BUG #5842: Memory leak in PL/Python when taking slices of results

The following bug has been logged online:

Bug reference: 5842
Logged by: Daniel Popowich
Email address: danielpopowich(at)gmail(dot)com
PostgreSQL version: 8.4.6
Operating system: x86_64-pc-linux-gnu (Ubuntu 10.04.1)
Description: Memory leak in PL/Python when taking slices of results
Details:

There is a memory leak in PL/Python when taking slices of results. This was
first discussed in pgsql-general:

http://archives.postgresql.org/pgsql-general/2011-01/msg00367.php

Thanks to Alex Hunsaker for pinpointing the problem to slices. The
following code (a modification of Alex's) demonstrates the problem well...in
a database with plpythonu installed:

-- leaks big time
CREATE or replace FUNCTION py_leak() RETURNS void
LANGUAGE plpythonu
AS $$
results = plpy.execute("""select generate_series(0, 1000000)""")
slice_creates_leak = results[:]
for r in slice_creates_leak:
pass
return
$$;

-- does not leak
CREATE or replace FUNCTION py_no_leak() RETURNS void
LANGUAGE plpythonu
AS $$
results = plpy.execute("""select generate_series(0, 1000000)""")
for noleak in results:
pass
return
$$;

I traced the bug to PLy_result_slice() in src/pl/plpython/plpython.c. That
function calls the python API function PyList_GetSlice() and erroneously
increments the reference count before returning the result to the caller.
PyList_GetSlice returns a *new* reference, not a borrowed one, so it should
just return the object as-is.

A patch is attached below.

Cheers,

Dan Popowich

----------------------------------------------------------------------

*** src/pl/plpython/plpython.c~ 2010-12-13 21:59:19.000000000 -0500
--- src/pl/plpython/plpython.c 2011-01-18 11:18:28.857831733 -0500
***************
*** 2328,2341 ****
static PyObject *
PLy_result_slice(PyObject *arg, Py_ssize_t lidx, Py_ssize_t hidx)
{
- PyObject *rv;
PLyResultObject *ob = (PLyResultObject *) arg;

! rv = PyList_GetSlice(ob->rows, lidx, hidx);
! if (rv == NULL)
! return NULL;
! Py_INCREF(rv);
! return rv;
}

static int
--- 2328,2336 ----
static PyObject *
PLy_result_slice(PyObject *arg, Py_ssize_t lidx, Py_ssize_t hidx)
{
PLyResultObject *ob = (PLyResultObject *) arg;

! return PyList_GetSlice(ob->rows, lidx, hidx);
}

static int

What has been done with this report/fix?

---------------------------------------------------------------------------

Daniel Popowich wrote:
>
> The following bug has been logged online:
>
> Bug reference: 5842
> Logged by: Daniel Popowich
> Email address: danielpopowich(at)gmail(dot)com
> PostgreSQL version: 8.4.6
> Operating system: x86_64-pc-linux-gnu (Ubuntu 10.04.1)
> Description: Memory leak in PL/Python when taking slices of results
> Details:
>
> There is a memory leak in PL/Python when taking slices of results. This was
> first discussed in pgsql-general:
>
> http://archives.postgresql.org/pgsql-general/2011-01/msg00367.php
>
> Thanks to Alex Hunsaker for pinpointing the problem to slices. The
> following code (a modification of Alex's) demonstrates the problem well...in
> a database with plpythonu installed:
>
> -- leaks big time
> CREATE or replace FUNCTION py_leak() RETURNS void
> LANGUAGE plpythonu
> AS $$
> results = plpy.execute("""select generate_series(0, 1000000)""")
> slice_creates_leak = results[:]
> for r in slice_creates_leak:
> pass
> return
> $$;
>
> -- does not leak
> CREATE or replace FUNCTION py_no_leak() RETURNS void
> LANGUAGE plpythonu
> AS $$
> results = plpy.execute("""select generate_series(0, 1000000)""")
> for noleak in results:
> pass
> return
> $$;
>
>
> I traced the bug to PLy_result_slice() in src/pl/plpython/plpython.c. That
> function calls the python API function PyList_GetSlice() and erroneously
> increments the reference count before returning the result to the caller.
> PyList_GetSlice returns a *new* reference, not a borrowed one, so it should
> just return the object as-is.
>
> A patch is attached below.
>
> Cheers,
>
> Dan Popowich
>
>
> ----------------------------------------------------------------------
>
>
> *** src/pl/plpython/plpython.c~ 2010-12-13 21:59:19.000000000 -0500
> --- src/pl/plpython/plpython.c 2011-01-18 11:18:28.857831733 -0500
> ***************
> *** 2328,2341 ****
> static PyObject *
> PLy_result_slice(PyObject *arg, Py_ssize_t lidx, Py_ssize_t hidx)
> {
> - PyObject *rv;
> PLyResultObject *ob = (PLyResultObject *) arg;
>
> ! rv = PyList_GetSlice(ob->rows, lidx, hidx);
> ! if (rv == NULL)
> ! return NULL;
> ! Py_INCREF(rv);
> ! return rv;
> }
>
> static int
> --- 2328,2336 ----
> static PyObject *
> PLy_result_slice(PyObject *arg, Py_ssize_t lidx, Py_ssize_t hidx)
> {
> PLyResultObject *ob = (PLyResultObject *) arg;
>
> ! return PyList_GetSlice(ob->rows, lidx, hidx);
> }
>
> static int
>
> --
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> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-bugs

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +

On Fri, Mar 11, 2011 at 6:02 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> What has been done with this report/fix?

AFAIK, nothing. Added to 9.1 open items list.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Thu, Mar 17, 2011 at 4:40 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Mar 11, 2011 at 6:02 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>> What has been done with this report/fix?
>
> AFAIK, nothing.  Added to 9.1 open items list.

The patch seems to do the right thing.

--
marko

Excerpts from Marko Kreen's message of jue mar 17 12:01:13 -0300 2011:
> On Thu, Mar 17, 2011 at 4:40 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Fri, Mar 11, 2011 at 6:02 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> >> What has been done with this report/fix?
> >
> > AFAIK, nothing.  Added to 9.1 open items list.
>
> The patch seems to do the right thing.

Looking into this. AFAICT this needs to be backported.

--
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Excerpts from Daniel Popowich's message of mar ene 18 13:30:04 -0300 2011:

> I traced the bug to PLy_result_slice() in src/pl/plpython/plpython.c. That
> function calls the python API function PyList_GetSlice() and erroneously
> increments the reference count before returning the result to the caller.
> PyList_GetSlice returns a *new* reference, not a borrowed one, so it should
> just return the object as-is.
>
> A patch is attached below.

Thanks, applied and backpatched all the way back to 8.2.

--
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support