Derive the label selector value from the target matchLabels

[splkforrest]

Hi Friends 😄 ,

This PR changes how the label selector value is derived. The way it is today, the label selector is derived from the service name defined in the Canary CR. This PR changes it to be derived from the value as it is already defined in the target Deployment's matchLabels.

This addresses a specific use case where the following occurs:

1. A pre-existing Deployment has labels that cannot be changed due to observability requirements in an organization
2. A pre-existing Service name cannot be changed due to downstream dependencies
3. The Deployment matchLabels are already defined and Deployment selectors are immutable
4. The Service name does not match the Deployment label value

The above points lead to a scenario where Flagger will create the service-canary with a selector based on the Service name defined in the Canary CR and thus not match the label/label-value on the original/canary Deployment. Initialization will pass and the primary Deployment will work fine because it is a newly created Deployment, but the canary Deployment will fail to receive traffic because of the mismatch in service selector and deployment label/label-value.

This situation can be worked around by deleting the Deployment (without deleting the associated Pods) and creating a new Deployment with updated matchLabels (using a new label and label-value). This works for a PoC, but, this doesn't seem to be a scalable workaround if we adopt Flagger in our organization with dozens and dozens of services. A lot of our services have significant traffic and it only takes one mistake to bring down the Pods with the Deployment.

The majority of the code changes here are tests. The actual change is really in getSelectorLabels and the associated interface changes. I would greatly appreciate any/all feedback. 😄 And thank you so much for this project! We are looking at adopting it more widely in our org. 👍

[splkforrest]

Hey @stefanprodan, does my latest commit address your feedback? Any other feedback before this can be merged? Thanks!

[stefanprodan]

@splkforrest what's the impact to current users?

[splkforrest]

Good question. My understanding is that for existing users, there should be no impact. The change will use the labelSelector value on the target Deployment/DaemonSet instead of the service name (or targetRef name) when creating the selector on the services and labelSelector values on the primary Deployment/DaemonSet. Everything else works the same way that it does today. In the case of existing users, the labelSelector value and service name should already match, since that is all that is currently allowed, so there is no affect to those users. In the case of new users, they can either match their labelSelector value to their service name or not.
That's my understanding. Please let me know if you see any problems or if I have misunderstood some implementation detail of Flagger.

[stefanprodan]

LGTM

Thanks Forrest 🏅

[splkforrest]

Awesome! Thanks so much @stefanprodan! Love the project and this will allow us to test out a lot of our other services. 👍