[eisen0419]

What type of PR is this?

/kind bug
/kind regression

What this PR does / why we need it:

Summary

Since v37.0.0, generated create_* and delete_* methods whose OpenAPI consumes is ["*/*"] serialize a JSON body without setting Content-Type, causing strict HTTP stacks/intermediaries (such as urllib3-future / niquests) to default str bodies to text/plain and fail with 415 Unsupported Media Type.

When RESTClientObject.request JSON-serializes a non-None body and no Content-Type header is present in any casing, default Content-Type to application/json on a shallow copy of headers, while keeping body-less requests unchanged and keeping scripts/rest_client_patch.diff and scripts/client_go_retry_patch.diff synchronized with scripts/update-client.sh.

  RESTClientObject.request(method, url, headers, body, ...)
    method in ['POST', 'PUT', 'PATCH', 'OPTIONS', 'DELETE']
-     content_type = headers.get('Content-Type')
+     content_type_key, content_type = find_header_case_insensitive(headers, 'content-type')
      if is_json or is_structured_yaml:
        if body is not None:
          request_body = json.dumps(body)
+         if content_type_key is None:
+           headers = dict(headers)
+           headers['Content-Type'] = 'application/json'

Evidence

• Before:

  BatchV1Api.create_namespaced_job(...) -> POST body JSON, Content-Type: None (415 under urllib3-future)
  BatchV1Api.delete_namespaced_job(..., body=V1DeleteOptions(...)) -> DELETE body JSON, Content-Type: None
  

• After:

  BatchV1Api.create_namespaced_job(...) -> Content-Type: application/json
  BatchV1Api.delete_namespaced_job(..., body=V1DeleteOptions(...)) -> Content-Type: application/json
  BatchV1Api.delete_namespaced_job(...) [no body] -> Content-Type: None
  pytest kubernetes/utils/rest_test.py kubernetes/utils/keepalive_test.py kubernetes/utils/retry_test.py -> 55 passed
  git apply -R / git apply (update-client.sh 3-patch stack) -> clean, 0 offset, 0 fuzz
  

Merge Danger

Door: two-way

Scoped to RESTClientObject.request and its corresponding regeneration patch stack (scripts/rest_client_patch.diff, scripts/client_go_retry_patch.diff).

Blast Radius: narrow

Only adds Content-Type: application/json when rest.py has already json.dumps-serialized a non-None request body and no Content-Type header (in any casing) was supplied; body-less requests and explicit media types are untouched.

Which issue(s) this PR fixes:

Fixes #2782

Special notes for your reviewer:

scripts/rest_client_patch.diff and the line offset in scripts/client_go_retry_patch.diff were verified via a full reverse/forward git apply round-trip matching scripts/update-client.sh.

Does this PR introduce a user-facing change?

Restore default `Content-Type: application/json` header in the synchronous REST client when sending JSON-serialized request bodies (such as `create_*` and `delete_*` calls with `consumes: ["*/*"]`).

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[kubernetes-prow]

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This pull-request has been approved by: eisen0419
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