Adding callback support.

Hi ppl,

I'm currently building some stored procedures in C that uses some internal
hash tables - It could be really nice to be able to deallocate those
correctly when e.g. a memctx is destroyed. Would it be possible to add this
as a postgresql feature and how should it be done.

I also have some other tasks where I would like to do specific actions when a
transaction completes or aborts. This could be used for maintaining a
pgmemcache correctly and many other cases. Could this be combined with the
above feature so a generic callback framework was added ?

I was thinking something like :
register_callback(my_func, (void*) myprivatedata,
CB_ON_TRNSACT_OK|CB_ON_TRNSACT_ABORT );

Other flags could be on CB_ON_QUERY_DONE/ABORTED, CB_ON_CONNETION_CLOSED

I wouldn't mind trying to do patches if they would be accepted for 8.1 or 8.2
but I would if we could settle for an API before I start coding anything.

---
Nicolai Petri