[PATCH] Abstraction for config resources, allowing file, URL, pipe, etc.

Hi list,

I have abstracted the gwlib/cfg.[ch] module in order not to handle 
simply a 'config file', but a 'config resource', which then can be 
either of:

- a config file
- a config URL, retrieving the config via the HTTP body
- the stdin pipe, allowing to pipe in a config
- a data block itself, interpreted as config

Now, to demonstrate the effect, here are some sample ways on how 
Kannel's daemon can be then started:

   $ bearerbox

will look for 'kannel.conf', as it does today.

   $ bearerbox smskannel.conf

will open the local 'smskannel.conf' file, parse and expand it, as it 
does today.

   $ bearebox http://host/smskannel.conf

will HTTP GET to the URL and pull the config from the HTTP response body.

   $ cat smskannel.conf | bearerbox pipe

will instruct bearerbox to read in the config from the stdin pipe.

Ok, sounds sweet, right? But is there more? Yes, there is. Due that the 
gwlib/cfg.[ch] module is abstracted on these "types" of resources for 
the config, we can mix them using the 'include = <resource>' statement.

I.e. I can have a local smskannel.conf that includes a remote URL config 
part, or I can fetch a remote URL config block, but refer again to local 
config files, etc.

Now you may ask what's this used for, you have been living quite fine 
with the normal local config style, or even have sync'ed configs via 
rsync to achieve HA setups?

Yes, sync'ing the filesystem is a way to make "remote changes" for a 
Kannel worker node. But I wanted something that is even more simple, 
like the HTTP URL call fetching of the config.

This allows a "controler node" to instruct how the Kannel "worker nodes" 
will operate. Meaning, to which SMSCs they connect, which sms-services 
they need to obey, etc.

It makes things simply more flexible, especially when you start to look 
at large scalability, HA and LB issues.

The patchset can be found at URL:
http://www.kannel.org/~tolj/patchsets/cfg/gateway-cfg.diff

NOTE: The user's guide part is still missing, I'll make sure this gets 
into the patchset before committing.

Reviews, comments, votes welcome.

Stipe

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