Add blocking sync row driver reusing the async parse body (L3)

[Saurabh Singh (saurabh500)]

Layer 3 (internal): blocking sync row driver + sync PLP collect

Part of the bottom-up sans-I/O stack. Based on L2 (dev/saurabh/sans-io-expose-l2-timeout-cancel, #201), not main.

Introduces, under the existing async client, a blocking synchronous row driver and a sync PLP collect that reuse the ONE parse body (TdsCore::step_row) and the ONE PLP leaf (plp_collect_step) verbatim. Only the refill wrapper differs (blocking vs .await). No new parse machine. No public API change (no TdsSyncClient yet — that is Layer 4). All new items are pub(crate).

What is added

• BlockingByteSource trait + assemble_tds_packet_blocking (byte_source.rs) — the packet framing body minus the await.
• BlockingRowReader trait, ensure_blocking, collect_plp_bytes_blocking, resolve_header_token_blocking, decode_blocking_async_column, and drive_row_over_buffer_blocking (token_stream.rs) — the async row driver loop minus awaits, calling the identical step_row.
• BlockingPacketReader shell (blocking_reader.rs) — buffer-owning reader whose refill blocks on a BlockingByteSource.

Tests (named)

• blocking_driver_matches_async_oracle_across_refill_boundary — mixed int4 + varchar + multi-chunk varbinary(max) PLP; refill boundary swept across every offset; byte-identical vs async oracle.
• blocking_driver_nbcrow_matches_async_oracle_across_refill_boundary — NBCROW, two-byte bitmap split swept.
• blocking_driver_truncation_errors_identically_to_async_oracle — truncation/underflow parity (identical error).
• blocking_driver_multichunk_plp_residency_ceiling — >16384 B varbinary(max) LOB; byte-identical and bounded peak_length() residency (<= 2x max_packet, < payload.len()).

Gates

• Public-API diff on tds_client.rs + cursor_ops.rs vs L2 tip: empty. No new non-pub(crate) pub.
• 7-cert nextest baseline preserved exactly (1718 run, 1711 passed, 7 failed = certificate_validator 4 + win_tls::validate 3).
• cargo bfmt / cargo bclippy / scripts\bfmt.ps1 / scripts\bclippy.ps1 clean.

Draft; do not merge.

[Copilot]

Pull request overview

Adds an internal blocking row-decoding path that reuses the existing synchronous parsing core.

Changes:

• Adds blocking packet assembly and buffered reading.
• Adds blocking row, token, and PLP drivers.
• Adds async/blocking parity and residency tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
mssql-tds/src/io/token_stream.rs	Adds blocking decoding logic and differential tests.
mssql-tds/src/io/byte_source.rs	Adds the blocking byte-source seam and packet assembler.
mssql-tds/src/io/blocking_reader.rs	Implements the blocking packet reader.
mssql-tds/src/io.rs	Registers the new internal module.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.