[coverage] Conformance findings: CLOUDFETCH-018

Beginne mit dem xfail-Test test_link_fetch_failure_surfaces_terminal_error im Coverage-PR-Diff unter tests/ und vergleiche dann das beabsichtigte Verhalten mit Referenz-PR #322. Führe das Protokollszenario mit einem Ergebnis in mehreren Chunks und einem persistenten Thrift-Fehler bei FetchResults aus. Fertig ist die Aufgabe, wenn der erste Drain innerhalb von 180 Sekunden eine Exception auslöst und ein wiederholter Drain umgehend mit einem Fehler oder dem Ende des Datenstroms zurückkehrt.

Summary

Surfaced by the multi-language coverage fan-out while conformance-testing these SPEC-IDs against databricks/databricks-sql-python. Each finding is committed as an expected-failure (xfail) test in the coverage PR — the test asserts the CORRECT (post-fix) behavior and stays red until THIS driver (databricks/databricks-sql-python) is fixed, then flips green as a tripwire.

Findings

• CLOUDFETCH-018 [thrift]: A permanently-failing CloudFetch link fetch (Thrift FetchResults) never surfaces a terminal error: the drain hangs indefinitely (>180s) instead of failing, stranding the application thread. The link fetch fails before any per-chunk state exists, so error handling keyed on per-chunk download/processing state never records the failure or wakes waiters (same class as kernel #319 / audit finding H03).
  • failing test: test_link_fetch_failure_surfaces_terminal_error (see the coverage PR diff under tests/)

Reproduce & Expected

CLOUDFETCH-018 — A permanently-failing CloudFetch LINK FETCH must surface a TERMINAL error to the consumer, and must do so in bounded time.

Reproduce:

• Enable this run's protocol scenario (see protocol_scenarios above) so the
  link-fetch call fails persistently. The result must be large enough to span
  MULTIPLE chunks, so the driver has to ask for links it does not already hold --
  otherwise every link arrives inline, no link fetch is issued, and the injected
  failure never fires (the protocol guard below catches that vacuous case).
• Drain a multi-chunk CloudFetch result whose link fetch fails permanently.
  Expect the drain to raise -- not to block, and not to report a clean
  end-of-stream over a truncated row set.
• Fetch again on the SAME result after the terminal error. Must return promptly
  (end-of-stream or the same error), not hang.

Expected (per the shared spec):

• full assertion contract:

result:
- label: first_drain
  exception_thrown: true
- label: first_drain
  elapsed_seconds_range:
    max: 180
- label: repeat_drain
  either_condition_true:
  - row_count: 0
  - exception_thrown: true
- label: repeat_drain
  elapsed_seconds_range:
    max: 30
protocol:
  thrift:
  - label: first_drain
    call_min:
      method: FetchResults
      min: 1
  sea:
  - label: first_drain
    call_min:
      operation: FetchResultChunk
      min: 1

Context

• The behavior was first fixed in a DIFFERENT driver — reference PR: https://github.com/databricks/databricks-sql-kernel/pull/322 — which seeded the shared language-neutral spec. This issue tracks the same conformance gap in databricks/databricks-sql-python; the reference PR is for cross-referencing the intended behavior, NOT a change to this repo.
• Coverage PR carrying the reproducing xfail test(s): https://github.com/databricks/databricks-driver-test/pull/1571