LISTEN/NOTIFY enhancement: Portable signal handling? - Mailing list pgsql-hackers

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
interfacethat can be used 
 
to assign different meanings when an SIGALRM is received.  Any other 
thoughts on the best way to proceed?

-sc

-- 
Sean Chittenden