Re: racing srfi-18 threads

[Neil Jerram <neil@ossau.uklinux.net>]

Neil Jerram <neil@ossau.uklinux.net> writes:

> Tristan Colgate <tcolgate@gmail.com> writes:
>
>> A similar, though much simpler, test case exhibits the same problem.
>> I'll raise a bug. report and
>> include a log of the issue.
>
> Hi Tristan,
>
> Thanks for reporting this.  FWIW I've checked that it still happens
> (exactly as you've described) with the current Git HEAD.  Not much clue
> yet about the underlying problem or fix, but I'll keep looking.

What happens is that each of the spawned threads throws an exception
before it gets going.  If I run test-broken in the (srfi srfi-18)
module, with the following code added at the end:

(write threads)
(newline)
(for-each (lambda (t)
	    (write (thread->exception t))
	    (newline))
	  threads)
(for-each thread-join! threads)

I see:

===============
build thread
build thread
build thread
build thread
build thread
build thread

(#<thread 3081010064 (8d308a0)> #<thread 3064224656 (8d305c0)> #<thread 3064224656 (8d30450)> #<thread 3064224656 (8d302e0)> #<thread 3064224656 (8d30170)> #<thread 3064224656 (8d30000)>)
((uncaught-exception) wrong-type-arg "with-exception-handler" "Wrong type argument: ~S" (#<program 8e13ac0>) ())
((uncaught-exception) wrong-type-arg "with-exception-handler" "Wrong type argument: ~S" (#<program 8e13a00>) ())
((uncaught-exception) wrong-type-arg "with-exception-handler" "Wrong type argument: ~S" (#<program 8e13940>) ())
((uncaught-exception) wrong-type-arg "with-exception-handler" "Wrong type argument: ~S" (#<program 8e13890>) ())
((uncaught-exception) wrong-type-arg "with-exception-handler" "Wrong type argument: ~S" (#<program 8e13800>) ())
((uncaught-exception) wrong-type-arg "with-exception-handler" "Wrong type argument: ~S" (#<program 8e13740>) ())
===============

This is apparently because of srfi-18.scm's `with-exception-handler'
including

    (check-arg-type thunk? thunk "with-exception-handler")

It seems that when run under the VM, (thunk? thunk) => #f.

scm_thunk_p depends on scm_i_program_arity, which depends on
scm_program_arities, and adding this -

       (write ((@ (system vm program) program-arities) thunk))
       (newline)
       (write (procedure-property thunk 'arity))
       (newline)

- into srfi-18's make-thread code, I get

#f
#f

every time.

So I think this is a good point to report and ask if this makes sense.
Andy / Ludo, can it be correct for (program-arities PROGRAM) to be #f ?
If not, any idea what is the root cause of this?

Another question here is why the thread-join! doesn't cause the uncaught
thread exceptions to be raised on the main thread.  I'll look further
into that.

Thanks,
        Neil