[CommonwealthPasta]

Title: fix(tds): read past the cancelled response for the Attention acknowledgement

Disclosure

Full transparency, this changeset was authored by Claude Opus 5.5. I instructed it to prove the bug existed before bothering to patch it. I am a professional software developer by trade and consume this library through the TDSQL library. I would loved to have authored this fix myself, but I have too many other projects to work on. I apologize! Feel free to reject as necessary.

Summary

After sending an Attention, cancel_query read only to the end of the current TDS message. SQL Server can send the acknowledgement (a DONE token with DONE_ATTN) as a message of its own, after the cancelled response (MS-TDS 3.2.5.9, 4.19.2). cancel_query then failed and left the acknowledgement unread, so the connection desynced:

cancel_query: Err(Protocol("Never got a DONE token acknowledging the Attention signal."))
SELECT 42 -> []
SELECT 43 -> [Some(42)]
SELECT 44 -> [Some(43)]

cancel_request now drains message by message until the acknowledgement arrives, discarding the cancelled response, including any server error in it.

Related issues

None.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing public API)
• Documentation / tooling / CI only
• Refactor (no functional change)

Checklist

• This PR targets the dev branch (feature work lands on dev, is promoted to qa, then main).
• cargo fmt --all has been run and the code is formatted.
• cargo clippy --all-targets --all-features -- -D warnings passes with no warnings. (--all-features doesn't compile on main because the TLS backends are mutually exclusive. --all-targets --features=all is clean.)
• cargo test passes. Integration tests against a live SQL Server (via TIBERIUS_TEST_CONNECTION_STRING / docker compose) were run if the change affects runtime behaviour.
• Supply-chain check passes: cargo deny check is green (advisories, licenses, bans, sources).
• CHANGELOG.md has been updated with a user-facing entry (unless this change is not user-visible).
• Public API changes are documented (rustdoc), and new features are gated behind the appropriate Cargo feature flag where applicable.
• Commits are signed off / attributed to me, and I agree to license my contribution under the project's terms (MIT OR Apache-2.0).

Testing

• New tests/cancel.rs, run on tokio and smol:
  • cancel_mid_stream cancels a batch mid-SELECT, then checks that the rest of the batch never ran and that the next query reads its own result.
  • cancel_after_a_server_error checks the next query's result after cancelling a response that contains a server error.
  • Both fail without the fix (10/10 runs). The second also fails with only the Err(crate::Error::Server(_)) arm removed.
• CI's integration matrix, run on Linux with Rust 1.99.1 and RUSTFLAGS=-Dwarnings:
  • Servers: SQL Server 2017, 2019, 2022, 2025 and azure-sql-edge, each built from docker/.
  • Feature sets: cargo test with each of --features=all, --no-default-features, and --no-default-features plus chrono, time, rustls or vendored-openssl.
  • All 30 runs pass. Without the fix, both cancel tests fail on every server.
• Also passes on Windows with default features against SQL Server 2022.
• cargo fmt --all --check, cargo clippy --all-targets --features=all -- -D warnings and cargo deny check advisories bans sources licenses are all clean.

Additional notes

• A server that never acknowledges now makes cancel_query wait indefinitely. Before, it errored at the end of the first message. The server is required to acknowledge (MS-TDS 3.3.5.9).
• The touched doc comment's Attention reference is corrected from section 2.2.1.6 (Remote Procedure Call) to 2.2.1.7. The same mistake remains in connection.rs, client.rs and header.rs, left for a separate change.