PHP crash in get() when using persistence

[doapp-ryanp]

I am getting intermittent problems when doing a get() - every few hundred requests PHP is crashing. I've narrowed it down to to a libmemcached/io.cc: No such file or directory error - but I only get this when PHP crashes. About 1% of my requests result in the crash (which caues HTTP 502).

My environment:
Ubuntu 12.04 64bit: Linux ryan 3.2.0-35-generic #55-Ubuntu SMP Wed Dec 5 17:42:16 UTC 2012 x86_64
nginx 1.2.4
php5-fpm:

/usr/sbin/php5-fpm -v
PHP 5.3.10-1ubuntu3.4 (fpm-fcgi) (built: Sep 12 2012 19:03:32)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies
    with Suhosin v0.9.33, Copyright (c) 2007-2012, by SektionEins GmbH

PECL memcached 2.1.0:

memcached

memcached support => enabled
Version => 2.1.0
libmemcached version => 1.0.15
Session support => yes
igbinary support => yes
json support => no

Directive => Local Value => Master Value
memcached.compression_factor => 1.3 => 1.3
memcached.compression_threshold => 2000 => 2000
memcached.compression_type => fastlz => fastlz
memcached.serializer => igbinary => igbinary
memcached.sess_binary => 0 => 0
memcached.sess_lock_wait => 150000 => 150000
memcached.sess_locking => 1 => 1
memcached.sess_prefix => memc.sess.key. => memc.sess.key.

Lots of memory:

free -m
             total       used       free     shared    buffers     cached
Mem:         15982      15712        270          0        462       9171
-/+ buffers/cache:       6078       9903
Swap:         3813          0       3813

Here is test script to reproduce (test.php):

<?php

$m = new Memcached('mc');
$m->addServer('localhost', 11211);

$d = $m->get('foo');
if(empty($d)){
    $m->set('foo', 100);
}
var_export($d);
echo $m->getResultMessage(),"\n";

Here is my GDB: https://gist.github.com/4b966fa47cd0515ddf25
Here is the core dump: https://dl.dropbox.com/u/1374786/core-php5-fpm.19325.bz2

then simply run ab -n 1000 -c 20 http://localhost/test.php

Any idea what is going on? Is this really a bug in libmemcached?

[doapp-ryanp]

I just found out that when using persistent connections, you should not re-add servers.

After making the following change things work without problem:

<?php

$m = new Memcached('mc');

if (!count($m->getServerList())) {
    $m->addServer('localhost', 11211);
}

$d = $m->get('foo');
if(empty($d)){
    $m->set('foo', 100);
}
var_export($d);
echo $m->getResultMessage(),"\n";