Thanks for the answer.

That is a simple moral, and I'm happy with it. Even if one could ask
if it's really impossible that OCaml manages to deal in a nicer way
with its internal locks, leaving to the user the task to manage his
locks. It's probably not worth working that issue, since people should
avoid doing that.. then why not issuing a warning ?

I said I was happy with the moral. Moving to a fork()+exec() was easy
and even made more sense. We didn't experience frozen downloads since
then. We get memory corruption, but that's another story :p

Cheers.
-- 
David