add default update stanza and max_parallel=0 disables deployments

[jazzyfresh]

Overview

This adds a default update stanza and makes max_parallel=0 a sentinel value that disables deployments.

• Jobs that do not have an update stanza present in the jobspec will default to a deployment of rolling updates with max_parallel=1
  • Previous behavior for jobs w/o an update stanza was forced updates
  • Jobs that have an update stanza (even an empty one) are not affected
• Jobs with max_parallel=0 in their update stanza will disable deployments (rolling and canary) explicitly, and make the job do forced updates

Behavior

Default update stanza

Service jobs have deployments when first submitted

Service jobs have rolling deployments when you submit a new version of the job

Batch jobs have no deployments and still perform destructive updates

---

max_parallel=0 disables deployments (i.e. the job will have destructive updates)

Implementation

• we set the default update strategy for a Job, which will get merged into the TaskGroup update strategy at api.Canonicalization in the command/agent
• the place to disable deployments for a job is when the alloc reconciler computes the deployment for the task group

Todo

• default update stanza
  • update validation
  • update docs with a note that max_parallel=0 is a sentinel value to indicate forced updates
  • update tests
  • set the default update strategy at Job canonicalization
    • double check w/ team: canonicalization should happen server side in structs#(*Job)Canonicalize?
      • no it should happen in the api#(*Job)Canonicalize which is called from the command/agent, that means we would set the defaults before raft
      • putting it in the structs#Canonicalize would result in surprising behavior, since we expect structs to be not nil
  • test that confirms it's only for service jobs
• max_parallel=0 should disable deployment patterns
  • confirm forced updates
  • confirm no deployments are present
• tests for max_parallel
• (separate PR) update docs: after an upgrade, a job update (not just job create) will result in an update stanza being added
• (separate PR) add e2e tests

[cgbaker]

LGTM (with minor error message change) 🎉

[endocrimes]

LGTM

[github-actions]

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