Dynamically Loaded Lookups
Problem Space
We currently have a problem packaging Exim because you have to decide at compile time which lookups you wish to support. This leads to either a binary with a lot of library dependancies, or forcing a subset of lookups on the end users.
Dynamically loading the lookups you need may help with this problem.
Proposal
Have a directory of lookup dynamically loaded modules, with a main config option pointing to them:-
lookup_module_directory = /usr/lib/exim/lookups
When you hit a lookup phrase:-
${lookup whoson {$sender_host_address}{yes}{no}}then you look through the list of currently known (loaded) lookups. If this lookup is not in the list then you dynamically load the module lookup_whoson.so, fix it up, and add it to the end of the lookup_list[] - it can now be used as per ususal.
We would need a method to pick up any additional data (think LDAP servers, MySql server/db/user/pass) for the lookup which would need to be done in a way that does not require exim to have foreknowledge of the lookup type.
Alternative
Have a set of initial directives of the form:-
lookup_function mysql = lookup_mysql.so args....
and then when mysql is seen it causes a load of the module. The args are then passed to the module init function (allowing module specific data such as db/user/server/pass to be specified.
This would also allow another lookup type to be defined
lookup_function othermysql = lookup_mysql.so args....
which would then allow multiple different mysql databases to be used, if that was appropriate.
Issues
We may wish to restrict what filters and user specified things can do - for example they do not need to see into any system authentication database!
Contributors
This came out of an IRC discussion between myself and David Woodhouse. Its a roughing out of the issues to get something down in semi-permanent form. Further refining or comments would be appreciated...
Comments
Some references: