output.yml

resources:
- name: my-resource
jobs:
- name: my-job
  plan:
  - get: my-resource
  serial: true
- name: my-job
  serial: false
  plan:
  - get: my-resource
- name: my-job
  serial_groups:
  - deploy
  plan:
  - get: my-resource
---
resources:
- name: my-resource
jobs:
- name: my-job
  serial: true
  plan:
  - get: my-resource
- serial: true # <<< This was unintended
---
resources:
- name: my-resource
jobs:
- name: my-job
  serial: false
  plan:
  - get: my-resource
- serial: true # <<< This was unintended

serial_overlay.yml

#@ load("@ytt:overlay", "overlay")

#@overlay/match by=overlay.all, expects="0+"
---
jobs:
#@overlay/match by=lambda indexOrKey, left, right: "serial_groups" not in left and "serial" not in left, expects="0+"
#@overlay/match-child-defaults missing_ok=True
- serial: true


#! This document works
---
resources:
- name: my-resource
  
jobs:
#! Jobs with neither serial nor serial_groups should get serial: true added
- name: my-job
  plan:
  - get: my-resource

#! Jobs with serial should be unchanged
- name: my-job
  serial: false
  plan:
  - get: my-resource

#! Jobs with serial_groups should be unchanged
- name: my-job
  serial_groups: ["deploy"]
  plan:
  - get: my-resource

#! This document gets a new blank job added?
---
resources:
- name: my-resource
  
jobs:
- name: my-job
  serial: true
  plan:
  - get: my-resource

#! This document gets a new blank job added?
---
resources:
- name: my-resource
  
jobs:
- name: my-job
  serial: false
  plan:
  - get: my-resource