[coverage] Conformance findings: CUSTOMHDR-002 #972

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

• CUSTOMHDR-002 [sea]: SEA statements client sends Accept-Encoding: gzip on ExecuteStatement instead of the explicit identity default, negotiating HTTP response compression over an already-LZ4-compressed base64 Arrow envelope (double compression: wasted server/client CPU, potentially larger payload on the wire)
  • failing test: test_sea_requests_decline_response_compression_by_default (see the coverage PR diff under tests/)

Reproduce & Expected

CUSTOMHDR-002 — SEA (Statement Execution API) inline results are LZ4-compressed Arrow, base64-encoded and wrapped in a JSON envelope.

Reproduce:

SELECT 1 AS value

Expected (per the shared spec):

• result has exactly 1 row(s)
• col 0, row 0 == 1 (type Int32)
• full assertion contract:

result:
- row_count: 1
- column:
    index: 0
    row: 0
    type: Int32
    equals: 1
protocol:
  sea:
  - request_header:
      operation: ExecuteStatement
      name: Accept-Encoding
      equals: identity

Context

• The behavior was first fixed in a DIFFERENT driver — reference PR: perf: skip gzip for SEA responses by default databricks-jdbc#1723 — 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/1765