[Dovecot] Problems with the IMAP proxy after upgrading from dovecot 1.1.16 to 1.211

[Holger Richter]

We have frequent timeout problems after upgrading our imap servers  
from dovecot 1.1.16 to dovecot 1.2.11. One server acts as proxy only,  
and the other one is the "real" imap server". The credentials for the  
proxy service are stored in a remote MYSQL database.

There were no trouble with dovecot 1.1.16. But now, with the most  
recent version, we get frequent login failures. It seems dovecot  
reconnects too late to the MySQL database after a timeout, after  
sending the SQL query. Here's a piece of my log file:

May  7 14:46:32 ttt dovecot: auth(default): new auth connection: pid=5136
May  7 14:46:42 ttt dovecot: auth(default): client in:  
AUTH^I1^IPLAIN^Iservice=imap^Isecured^Ilip=xxx.xxx.xxx.xxx^Irip=yyy.yyy.yyy.yyy^Ilport=sss^Irport=26480^Iresp=<hidden>
May  7 14:46:42 ttt dovecot: auth-worker(default):  
sql(uid,yyy.yyy.yyy.yyy): query: SELECT NULL as password, destuser,  
host, 'zzz' as port, 'Y' as proxy, '0' as proxy_timeout, 'Y' as  
nopassword, 'Y' as nodelay from accounts where user ='uid';
May  7 14:47:42 ttt dovecot: auth(default):  
worker-server(uid,yyy.yyy.yyy.yyy): Aborted: Lookup timed out
May  7 14:47:42 ttt dovecot: auth-worker(default): mysql: Connected to  
lll.lll.lll.lll (imapauth)
May  7 14:47:44 ttt dovecot: auth(default): client out:  
FAIL^I1^Iuser=uid^Itemp
May  7 14:47:49 ttt dovecot: imap-login: Disconnected (auth failed, 1  
attempts): user=<uid>, method=PLAIN, rip=yyy.yyy.yyy.yyy,  
lip=xxx.xxx.xxx.xxx, TLS
May  7 14:48:26 ttt dovecot: auth(default): new auth connection: pid=5159
May  7 14:48:47 ttt dovecot: auth(default): client in:  
AUTH^I1^IPLAIN^Iservice=imap^Isecured^Ilip=xxx.xxx.xxx.xxx^Irip=yyy.yyy.yyy.yyy^Ilport=sss^Irport=6243^Iresp=<hidden>
May  7 14:48:47 ttt dovecot: auth-worker(default):  
sql(uid,yyy.yyy.yyy.yyy): query: SELECT NULL as password, destuser,  
host, 'zzz' as port, 'Y' as proxy, '0' as proxy_timeout, 'Y' as  
nopassword, 'Y' as nodelay from accounts where user ='uid';
May  7 14:48:47 ttt dovecot: auth(default): client out:  
OK^I1^Iuser=uid^Idestuser=luid^Ihost=mmm.mmm.mmm.mmm^Iport=zzz^Iproxy^Iproxy_timeout=0^Ipass=<hidden>
May  7 14:48:47 ttt dovecot: imap-login: proxy(uid): started proxying  
to mmm.mmm.mmm.mmm:zzz/luid: user=<uid>, method=PLAIN,  
rip=yyy.yyy.yyy.yyy, lip=xxx.xxx.xxx.xxx, TLS
May  7 14:48:48 ttt dovecot: imap-login: proxy(uid): disconnecting  
yyy.yyy.yyy.yyy

As you can see, the second login attempt is always successful. Of  
course I've tried out various values (0, 60, 90, 300) for the  
proxy_timeout parameter - but without any success.