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Re: [BUGS] BUG #5842: Memory leak in PL/Python when taking slices of results

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Daniel Popowich <danielpopowich(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgreSQL(dot)org>
Subject: Re: [BUGS] BUG #5842: Memory leak in PL/Python when taking slices of results
Date: 2011-03-11 11:02:26
Message-ID: 201103111102.p2BB2R705235@momjian.us (view raw, whole thread or download thread mbox)
Thread:
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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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-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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

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