support parameterization for steps[].onError

[pritidesai]

Changes

In addition to the static values supported (continue and stopAndFail) for onError, allow specifying these valid values as a task parameter for example, onError: $(params.CONTINUE). Also, validate the specified value is supported otherwise return an error for an invalid value.

Submitter Checklist

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• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
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  functionality, content, code)
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Release Notes

Support variables in steps[].onError, for example, $(params.CONTINUE)

/kind feature

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1beta1/task_validation.go	97.3%	97.3%	-0.0
pkg/container/step_replacements.go	66.7%	70.0%	3.3
pkg/pod/entrypoint.go	89.3%	89.5%	0.2

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1beta1/task_validation.go	97.3%	97.3%	-0.0
pkg/container/step_replacements.go	66.7%	70.0%	3.3
pkg/pod/entrypoint.go	89.3%	89.5%	0.2

[afrittoli]

Thank you for this!
It's a nice feature to have, it allows running a task/pipeline in different modes depending on the environment.
The only NIT I have is that it would still be possible to have early validation of the values when no variables are used, but I'll leave it up to you if you want to add it back.

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: afrittoli

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[afrittoli]

/retest

[afrittoli]

The plumbing change that broke the build test has been reverted now, but there is still one issue left:

=========================================
==== TESTING IF GOLINT HAS BEEN DONE ====
=========================================
pkg/pod/entrypoint.go:142:19: errorf: should replace errors.New(fmt.Sprintf(...)) with fmt.Errorf(...) (revive)
						return nil, errors.New(fmt.Sprintf("task step onError must be either %s or %s but it is set to an invalid value %s",
							entrypoint.ContinueOnError, entrypoint.FailOnError, taskSpec.Steps[i].OnError))

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1beta1/task_validation.go	97.3%	97.3%	0.0
pkg/container/step_replacements.go	66.7%	70.0%	3.3
pkg/pod/entrypoint.go	89.3%	89.5%	0.2

[pritidesai]

/test pull-tekton-pipeline-alpha-integration-tests

[abayer]

/lgtm