LISTEN/NOTIFY enhancement: Portable signal handling?

Howdy. I'm starting some work on our favorite LISTEN/NOTIFY subsystem
in the hopes of more advanced functionality. Right now I'm using a
bastardized (RAISE NOTICE + tailing error logs) + NOTIFY to get what
should just be built in to the LISTEN/NOTIFY interface. Here's the
syntax for the proposed functionality. I've updated the grammar to
support the following:

LISTEN name [BLOCK] [WITH] [TIMEOUT [=] ''::INTERVAL];
NOTIFY name a_expr;

But am having something of a think-o when it comes to the semantics of
the BLOCK keyword for the LISTEN command. Let me first explain my
goals for this exercise:

* Allow passing of a data type in a NOTIFY. Something like:

NOTIFY 'relname' a_expr;

Should be able to pass a text, INET, etc... shouldn't matter... not
sure if this is possible though given that OIDs don't travel with data
types... I may have to settle for a TEXT arg, which is acceptable.

* Allow LISTEN to block until a value changes. LISTEN [BLOCK|WAIT]
'relname'

* Allow LISTEN to have a timeout

LISTEN name [BLOCK] [WITH] [TIMEOUT [=] ''::INTERVAL];

* Allow blocking LISTEN queries to update the status of the proc title
while blocking.

Basically I want to introduce formal support for turning PostgreSQL
into a message bus. To start with, it won't be a scalable message bus
that can scale to thousands of concurrent connections (that's something
I'd like to do long term(tm), but I digress). The problem is with the
BLOCK'ing semantics or implementation. There are two ways that I can
do this, IMHO, but I'm out to solicit alternatives as I'm still getting
a handle on what the best interfaces/APIs are internally to get
something done.

Option 1) Use sleep(3) for the given timeout and wake up on some
interruptible a signal (USR2?). This is the simplest solution, but
likely the least portable to win32. Given the new world order of 8.0
and it's portability headaches, it's something I'm aware of.

Option 2) block on an exclusive lock. Check to see if relname has been
registered. If it has, block on the existing exclusive lock (can I
block on a lock for a given number of sec/ms?). If it hasn't, create
an exclusive lock, but give the lock away (to the parent postmaster, a
lockmgr proc, etc) so that a notify can remove the remove and unlock
the exclusive lock so that all of the blocking children wake up.

The async interface is nice, but not really useful to me as it requires
polling, instead of unblocking when an event comes through, which would
create a vastly more real time interface that should be easier on the
database. Does anyone think there would be any conflicts with the use
of the existing alarm code from storage/lmgr/proc.c for the
LISTEN/NOTIFY interface? It looks like SIGALRM has a reserved purpose.
I haven't found any global alarm handling interface that can be used
to assign different meanings when an SIGALRM is received. Any other
thoughts on the best way to proceed?

-sc

--
Sean Chittenden