unable to generate controller for the core group

[raffaelespazzoli]

Bug Report

What did you do?
A clear and concise description of the steps you took (or insert a code snippet).
here is the command I issued and corresponding error:

operator-sdk add controller --api-version=core/v1 --kind=Namespace
INFO[0000] Generating controller version core/v1 for kind Namespace. 
Error: group cannot be empty
Usage:
  operator-sdk add controller [flags]

Flags:
      --api-version string   Kubernetes APIVersion that has a format of $GROUP_NAME/$VERSION (e.g app.example.com/v1alpha1)
  -h, --help                 help for controller
      --kind string          Kubernetes resource Kind name. (e.g AppService)

What did you expect to see?
A clear and concise description of what you expected to happen (or insert a code snippet).
controller correclty generated

What did you see instead? Under which circumstances?
A clear and concise description of what you expected to happen (or insert a code snippet).
see error above

Environment

• operator-sdk version:

operator-sdk --version
operator-sdk version v0.5.0+git

notice that this version is misleading. I am actually using the binary resulting from the compilation of the master branch.

Possible Solution

this worked:

operator-sdk add controller --api-version=core.com/v1 --kind=Namespace
INFO[0000] Generating controller version core.com/v1 for kind Namespace. 
INFO[0000] Created pkg/controller/namespace/namespace_controller.go 
INFO[0000] Created pkg/controller/add_namespace.go      
INFO[0000] Controller generation complete. 

I can fix the generated code manually.

Additional context
Add any other context about the problem here.

[hasbro17]

@raffaelespazzoli Thanks for reporting the issue.

We have the check for group names to prevent users from adding single group names for their CRDs.
From the kubernetes API conventions guidelines:

Group names are typically in domain name form - the Kubernetes project reserves use of the empty group, all single word names ("extensions", "apps"), and any group name ending in "*.k8s.io" for its sole use. When choosing a group name, we recommend selecting a subdomain your group or organization owns, such as "widget.mycompany.com".

But I guess that only makes sense for defining new apis. If you're adding a controller to watch a k8s resource(with a single word group) as the primary resource then that'll be a problem.

As a work around for now, you can fix it in after the fact. add controller --api-version=core.com/v1 and then change the group to core the file.

@estroz Since that was added in #815 I was wondering if we can somehow relax this check for when we add controllers, or altogether with a warning?

Side note: Truncated the issue for clarity and removing the prompt for Gopkg.lock in the bug report template #1194

[hasbro17]

@estroz Just pinging again to get your input before I make the fix which should be a small one.