Possible issues in master.c (centry_add hiding entries, snmp_timeout applied when it shouldn't)

[Jens Erat]

Hi all,

the only feedback was for the snmp_timeout (probably should have better
sent out two mails). Did you look into the ctable_add issue?

> ## centry_add hides older entries
> 
> This seems like a minor issue as it might result in broken state in rare
> cases, which repairs itself after "hiding" process is terminated again
> (which might take days or even weeks, though).
> 
> `centry_add` does simply add a new entry to the ctable, but does not
> verify whether it already exists:
> 
>     /* add a centry to the global table of all
>      * centries, using the given pid as the key */
>     static void centry_add(struct centry *c, pid_t p)
>     {
>         c->pid = p;
>         c->next = ctable[p % child_table_size];
>         ctable[p % child_table_size] = c;
>     }
> 
> While debugging, we added a `centry_find` call before adding and indeed
> observed sporadic events when entries have been added, but not cleaned
> up yet. Mostly they occurred when Cyrus was stuck because of the
> `select` interrupting the master loop, but very rarely this also
> happened during normal operation. I must admit we had a situation with
> heavily increased probability for for such issues:
> 
> - Solaris limited to 30k PIDs by default, while we're running 10-12k
> processes during normal operation
> - `-T 60` joined with a high prefork resulted in imapd processes
> terminated frequently
> - Single active, rather large machine serving a large number of users,
> lots of them using a webmail system with very short IMAP sessions
> - Because of the `select` bug, we often didn't have the child janitor
> cleaning up the ctable for seconds and minutes
> 
> Generally, this might happen on every setup when having bad luck.
> 
> To fix this issue and completely prevent such hiding of older processes,
> I'd propose to run `centry_find` before adding a new entry, and run the
> child janitor if needed.

Regards,
Jens

-- 
Jens Erat
Universität Konstanz
Kommunikations-, Infomations-, Medienzentrum (KIM)
Abteilung Basisdienste
D-78457 Konstanz
Mail: jens.erat at uni-konstanz.de

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