Cannot define cache pool on per-client basis without configuring dummy cache plugin

[ostrolucky]

Q	A
Bug?	no
New Feature?	yes

I was happy to learn I can configure cache plugins on per-client basis!
However, it makes me configure default cache plugin anyway. This doesn't work:

httplug:
  clients:
    app:
      plugins:
        - cache:
          cache_pool: 'app.cache.google'

It will result in error: Service "httplug.client.app.plugin.cache": Parent definition "httplug.plugin.cache" does not exist.

Ok let's add it:

httplug:
        cache: ~

Doesn't work either: The child node "cache_pool" at path "httplug.plugins.cache" must be configured.

At this point I have to configure dummy pool

-       cache: ~
+       cache:
+         cache_pool: 'app.cache.void'

Only now it works. What I am asking is to not force to configure default cache plugin.

[dbu]

i think the problem is here where we remove the main cache plugin service if cache is not enabled.

i wonder how to solve that. we should not make cache_pool fully optional, as it would mean you can end up with invalid configuration. maybe we could allow it to be empty, but register that information and require individual plugins to all specify the pool in that case. do you want to try a pull request for that?