.load_packages from findlib.dynload package to load dynamically packages: packages are found but no dynamic modules seem to be loaded. The problem only appears inside a PPX transformer: if the same loading is made in a simple executable, it works as expected.

I put a simple example to illustrate the problem in

A module Plugin is defined in the directory plugin/, which relies on the library pcre.
The dynamic library plugin/plugin.cmxs can be built with dune build plugin/plugin.cmxs.

There is no problem when plugin/plugin.cmxs is loaded by a simple executable, such as the one defined in test_direct/. The code of test_direct is fairly simple.

let () =
  Findlib.init ();
  Fl_dynload.load_packages ~debug:true ["pcre"];
  Dynlink.loadfile "plugin/plugin.cmxs"

By building dune build test_direct/test_direct.exe, one can check that executing test_direct/test_direct.exe in the directory _build/default succeeds without error (it is important to execute the program with _build/default as current directory, in order to let the program find plugin/plugin.cmxs). The debugging log is reproduced below. In particular, we can notice that pcre.cmxs is loaded.

[DEBUG] Fl_dynload: about to load: unix
[DEBUG] Fl_dynload: files="unix.cmxs"
[DEBUG] Fl_dynload: loading "unix.cmxs"
[DEBUG] Fl_dynload: about to load: threads.posix
[DEBUG] Fl_dynload: files=""
[DEBUG] Fl_dynload: about to load: threads
[DEBUG] Fl_dynload: files=""
[DEBUG] Fl_dynload: about to load: pcre
[DEBUG] Fl_dynload: files="pcre.cmxs"
[DEBUG] Fl_dynload: loading "pcre.cmxs"

By contrast, if the same code as in test_direct is executed inside a PPX, pcre.cmxs is not loaded and, therefore, loading plugin.cmxs fails. One can check this by trying to compile dune build test_ppx/test_ppx.exe, which invokes the PPX transformer defined in ppx/. The latter just wraps the test above in an Ast_mapper.mapper. The log is reproduced below, where it appears that pcre.cmxs does not seem to be loaded.

[DEBUG] Fl_dynload: about to load: pcre
[DEBUG] Fl_dynload: files=""
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll "Dynlink.Error (Dynlink.Cannot_open_dll \"Failure(\\\"/home/tmartine/tmp/test_findlib/_build/default/plugin/plugin.cmxs: undefined symbol: camlPcre__exec_583\\\")\")")  

It appears that to make findlib.dynload work, dune generates a file to initialize findlib predicates and record packages linked with the executable. But this file is generated for executable target only, and is not generated for the intermediate executable generated for ppx transformations. In the directory ppx_fixed/ of, the module Findlib_for_ppx provides a function init_predicates to initialize findlib predicates, and a function load_packages that does not fail if a module is already linked (since we do not know which packages have already been linked).

I filled an issue for dune: