threads and mt predicate

[bobot]

The mt predicate is forced by jbuilder.

let preds = ["ppx_driver"; "mt"; "mt_posix"] in

What is the rational for that? For example Lablgtk can be used without threads, jbuilder force the user to use them (or perhaps it change nothing when you don't use them).

ocamlfind has some heuristics for adding missing threads dependency (eg. lablgtk with mt doesn't depend on thread):

let fixup_thread_needed_1 predlist =
  (* When the thread fixup is required to apply, 1st criterion *)
  List.mem "mt" predlist
;;

let fixup_thread_needed_2 pkg =
  (* When the thread fixup is required to apply, 2nd criterion *)
  (pkg <> "unix" && pkg <> "threads" && not (has_prefix pkg "threads."))
;;

It seems to me that jbuilder doesn't have them, even if ,maybe by chance , program that use lablgtk and threads can be linked well.

[ghost]

So far predicates were not needed so they are not supported to simplify things, it's one less concept to understand for users.

I think supporting this would be a lot of work for little benefit, if any. Especially given that support for multicore is coming.

[bobot]

I think supporting this would be a lot of work for little benefit, if any. Especially given that support for multicore is coming.

Ok it is reasonable. I think something should be said in the manual for advanced user. And shouldn't thread be always linked in that case?

[ghost]

Ok, I added a comment about it in the manual.

And shouldn't thread be always linked in that case?

they will be linked if you depend directly or indirectly on the threads library.

[bobot]

Ok so I'm going to propose an MR that adds ocamlfind heuristics for missing thread dependency (and I should also try to ask lablgtk to fix its meta).

[ghost]

I'm a bit confused, what would such a heuristic do?

[bobot]

The ocamlfind heuristic is when mt is used "for all package add a dependency to threads except for unix and threads.* packages".

If you want that binary for which no packages use the mt predicate, the heuristic could be, if a package use the predicate +mt for the definition of a variable then add a dependency to threads. For example for

description = "Bindings for gtk2"
requires = ""
version = "2.18.4"

archive(byte) = "lablgtk.cma"
archive(native) = "lablgtk.cmxa"
archive(byte,mt) += "gtkThread.cmo"
archive(native,mt) += "gtkThread.cmx"

the heuristic adds requires += threads. If ,mt) is not present the requires is not added.

But I think that if mt is always present the threads package must be always present.

[ghost]

Seems to me that lablgtk should just say:

requires(mt) += "threads"

If only a handful of packages are affected, i'd rather document that people need to add an explicit dependency on threads when using such packages rather than add some magic.

[ghost]

However i'm happy to detect such cases and print a warning, to help users understand what's happening and fix their linking errors