Exception uncaught by findlib when a package exists but the type_of_threads key is absent

[palainp]

Dear developers,
I recently attempted to update the ecosystem for compilation of qubes-mirage-firewall, but failed (see mirage/qubes-mirage-firewall#229 for context). To compile for this target, I understand that ocaml-solo5 uses empty META files (see #45). From what I understand, since ocaml/ocamlfind@aedee07, if package_property raises a Not_found exception (to my understanding, because the thread package exists from findlib’s perspective, but a key/some keys are missing from the META file), the exception is no longer caught (I think this is where my problem lies, but I haven’t yet looked into it in more detail). See ocaml/ocamlfind#99 for more details.
A minimal reproducible test:

$ ocamlfind -toolchain solo5 ocamlopt -version ;  \
printf 'type_of_threads = "posix"\n' > $OPAM_SWITCH_PREFIX/lib/ocaml-solo5/lib/threads/META ; \
ocamlfind -toolchain solo5 ocamlopt -version
Uncaught exception: Not_found
5.5.0

So the possible solutions would therefore be as follows: either there is a problem with my setup, and that uncaught exception shouldn’t happen; or we would need to add the minimum line to the thread’s META file instead of an empty file; or add a |Not_found to the try block in findlib. I’d tend to favour, if my installation is not broken, the last solution, as that was how it behaved before the latest patch at findlib, but I don’t know if my case is an isolated one.
/cc @shym @hannesm @dinosaure

[dinosaure]

or add a |Not_found to the try block in findlib. I’d tend to favour, if my installation is not broken, the last solution, as that was how it behaved before the latest patch at findlib, but I don’t know if my case is an isolated one.

I think that's the right solution but, on my case, the META file is empty and everything works. So the question is: why you need to add type_of_threads = "posix"\n into the META file?

[palainp]

With everything fine for you suggests my environnement is broken (but as well the qubes-fw Dockerfile :/). I'll dig there. Would you mind to try out the Dockerfile compilation for qubes-fw ?

[dinosaure]

I think you find a bug 👍 but I suspect that type_of_threads should not appear, that's the way ocaml-solo5 install META file:

ocaml-solo5/gen_dot_install.sh

Lines 42 to 45 in 8e9edf9

	# dummy packages
	for pkg in nolibc threads is_solo5; do
	install_file _build/empty-META lib/$pkg/META
	done

[hannesm]

I have some questions:

• (a) @shym what is the way forward? should we add the type_of_threads to the META files until ocamlfind is released with your patch Fix the exception to catch when threads is not available ocaml/ocamlfind#99?
• (b) why do we install lib/is_solo5/META?

I'm still curious during build of what this is an issue, since this issue didn't appear for me... (but the reproducible case ocamlfind -toolchain solo5 ocamlopt -version errors here)

[hannesm]

Ah, likely mirage-xen... (though mirage-solo5 as well uses these invocations in lib/bindings/Makefile)...

[palainp]

Sorry, yes that was when compiling Xen related unikernels.

[hannesm]

see 6d6f283

[hannesm]

I somehow ended up in the same situation. I'll merge #186 and cut a release later today.

[shym]

• (a) @shym what is the way forward? should we add the type_of_threads to the META files until ocamlfind is released with your patch Fix the exception to catch when threads is not available ocaml/ocamlfind#99?

Makes sense to me.

• (b) why do we install lib/is_solo5/META?

I’m not entirely sure, to be honest. When rewriting that part, I kept the generation of the META files that were produced before, assuming some packages of the ecosystem might look for them (I think I tried to look for such packages but didn’t find them; but I’m not completely sure I recall correctly).