Libmemcache/php-mcache issues

[Russ Garrett]

OK, today I tried moving our live site over to the mcache PHP extension. 
Previously we were using the PECL extension with a custom multi-server 
wrapper, however this caused problems with failover (i.e., it doesn't), 
and we'd much rather be running on libmemcache which seems like a much 
more reliable solution.

So the setup is:

mcache version 1.2.0-beta8
libmemcache version 1.4.0.b5
memcached version 1.1.11

We are (or rather, were, since we've rolled back) getting loads of 
errors like this (probably 5 per second):

Sep  4 15:54:20 local at www5 httpd[3256]: [error] PHP Fatal error:  
memcache(4) protocol error - \r\nWEEKLY UK AUDIOSCROBBLER ARTIST 
CHART\r\nW...
Sep  4 15:54:16 local at www3 httpd[27668]: [error] PHP Fatal error:  
memcache(4) protocol error - VALUE c012ba4abdd9f4f8b4ab1f1386957dd5 1 
4043\r\nO:5:"group":..
Sep  4 15:54:17 local at www2 httpd[3275]: [error] PHP Fatal error:  
read(2) failed - server unexpectedly closed connection in 
/nfsroot/web/site/www.last.fm/core/memcached.php on line 43
Sep  4 15:53:45 local at www3 httpd[27673]: [error] PHP Fatal error:  
memcache(4) protocol error - no \\r before \\n in 
/nfsroot/web/site/www.last.fm/core/memca
ched.php on line 43

Firstly, why are these fatal errors? Surely libmemcache should simply 
log a notice, and then close/reopen the connection to reset it, hence 
not completely breaking the site if something goes wrong with memcached.

Is this some sort of version incompatibility, or an issue in libmemcache 
itself? It only seems to happen on a certain (small) subset of values. I 
hope to isolate these keys and see if I can reproduce these errors on 
our development site.

Thanks in advance,

Russ Garrett
Last.fm Ltd
russ at last.fm