Inconsistent validation for the image field for Sidecar versus Step

[JeromeJu]

Expected Behavior

Without specific use cases(i.e. a ref is used in Step for referencing StepAction), users shall assume that Sidecars and Steps undergo identical validation processes.

Actual Behavior

For Step validation:

Error from server (BadRequest): error when creating "sidecar-step-validation.yaml": admission webhook "validation.webhook.pipeline.tekton.dev" denied the request: validation failed: missing field(s): spec.taskSpec.steps[0].Image

While for Sidecar it can only be observed via taskRun status:

tkn tr logs
? Select taskrun: sidecar-ready-x8nnj started 34 seconds ago
task Task 0 has failed: failed to create task run pod "sidecar-step-validaiton-x8nnj": Pod "sidecar-ready-x8nnj-pod" is invalid: spec.containers[1].image: Required value. Maybe invalid TaskSpec
Error: pod for taskrun sidecar-ready-x8nnj not available yet

There are two questions:

• Image field for both Step and Sidecar are marked as optional, but Step Image is being validated at wehook while for Sidecar the error messages were unclear at runtime. Shall we mark them both as non-optional?
• Why does the static validation at admission webhook for Sidecar differentiate from that for Step, which made Sidecar validation error observable at runtime while Step validation failing admission webhook?

Steps to Reproduce the Problem

Apply the following yaml with the image field of sidecar and step commented out one at a time.

apiVersion: tekton.dev/v1
kind: TaskRun
metadata:
  generateName: sidecar-step-validation-
spec:
  taskSpec:
    sidecars:
    - name: slow-sidecar
       # image: ubuntu
      script: |
        echo "hello from sidecar" > /shared/message
      volumeMounts:
      - name: shared
        mountPath: /shared
    steps:
    - name: check-ready
      # image: ubuntu
      script: cat /shared/message
      volumeMounts:
      - name: shared
        mountPath: /shared
    # Sidecars don't have /workspace mounted by default, so we have to define
    # our own shared volume.
    volumes:
    - name: shared
      emptyDir: {}

Additional Info

• Tekton Pipeline version:
  devel
  Client version: 0.33.0
  Pipeline version: v0.54.0