[seonghobae]

Summary

Stacked follow-up to #548 at dd5ebcfac3f0163fb083741ae834d4f304cb4797.
Retain its existing authenticated, non-waiting parser lease until actual
synchronous worker completion after request cancellation. No new queue,
capacity setting, dependency, workflow or authentication policy.

Root cause and repair

Starlette runs the endpoint in a task separate from outer middleware.
Cancelling the outer ASGI request released the lease while its parser thread
continued. The actual ASGI regression observed a second 200 instead of 429.

Store the guarded default-executor completion future in shared request state
before yielding. Defer the existing lease release to exactly one completion
callback. Preserve copied context, temporary uploads, repeated cancellation and
orderly task-shutdown behavior. A completed future can conservatively retain
capacity for one additional loop turn.

Verification

• Actual ASGI cancellation + live synchronous worker, both success and failure:
  second request remains 429/Retry-After 1/no-store, no extra worker starts,
  then capacity recovers with 200 after terminal work.
• Repeated direct task cancellation, actual AnyIO cancel scopes and cancellation
  of all remaining event-loop tasks. Old worker-task implementation failed
  shutdown cleanup ordering; guarded executor completion passes.
• Full suite: 536 passed, warnings as errors; required production API branch
  coverage 100% (893 statements, 264 branches).
• New test module Ruff and git diff checks pass. Independent source review
  found no blocking issue; it did not rerun tests.

Acceptance limits

Draft while parent #548 remains CHANGES_REQUESTED and hosted checks/review are
unaccepted. This is a local authenticated ASGI fixture, not deployment or real
OCR acceptance. Capacity remains per application/process, not global across
replicas or shared model servers. Forced process termination is outside the
cleanup guarantee. Separate native OCR migration continues independently.

Optional wider API-plus-tools coverage was 96.15%, solely in unchanged
tools/benchmark_upload_ingestion.py; it is not claimed as passing. Both hosted
repository test gates explicitly require API-only coverage; no gate was changed.

Durable RCA: docs/doctoring/parser-cancellation-work-lifetime-20260928.md.

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