[TEP-0075] Validate against using the whole object in task steps

[chuangw6]

Changes

According to TEP-0075:
When providing values for strings, Task and Pipeline authors can
access individual attributes of an object param; but they cannot access
the object as whole.

This PR makes sure that the object param cannot be used as whole when providing values for strings in task steps.

Also modified extractVariablesFromString function a bit to recognize
params.foo.bar reference that was treated as invalid previously.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Docs included if any changes are user facing
• Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been filled in
  (if there are no user facing changes, use release note "NONE")

Release Notes

Added validation against using the whole object in fields of a task step.
For example, the following reference to the whole object param is not allowed.
steps:
  - name: my-step
     image: $(params.myObjectParam[*])


This is part of TEP-0075. In order to have the changes in this PR to be applied, 
users have to enable the `alpha` feature flag.

[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/substitution/substitution.go	54.5%	43.3%	-11.2

[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	97.6%	97.4%	-0.2
pkg/substitution/substitution.go	54.5%	43.3%	-11.2

[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	97.6%	97.4%	-0.2
pkg/substitution/substitution.go	54.5%	62.9%	8.3

[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	97.6%	97.4%	-0.2
pkg/substitution/substitution.go	54.5%	62.9%	8.3

[ywluogg]

/assign ywluogg

[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	97.7%	97.4%	-0.2
pkg/substitution/substitution.go	54.5%	62.9%	8.3

[chuangw6]

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

[ywluogg]

Just wanted to confirm that this PR will still be focused on substituted field is a string, and the substituting param is an object, is it?

[chuangw6]

Just wanted to confirm that this PR will still be focused on substituted field is a string, and the substituting param is an object, is it?

This PR makes sure object param cannot be used as whole in any part of a task step.
As tep75 proposed, the whole object can only be used when providing value for other objects. That validation will be part of pipeline validation (not in this pr).

[ywluogg]

This PR makes sure object param cannot be used as whole in any part of a task step.
As tep75 proposed, the whole object can only be used when providing value for other objects. That validation will be part of pipeline validation (not in this pr).

Oh I think I meant to ask - if this PR would be focused on validations of whole object substitutions, when the substituted field is a string?

You can close this thread, as I found where you add the validations of substituted param is an array and substituting param is an object. I updated a comment in test cases to help reviewers quickly identify what validations you added more easily.

[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	97.7%	97.5%	-0.2
pkg/substitution/substitution.go	57.9%	63.6%	5.7

[chuangw6]

/test pull-tekton-pipeline-integration-tests

[ywluogg]

/assign @lbernick

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lbernick, ywluogg

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [lbernick]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[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	97.7%	97.5%	-0.2
pkg/substitution/substitution.go	57.9%	64.9%	7.0

[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	97.7%	97.5%	-0.2
pkg/substitution/substitution.go	57.9%	64.9%	7.0

[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	97.7%	97.5%	-0.2
pkg/substitution/substitution.go	57.9%	64.9%	7.0

[ywluogg]

Link this to #4723

[abayer]

This needs a release note.

[abayer]

/lgtm

[chuangw6]

/test pull-tekton-pipeline-integration-tests

[afrittoli]

/kind feature