add placement spread assertions

[jaypipes]

The assert.placement field of a gdt-kube test Spec allows a test author to specify the expected scheduling outcome for a set of Pods returned by the Kubernetes API server from the result of a kube.get call.

Suppose you have a Deployment resource with a TopologySpreadConstraints that specifies the Pods in the Deployment must land on different hosts:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
       - name: nginx
         image: nginx:latest
         ports:
          - containerPort: 80
      topologySpreadConstraints:
       - maxSkew: 1
         topologyKey: kubernetes.io/hostname
         whenUnsatisfiable: DoNotSchedule
         labelSelector:
           matchLabels:
             app: nginx

You can create a gdt-kube test case that verifies that your nginx Deployment's Pods are evenly spread across all available hosts:

tests:
 - kube:
     get: deployments/nginx
   assert:
     placement:
       spread: kubernetes.io/hostname

If there are more hosts than the spec.replicas in the Deployment, gdt-kube will ensure that each Pod landed on a unique host. If there are fewer hosts than the spec.replicas in the Deployment, gdt-kube will ensure that there is an even spread of Pods to hosts, with any host having no more than one more Pod than any other.

Debug/trace output includes information on how the placement spread looked like to the gdt-kube placement spread asserter:

jaypipes@lappie:~/src/github.com/gdt-dev/kube$ go test -v -run TestPlacementSpread ./eval_test.go
=== RUN   TestPlacementSpread
=== RUN   TestPlacementSpread/placement-spread
[gdt] [placement-spread] kube: create [ns: default]
[gdt] [placement-spread] create-deployment (try 1 after 1.254µs) ok: true
[gdt] [placement-spread] using timeout of 40s (expected: false)
[gdt] [placement-spread] kube: get [ns: default]
[gdt] [placement-spread] deployment-ready (try 1 after 2.482µs) ok: false
[gdt] [placement-spread] deployment-ready (try 1 after 2.482µs) failure: assertion failed: match field not equal: $.status.readyReplicas not present in subject
[gdt] [placement-spread] kube: get [ns: default]
[gdt] [placement-spread] deployment-ready (try 2 after 307.618472ms) ok: false
[gdt] [placement-spread] deployment-ready (try 2 after 307.618472ms) failure: assertion failed: match field not equal: $.status.readyReplicas not present in subject
[gdt] [placement-spread] kube: get [ns: default]
[gdt] [placement-spread] deployment-ready (try 3 after 1.245091704s) ok: false
[gdt] [placement-spread] deployment-ready (try 3 after 1.245091704s) failure: assertion failed: match field not equal: $.status.readyReplicas not present in subject
[gdt] [placement-spread] kube: get [ns: default]
[gdt] [placement-spread] deployment-ready (try 4 after 2.496969168s) ok: false
[gdt] [placement-spread] deployment-ready (try 4 after 2.496969168s) failure: assertion failed: match field not equal: $.status.readyReplicas had different values. expected 6 but found 3
[gdt] [placement-spread] kube: get [ns: default]
[gdt] [placement-spread] deployment-ready (try 5 after 3.785007183s) ok: true
[gdt] [placement-spread] kube: get [ns: default]
[gdt] [placement-spread/assert-placement-spread] domain: topology.kubernetes.io/zone, unique nodes: 3
[gdt] [placement-spread/assert-placement-spread] domain: topology.kubernetes.io/zone, pods per node: [2 2 2]
[gdt] [placement-spread] deployment-spread-evenly-across-hosts (try 1 after 3.369µs) ok: true
[gdt] [placement-spread] kube: delete [ns: default]
[gdt] [placement-spread] delete-deployment (try 1 after 1.185µs) ok: true

--- PASS: TestPlacementSpread (4.98s)
    --- PASS: TestPlacementSpread/placement-spread (4.96s)
PASS
ok  	command-line-arguments	4.993s

Issue #7