[Dovecot] PAM, NSS, and a new blocking=yes setting

[Timo Sirainen]

I wondered for a while if I should do this before v1.0, but since there
seem to be problems with some PAM/NSS users and this was way easier to
implement than I realized, here it is: passdb pam and userdb passwd now
support blocking=yes setting in their args.

For PAM the default has been to fork the dovecot-auth process and talk
via a pipe to the main dovecot-auth. I've never really liked this
approach, but didn't bother fixing it before. However it is also what
has caused the bugs with nss_ldap, and maybe with other NSS backends
too.

NSS lookups then have been done all in the same dovecot-auth process
without any forking. This meant that if it got stuck (or delayed a bit
in LDAP queries and such), the whole dovecot-auth got stuck and all
authentication attempts got stuck.

With blocking=yes the lookups are handled in dovecot-auth worker
processes. Earlier these workers have been used only with MySQL.
Initially there is only a single worker process, but whenever
dovecot-auth notices that there are no free workers while a new auth
request comes, a new worker is created. auth_worker_max_count specifies
the maximum number of those processes.

This blocking=yes will probably work great with NSS. With PAM I'm not so
sure. Mostly because the worker never exits, it just does one PAM auth
after another. I think usually PAM is used only once in a process, so
there might be memory leaks and maybe other bugs.. Then again, maybe
not. I'll add some auth_worker_max_requests setting which can be set to
1 if this is a problem.

Also for blocking=no case I added code to send KILL signal to PAM child
processes which haven't replied for 2 minutes. With blocking=yes this
condition can't be checked currently.

http://dovecot.org/list/dovecot-cvs/2007-February/007639.html
http://dovecot.org/list/dovecot-cvs/2007-February/007642.html
http://dovecot.org/list/dovecot-cvs/2007-February/007646.html
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