Remaining issues in the OCaml 5 systhread implementation

[gadmm]

The new systhreads implementation now contains 4 FIXMEs/TODOs in systhreads/st_stubs.c.

• /* Cleanup the thread machinery when the runtime is shut down. Joining the tick thread take 25ms on average / 50ms in the worst case, so we don't do it on program exit. (FIXME: not implemented in OCaml 5 yet) */ Note: the safe solution for multicore is likely more involved, one idea is to use condition variables with timedwait.
• caml_thread_domain_initialize_hook: /* FIXME: this should return an encoded exception for use in domain_thread_func, but the latter is not ready to handle it yet. */ (related: possible memory-leak on failed domain backup thread installation #12269)
• caml_thread_reinitialize (best-effort fork with multiple threads): /* FIXME: The same should be done (or not) for IO mutexes (as in the pre-multicore world). However there they might be locked by someone else, and we are probably in an inconsistent state. */
• Unclear initialization of domain variables related to exception handling in bytecode inside caml_thread_new_info. Ideally, one should find a way to factor caml_thread_new_info and domain_create regarding the initialization of relevant domain variables. See the discussion starting here: Refactor caml_c_thread_(un)register to fix various bugs #12834 (comment)
• C threads registered as systhread do not behave as expected in case of an uncaught exception. If an exception is uncaught, caml_fatal_uncaught_exception is called and the program stops. This is in contrast with regular systhreads: in case of an uncaught exception, the thread stops and an error is output on stderr, and Thread.set_uncaught_exception_handler is obeyed. (Reported here: Refactor caml_c_thread_(un)register to fix various bugs #12834 (comment))

[gadmm]

The second one is now treated in a series of commits (#12409, #12410). I would prefer to let other people deal with the other issues and I am unsubscribing from this issue, notably to avoid being pinged by the stale bot in case nothing is done (just so you know in case you need to ping me).

[gadmm]

Added the bug reported as a comment here: #12834 (comment)