kubernetes: kubectl apply -o yaml does not output proper yaml when changing multiple objects

Is this a BUG REPORT or FEATURE REQUEST?: Bug report

/kind bug

What happened: Given the following file:

kind: ConfigMap
apiVersion: v1
metadata:
  name: test
data:
  key1: apple
---
kind: ConfigMap
apiVersion: v1
metadata:
  name: test2
data:
  key2: apple

returns

apiVersion: v1
data:
  key1: apples
kind: ConfigMap
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","data":{"key1":"apples"},"kind":"ConfigMap","metadata":{"annotations":{},"name":"test","namespace":"some-namespace"}}
  creationTimestamp: 2018-01-25T22:43:35Z
  name: test
  namespace: somenamespace
  resourceVersion: "12144"
  selfLink: /api/v1/namespaces/some-namespace/configmaps/test
  uid: 2dc1c468-0221-11e8-bce4-8ece3fb0c49d
apiVersion: v1
data:
  key2: apples
kind: ConfigMap
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","data":{"key2":"apples"},"kind":"ConfigMap","metadata":{"annotations":{},"name":"test2","namespace":"some-namespace"}}
  creationTimestamp: 2018-01-25T22:43:35Z
  name: test2
  namespace: some-namespace
  resourceVersion: "12145"
  selfLink: /api/v1/namespaces/some-namespace/configmaps/test2
  uid: 2dc2d982-0221-11e8-bce4-8ece3fb0c49d

which is not valid yaml (needs a document separator) What you expected to happen: For the output to be a List object containing the 2 ConfigMaps

How to reproduce it (as minimally and precisely as possible): Described above

Anything else we need to know?: The -o json flag returns something similar, where it’s just JSON bodies concatenated one after another. Not sure if that’s considered valid json.

Environment:

• Kubernetes version (use kubectl version): 1.8.6 client/server
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release): N/A
• Kernel (e.g. uname -a): 17.3.0 Darwin Kernel Version 17.3.0: Thu Nov 9 18:09:22 PST 2017; root:xnu-4570.31.3~1/RELEASE_X86_64 x86_64
• Install tools:
• Others: