Add experimental TDS performance baseline

[Saurabh Singh (saurabh500)]

Description

Creates an experimental, independently reviewable performance baseline on current main by duplicating the PR-level changes from #291 and the updated #299.

• forwards ResultSet row reads through native futures and keeps row-fetch futures within the size budget
• expands synchronous buffered probes for fixed-width scalar decoding, falling back to async reads only when data is unavailable
• preserves current main behavior unrelated to the source changes

This is a separate performance exploration. It does not replace, close, retarget, or modify #291 or #299, and it is not part of stack #287.

Standalone adaptation: #291's generic RowWriter dispatch depends on AnyTransport from its separate native-stack prerequisite. Current main intentionally stores Box<dyn TdsTransport>, so this layer retains the boxed transport and dynamic writer boundary while preserving #291's native ResultSet future forwarding, future-size checks, and explicit error cleanup.

Related Issues

Source context only (does not close either PR):

• Remove ResultSet async_trait boxing #291
• Improve decoder throughput with buffered scalar reads #299

Checklist

• cargo bfmt passes
• cargo bclippy passes
• cargo btest passes (1666/1670 mssql-tds library tests pass; four unrelated certificate tests cannot find the repository's absent tests/test_certificates/valid_cert.{pem,der} fixtures)
• New/changed functionality has tests
• Public API changes are documented

[github-actions]

📊 Code Coverage Report

🔥 Diff Coverage 98%

🎯 Overall Coverage 91.6%

📦 Project: mssql-tds + mssql-odbc + mssql-py-core ℹ️ Note: diff coverage is reported, not enforced.

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Diff Coverage

Diff: main...HEAD, staged and unstaged changes

• mssql-tds/src/connection/tds_client.rs (94.9%): Missing lines 3631-3633,3666
• mssql-tds/src/connection/transport/buffers.rs (100%)
• mssql-tds/src/connection/transport/network_transport.rs (100%)
• mssql-tds/src/datatypes/decoder.rs (97.2%): Missing lines 866,1867,1873
• mssql-tds/src/io/packet_reader.rs (100%)

Summary

• Total: 441 lines
• Missing: 7 lines
• Coverage: 98%

mssql-tds/src/connection/tds_client.rs

  3627 
  3628         let metadata = Arc::clone(self.current_metadata.as_ref().unwrap());
  3629         let decryptor = match self.resolve_cell_decryptor(&metadata).await {
  3630             Ok(decryptor) => decryptor,
! 3631             Err(error) => {
! 3632                 self.abort_pending_prepare_capture();
! 3633                 return Err(error);
  3634             }
  3635         };
  3636         let parser_context = ParserContext::ColumnMetadata(metadata, decryptor);
  3637         loop {

---

  3662                     return Ok(true);
  3663                 }
  3664                 RowReadResult::RowPaused(_) | RowReadResult::PlpPaused(_) => {
  3665                     // DecodeAll never pauses; a pause here is a protocol/logic error.
! 3666                     self.abort_pending_prepare_capture();
  3667                     return Err(crate::error::Error::ProtocolError(
  3668                         "Unexpected pause while decoding a full row (ColumnPolicy::DecodeAll)"
  3669                             .to_string(),
  3670                     ));

---

mssql-tds/src/datatypes/decoder.rs

  862         let value: ColumnValues = match byte_len {
  863             1 => ColumnValues::TinyInt(read_sync_first!(reader, try_read_byte, read_byte)),
  864             2 => ColumnValues::SmallInt(read_sync_first!(reader, try_read_int16, read_int16)),
  865             4 => ColumnValues::Int(read_sync_first!(reader, try_read_int32, read_int32)),
! 866             8 => ColumnValues::BigInt(read_sync_first!(reader, try_read_int64, read_int64)),
  867             0 => ColumnValues::Null,
  868             _ => {
  869                 return Err(crate::error::Error::from(Error::new(
  870                     std::io::ErrorKind::InvalidData,

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  1863             // Allocates buffer and reads data
  1864             // Creates SqlString with appropriate encoding type
  1865             // NULL handling works (textptr_len = 0)
  1866             // LCID-based decoding implemented (see sql_string.rs)
! 1867             let text_ptr_len = read_sync_first!(reader, try_read_byte, read_byte) as usize;
  1868 
  1869             let length = if text_ptr_len > 0 {
  1870                 const TIMESTAMP_BYTE_COUNT: usize = 8;
  1871                 reader.skip_bytes(text_ptr_len).await?;

---

  1869             let length = if text_ptr_len > 0 {
  1870                 const TIMESTAMP_BYTE_COUNT: usize = 8;
  1871                 reader.skip_bytes(text_ptr_len).await?;
  1872                 reader.skip_bytes(TIMESTAMP_BYTE_COUNT).await?;
! 1873                 read_sync_first!(reader, try_read_uint32, read_uint32) as usize
  1874             } else {
  1875                 // text_ptr_len == 0 means NULL value
  1876                 return Ok(ColumnValues::Null);
  1877             };

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