[Dovecot] Couldn't open INBOX: Timeout while waiting for lock

[Eric Sammons]

Hi,

So I've been through google and the dovecot source and have not yet found
a reasonable explanation for the 'Timeout while waiting for lock' error. 
My customer's configuration is utilizes mbox with the following mbox
configs.

[snip]
pop3_lock_session = yes
mbox_read_locks = fcntl
mbox_write_locks = fcntl
mbox_lock_timeout = 300
[/snip]

I see from the source that this error can occur in one of two places:

 mbox_lock_dotlock()
 mbox_update_locking()

Given my customer is using fcntl locking I automatically assume that it is
the later where they are hitting the error.

While I see in the source these errors are Temporary Errors; I do wonder
what is the specific cause and what should we be looking at to address the
errors.

One thing we have noticed is that the errors do not occur regulararly. 
For exapmle, there were no such errors reported on June 9 or June 10;
however on June 11 and 12 the errors started to appear.

Any direction / pointers are greatly appreciated.

Thank you!
-- 
Eric L. Sammons, RHCE