fix: prevent data race in tdsBuffer from concurrent processSingleResponse goroutines

[dlevy-msft-sql]

Summary

Prevents the data race in tdsBuffer caused by concurrent processSingleResponse goroutines reading from the same session buffer.

Problem

When an INSERT...OUTPUT or similar query produces results, startReading() spawns a processSingleResponse goroutine that reads from sess.buf. If the caller doesn't fully drain the rows before executing the next statement, startReading() spawns a second goroutine that reads from the same sess.buf concurrently. Both goroutines write to tdsBuffer.rpos, tdsBuffer.rsize, etc. without synchronization, causing a data race detected by go test -race.

Fix

Add a readDone chan struct{} field to tdsSession. Each processSingleResponse goroutine closes its readDone channel on exit (via defer close(readDone)). The next startReading() call waits on the previous readDone channel before spawning a new goroutine.

The same gate is applied to the cancellation retry path in nextToken() where a second processSingleResponse goroutine is spawned to read the cancellation confirmation.

Why this is safe

• readDone is nil on the first call, so no blocking occurs
• Each subsequent call blocks until the previous goroutine exits
• In normal operation (rows fully consumed before next query), the channel is already closed and the receive returns immediately with zero added latency
• startReading is only called from the connection's serial execution path, so no mutex is needed
• The goroutine wrapper uses defer close(readDone) to guarantee the channel is always closed, even on panic recovery

Risk

Low. The wait only adds latency in the exact scenario that causes the data race today.

Fixes #171

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.60%. Comparing base (65e137f) to head (6ecb7e4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #357       +/-   ##
===========================================
+ Coverage   80.62%   96.60%   +15.98%     
===========================================
  Files          35       92       +57     
  Lines        6910    74351    +67441     
===========================================
+ Hits         5571    71829    +66258     
- Misses       1068     2187     +1119     
- Partials      271      335       +64     

Flag	Coverage Δ
unittests	96.53% <ø> (?)

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[Copilot]

Pull request overview

This PR addresses a go test -race data race in the TDS response-reading path by ensuring only one processSingleResponse goroutine can read from a session’s shared tdsBuffer at a time (fixes #171).

Changes:

• Add readDone chan struct{} to tdsSession to track completion of the active response-reader goroutine.
• Introduce (*tdsSession).startResponseReader(...) to wait for the previous reader to exit before starting the next one, and use it from both startReading() and the cancellation retry path in nextToken().
• Add a unit test intended to validate the new serialization behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tds.go	Adds tdsSession.readDone used as a completion gate for response readers.
token.go	Centralizes spawning of processSingleResponse via startResponseReader, serializing readers to prevent concurrent buffer reads.
token_test.go	Adds a test for response-reader serialization behavior.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

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[Copilot]

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Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

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[Copilot]

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[Copilot]

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