[Dovecot] Upcoming configuration file changes

[Timo Sirainen]

For those who might be interested. Comments welcome.

A few weeks ago I had started redesigning how Dovecot handles
configuration files. I wrote a new parser and converted dovecot-auth to
use the new way. Today I finally managed to convert imap process as
well, which wasn't that easy because I didn't want mbox/maildir-specific
settings to be hardcoded in any common code.

So, the changes will be:

 - Dovecot master process will need a new completely different
configuration file which tells what processes to run and their
uids/chroots/memory limits. Very much like Postfix's master.cf. I'm not
sure about its format yet (suggestions welcome), but maybe something
like it's already:

process imap-login {
  executable = /usr/lib/dovecot/imap-login
  chroot = /var/run/dovecot/login
  uid = dovecot
  vsz_limit = 32M
}
...

Hopefully after these changes the master process will become very small
and simple.

 - One of the processes will be a configuration process, which is the
one that reads and parses the configuration for non-master processes.
The default configuration file format will be pretty similar to what we
have now, but it can of course be replaced with SQL backends and
whatever.

Config process will listen in some unix socket waiting for requests from
other processes, which connect to it just after they start up and maybe
also sometimes later (eg. to get user/IP-specific settings when a new
client connects).

 - The data that config process sends and what other processes parse is
based on simple key=value pairs, so it's possible to still store and
parse them from environment if needed. Also it makes it possible to
override settings from command line (eg. imap -o mail_debug=yes) or give
user-specific settings in userdb. Here's an example:

# config process reads something like:
mmap_disable = yes
namespace foo {
  type = private
  separator = / 
  mbox {
    dir = ~/mail
    mbox_read_locks = dotlock
  }
}

# sends out:
mmap_disable = yes
namespaces = foo
namespaces_foo_type = private
namespaces_foo_separator = / 
namespaces_foo_mbox = 1
namespaces_foo_mbox_1_dir = ~/mail
namespaces_foo_mbox_1_mbox_read_locks = dotlock

 - Config process should verify that all given settings are valid so
that typos are noticed. This may not be that easy because the setting
definitions are distributed all around the source code. I think there
needs to be at least a (Perl) script that finds out all the setting
definitions and writes them to some all-settings.c for config process so
it knows them.

Settings for plugins are more difficult though.. Should they somehow be
marked in config file that they're plugins (plugin foo { .. })? Or maybe
config process could dynamically load all the plugins itself and request
their setting definitions?

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