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