Interrupting long external library calls

Hi all,

One of the issues we've been looking at with PostGIS is how to interrupt
long-running processing tasks in external libraries, particularly GEOS.
After a few tests here, it seems that even the existing SIGALRM handler
doesn't get called if statement_timeout is reached when in an external
library, e.g. with PostgreSQL 9.0/PostGIS 2.0 trunk/GEOS:

pg90(at)kentang:~$ ./rel/bin/postgres --single -D data postgis

PostgreSQL stand-alone backend 9.0.7
backend> create database foo;
backend> drop database foo;
backend> show statement_timeout;
1: statement_timeout (typeid = 25, len = -1, typmod = -1,
byval = f)
----
1: statement_timeout = "500ms" (typeid = 25, len = -1, typmod
= -1, byval = f)
----
backend> set statement_timeout = '10ms';
backend> create database foo;
ERROR: canceling statement due to statement timeout
STATEMENT: create database foo;

(note the following command takes about 3-4s to run but doesn't get
cancelled)

backend> select st_segmentize('LINESTRING(0 0, 10 0)', 1e-12);
ERROR: invalid memory alloc request size 1073741824

Is there a way in which we can interrupt long-running external library
calls that are unable to call the PostgreSQL CHECK_FOR_INTERRUPTS()
macro directly? If it is possible for standard alarms to work in this
way then something along the lines of an alarm within PostgreSQL itself
that manually invokes CHECK_FOR_INTERRUPTS() every 1s while the query is
running could potentially solve the issue for us.

Many thanks,

Mark.