Change injector overriding logic to be more generic

[mateiidavid]

The proxy-injector package has a ResourceConfig type that is responsible for parsing resources, applying overrides, and serialising a series of configuration values to a Kubernetes patch. The functionality is very concrete in its assumption; it always relies on a pod spec and it mutates inner state when deciding on which overrides to apply.

This is not a flexible way to handle injection and configuration overriding for other types of resources. We change this by turning methods previously defined on ResourceConfig into free-standing functions. These functions can be applied for any type of resources in order to compute a set of configuration values based on annotation overrides. Through the change, the functions can be used to compute static configuration for non-Pod types or can be used in tests.

Some of the functionality relies on concrete types, e.g. ContainerPorts. To allow the free-standing functions to be generic, we introduce an interface to abstract away the details of how the configuration value is produced.

[mateiidavid]

Tested in a k3d cluster to ensure everything still works as expected, particularly around opaque ports, inbound ports and namespace overrides.

An example ns and nginx workload:

:; k get ns default -o yaml | yq .metadata.annotations
config.linkerd.io/proxy-log-level: debug

:; k get deploy nginx -o yaml | yq '.spec.template.metadata.annotations'
config.linkerd.io/opaque-ports: http,2000-2050,1990-1992a
kubectl.kubernetes.io/restartedAt: "2024-04-10T14:30:53Z"
linkerd.io/inject: enabled


:; k get deploy nginx -o yaml | yq '.spec.template.spec.containers[].ports'
- containerPort: 80
  name: http
  protocol: TCP
- containerPort: 8080
  name: 1990-1992a
  protocol: TCP

We expect to see:

• Inbound ports = 80, 8080.
• Opaque ports = 80,8080 and 2000 - 2050
• Debug log level (inherited from namespace)

:; k get po nginx-85864797b7-9l9hh -oyaml | yq '.spec.containers[] | select(.name == "linkerd-proxy").env'
- name: LINKERD2_PROXY_LOG
  value: debug
- name: LINKERD2_PROXY_LOG_FORMAT
  value: plain
- name: LINKERD2_PROXY_INBOUND_PORTS
  value: 80,8080
- name: LINKERD2_PROXY_INBOUND_PORTS_DISABLE_PROTOCOL_DETECTION
  value: 80,2000-2050,8080