Re: signal handling in plpython

On 10/13/2016 08:57 PM, Mario De Frutos Dieguez wrote:
> I come here asking for some advice/help because we're facing some
> unexpected behavior when we want to interrupt functions doing CPU intensive
> operations in plpython.
>
> Our problem is that we're not able to interrupt them when they're making
> CPU intensive operations. For example, when calculating Moran using PySAL,
> the SIGINT handler of Postgres is not able to cancel it.

Python code isn't interruptible, but any queries you run within a python
function are. So if you have a loop in your function that you know will
run for a long time, you could issue a dummy "SELECT 1" query every once
in a while. However, that doesn't help, if the long loop is in a library
function that you have no control over, rather than the PL/python
function itself.

It would be nice to have a solution for this in plpython itself, so that
the query cancel was turned into a Python exception. Patches for that
would be welcome. I think you could use Py_AddPendingCall() from
PostgreSQL's signal handler, to schedule a call to a function that in
turn throws a Python exception. That'll need some changes to
PostgreSQL's normal signal handlers, like die() and
StatementCancelHandler() in postgres.c, but it seems doable.

- Heikki