Add new cfg module

As described here:

  http://wiki.openstack.org/CommonConfigModule

The module implements an API for defining configuration options and
reading values for those options that a user may have set in a config
file or on the command line.

The module is successfully in use in both Nova and Glance. Some work
remains in Nova to switch from using it under a gflags compatible
shim layer, but Glance is using it fully.

There doesn't appear to be any blockers to other projects moving over
to it fairly easily. Swift would perhaps be the next project to tackle.

Just to go through potential future compatibility concerns:

  - Nova (the scroundrel) hackily uses the private ConfigOpts::_oparser
    in order to disable interspersed args. This was just for nova-manage
    and can probably be resolved some other way. In any case, Nova
    shouldn't switch to openstack-common's cfg API until it removes this
    hack.

  - the CommonConfigOpts subclass set of logging related options is
    perhaps assuming too much about what configuration options should
    be common across all the projects. However, it seems a fairly sane
    set and the worst that can happen is that projects avoid using it.

  - the parameters to the Opt constructor fairly closely mirror
    optparse, but they're fairly generic and shouldn't prevent us from
    switching to e.g. argparse

  - stuff like %prog expansion in the ConfigOpt's usage ctor param is a
    similar concern, but it's a very minor concern.

  - find_config_files() search path is perhaps too much policy for
    openstack-common; however, it is probably as generic as it could
    be and projects which need a different policy can just not use
    the function

On the whole, I think we're in good shape wrt future compatibility.

Change-Id: I279a9db7806d80aff3b9b085b4a9e4fb193662f9

0 files changed, 0 insertions, 0 deletions