srfi-18 requirements

[Julian Graham]

Hi guys,

While I was waiting to get my copyright assignment sorted out, I
started trying to figure out what it would take to add SRFI-18
(http://srfi.schemers.org/srfi-18/srfi-18.html) support to Guile.  I
think a lot of it can be safely done in Scheme (mostly by mapping the
SRFI's proposed API onto what's already in Guile), but there are a few
things that'd require further modification to the C core.
Specifically:

* Type predicates: thread?, mutex?, and condition-variable?.

* Mutex state: This kind of needs to happen in C, since we need to
hook the actual lock / unlock calls in order to record the state
changes.

* Mutex unlocking outside the owner thread: The existing
implementation claims the result of doing this is undefined, but what
it actually does is mark the mutex unlocked and then throw an
exception.  I don't really think there's a technical reason to
prohibit this, especially since Guile doesn't use the pthreads mutex
implementation directly.

* Mutex lock timeout: Not really sure how to do this -- maybe add a
pthreads condition variable to the definition of fat_mutex.
Supporting this with the existing lock-mutex code would also require a
breaking change to the C API, and I don't know what the conditions are
(if any) under which that's acceptable.

All this kind of presumes that SRFI-18 is something that the Guile
maintainers care about supporting.  Is it?  I'm afraid I don't know
the Guile project's attitude towards implementing SRFIs.


Regards,
Julian