Feature: Support NotDuring constraints for JobConfig

[irvinlim]

Users may want to have a periodically scheduled job that runs regularly on interval, except for some explicitly defined time ranges. For example, we may want to specify that during deploy freezes or some other real-world event, a cron job should not be run.

Users should be able to specify a list of time ranges (start and end time, inclusive), during which schedules are not allowed.

API Design

apiVersion: execution.furiko.io/v1alpha1
kind: JobConfig
metadata:
  name: my-job-config
  namespace: my-namespace
spec:
  schedule:
    # Schedule every hour from 10AM to 6PM.
    cron:
      expression: 0 10-18 * * *
      timezone: Asia/Singapore
    constraints:
      # Example of multiple notDuring freeze periods, inclusive.
      notDuring:
        - start: 2022-04-01T00:00:00+08:00
          end: 2022-04-02T11:59:59+08:00
        - start: 2022-04-28T00:00:00+08:00
          end: 2022-04-28T15:59:59+08:00

Using the above example:

• On 1st April, no schedules will be created at all.
• On 2nd April, the first schedule that day will be at 12:00:00.
• On 28th April, the first schedule that day will be at 16:00:00.

Possible Extensions

• Sharing constraints across multiple JobConfigs
• Regular constraints instead of fixed time range: Using a cron expression to define exclusions to the main cron schedule, similar to GitLab