feat: make PodTemplateSpec more robust and less error prone

[pmalek]

What this PR does / why we need it:

This PR makes strategic merge patch more robust and less error prone by preventing several problematic use cases:

• users needing to restate the base manifest (generated by KGO) which is not known to them before hand.
  With this PR, this won't be necessary anymore as. Users won't need to provide elements form base manifests.

  So this:

  apiVersion: gateway-operator.konghq.com/v1beta1
  kind: DataPlane
  metadata:
    name: konnect-example
  spec:
    deployment:
      podTemplateSpec:
        spec:
          containers:
          - name: proxy
            image: kong/kong-gateway:3.6
            env:
              - name: KONG_ROLE
                value: data_plane
              ...
            volumeMounts:
            # We need to specify the cluster-certificate volume-mount because otherwise
            # strategic merge patch would merge its entry with provided
            # konnect-client-tls volume mount.
            - name: cluster-certificate
              mountPath: /var/cluster-certificate
            - name: konnect-client-tls
              mountPath: /etc/secrets/kong-cluster-cert/
              readOnly: true
          volumes:
          - name: cluster-certificate
          - name: konnect-client-tls
            secret:
              secretName: konnect-client-tls
  

  becomes:

  apiVersion: gateway-operator.konghq.com/v1beta1
  kind: DataPlane
  metadata:
    name: konnect-example
  spec:
    deployment:
      podTemplateSpec:
        spec:
          containers:
          - name: proxy
            image: kong/kong-gateway:3.6
            env:
              - name: KONG_ROLE
                value: data_plane
              ...
            volumeMounts:
            - name: konnect-client-tls
              mountPath: /etc/secrets/kong-cluster-cert/
              readOnly: true
          volumes:
          - name: konnect-client-tls
            secret:
              secretName: konnect-client-tls
  

• using Go API users could experience merging mutually exclusive fields e.g. when setting env vars:

  gateway-operator/pkg/utils/kubernetes/resources/strategicmerge_test.go

  Lines 752 to 807 in 3876430

  	{
  	Name: "add envs with fieldRef",
  	Patch: &corev1.PodTemplateSpec{
  	Spec: corev1.PodSpec{
  	Containers: []corev1.Container{
  	{
  	Name: consts.ControlPlaneControllerContainerName,
  	Env: []corev1.EnvVar{
  	{
  	Name: "LIMIT",
  	ValueFrom: &corev1.EnvVarSource{
  	ResourceFieldRef: &corev1.ResourceFieldSelector{
  	Resource: "limits.cpu",
  	},
  	},
  	},
  	},
  	},
  	},
  	},
  	},
  	Expected: func() corev1.PodTemplateSpec {
  	d, err := makeControlPlaneDeployment()
  	require.NoError(t, err)
  	d.Spec.Template.Spec.Containers[0].Env = []corev1.EnvVar{
  	{
  	Name: "LIMIT",
  	// NOTE: this is an artifact of the strategic merge patch at work.
  	// This values comes from the first entry in the base patch.
  	// In order to overcome this we need to re-state the base values
  	// in the slice.
  	Value: "VALUE1",
  	ValueFrom: &corev1.EnvVarSource{
  	ResourceFieldRef: &corev1.ResourceFieldSelector{
  	Resource: "limits.cpu",
  	Divisor: *resource.NewQuantity(1, resource.DecimalSI),
  	},
  	},
  	},
  	{
  	Name: "ENV1",
  	Value: "VALUE1",
  	},
  	{
  	Name: "ENV2",
  	Value: "VALUE2",
  	},
  	{
  	Name: "ENV3",
  	Value: "VALUE3",
  	},
  	}
  	return d.Spec.Template
  	},
  	},

  
  This PR fixes this as well. Users are not required anymore to restate the elements from the base manifest array as in e.g.

  gateway-operator/pkg/utils/kubernetes/resources/strategicmerge_test.go

  Lines 808 to 870 in 3876430

  	{
  	Name: "add envs with fieldRef re-stating the base",
  	Patch: &corev1.PodTemplateSpec{
  	Spec: corev1.PodSpec{
  	Containers: []corev1.Container{
  	{
  	Name: consts.ControlPlaneControllerContainerName,
  	Env: []corev1.EnvVar{
  	{
  	Name: "ENV1",
  	Value: "VALUE1",
  	},
  	{
  	Name: "ENV2",
  	Value: "VALUE2",
  	},
  	{
  	Name: "ENV3",
  	Value: "VALUE3",
  	},
  	// We're moving the `LIMIT` entry from the 1st to the 4th place in the slice to make sure it will not end up with both `Value` and `ValueFrom` fields.
  	{
  	Name: "LIMIT",
  	ValueFrom: &corev1.EnvVarSource{
  	ResourceFieldRef: &corev1.ResourceFieldSelector{
  	Resource: "limits.cpu",
  	},
  	},
  	},
  	},
  	},
  	},
  	},
  	},
  	Expected: func() corev1.PodTemplateSpec {
  	d, err := makeControlPlaneDeployment()
  	require.NoError(t, err)
  	d.Spec.Template.Spec.Containers[0].Env = []corev1.EnvVar{
  	{
  	Name: "ENV1",
  	Value: "VALUE1",
  	},
  	{
  	Name: "ENV2",
  	Value: "VALUE2",
  	},
  	{
  	Name: "ENV3",
  	Value: "VALUE3",
  	},
  	{
  	Name: "LIMIT",
  	ValueFrom: &corev1.EnvVarSource{
  	ResourceFieldRef: &corev1.ResourceFieldSelector{
  	Resource: "limits.cpu",
  	Divisor: *resource.NewQuantity(1, resource.DecimalSI),
  	},
  	},
  	},
  	}
  	return d.Spec.Template
  	},

• this PR additionally catches previously uncaught errors like setting not fully spec's readinessProbe on a sidecar:

  gateway-operator/config/samples/dataplane-sidecar.yaml

  Lines 44 to 46 in 36c58ab

  	readinessProbe:
  	initialDelaySeconds: 1
  	periodSeconds: 1

  .
  This now properly yields an error and is not silenced:

  2024-05-10T15:00:17.681+0200 - ERROR - Reconciler error - {"controller": "dataplane", "controllerGroup": "gateway-operator.konghq.com", "controllerKind": "DataPlane", "DataPlane": {"name":"sidecar-example","namespace":"default"}, "namespace": "default", "name": "sidecar-example", "reconcileID": "ccf8917a-c101-479e-8b2b-c449fd51e743", "error": "could not build Deployment for DataPlane default/sidecar-example: failed creating Deployment for DataPlane sidecar-example: Deployment.apps \"dataplane-sidecar-example-4mxqs\" is invalid: spec.template.spec.containers[0].readinessProbe: Required value: must specify a handler type"}
  

Which issue this PR fixes

Related #128

Special notes for your reviewer:

Prior work: https://github.com/Kong/gateway-operator-archive/pull/1489/

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect significant changes