conn: expose the OK/EOF packet warning count

[morgo]

What

MySQL reports how many warnings a statement raised in the packet that terminates its response. The driver dropped the field on the floor — a literal // warning count [2 bytes] comment in handleOkPacket with no parse behind it, and three copies of the resultset-terminator reader that skipped straight past it to the status flags.

This records the count on the connection, clears it in clearResult(), and exposes it as (*mysqlConn).Warnings() for callers reaching the connection through (*sql.Conn).Raw.

Why

Without the count, a caller that wants to surface warnings has only bad options. The warnings themselves live in per-connection state that only SHOW WARNINGS can read, so reading them costs a round trip; with no count to gate on, that round trip has to be spent after every statement, warnings or not.

The count is what makes it affordable — and for a proxy it is also what has to go into its own OK packet, since clients such as Connector/J only issue SHOW WARNINGS when the count they were handed is non-zero.

Concretely: strata (a Vitess-compatible MySQL proxy) has no backend connection affinity outside transactions, so it must read a statement's warnings eagerly, while it still holds the connection. Gating that on the count keeps the cost at one integer comparison for the overwhelming majority of statements, which raise nothing.

The terminator refactor

The three inlined resultset-terminator readers (textRows.readRow, binaryRows.readRow, skipRows) collapse into one readResultsetTerminator method. They were already byte-for-byte identical; the warning count made keeping them identical harder, because an EOF packet orders warnings before status and an 0xFE-headered OK packet orders them after — so each copy would have had to get that backwards independently.

Validity contract

Documented on the accessor: valid from the moment the statement's response is complete until the next statement starts on that connection. For a resultset that means after the rows are drained or Closed — the terminating packet carrying the count has not arrived before then. An error packet carries no count, so a failed statement leaves the previous one's value in place.

Tests

• TestReadWarnings — little-endian decode and the short-buffer guard.
• TestHandleOkPacketWarnings — OK-packet parse, and that clearResult() resets it so one statement's count cannot be read as the next one's.
• TestHandleOkPacketWarningsWithMoreResults — the count is recorded before the statusMoreResultsExists early return, so each statement of a multi-statement leaves its own behind.
• TestReadResultsetTerminatorWarnings — both packet layouts, pinning the field order that differs between them.
• TestWarnings — integration: clean INSERT (0), DROP TABLE IF EXISTS on a missing table (note 1051, via the OK packet), a truncating CAST (via the resultset terminator), and a clean SELECT (0).

Full suite passes against MySQL 8. TestLoadData fails on my local server only because it has local_infile=OFF; it fails identically on master.

[coveralls]

Coverage Report for CI Build 33120091003

Coverage increased (+0.05%) to 82.659%

Details

• Coverage increased (+0.05%) from the base build.
• Patch coverage: 34 of 34 lines across 3 files are fully covered (100%).
• 4 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

4 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
compress.go	4	78.42%

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Coverage Stats

Relevant Lines:	4175
Covered Lines:	3451
Line Coverage:	82.66%
Coverage Strength:	2569384.31 hits per line

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💛 - Coveralls

[Copilot]

Pull request overview

This PR adds support for tracking and exposing the MySQL protocol warning-count field (from OK / EOF / 0xFE-OK packets) on a per-connection basis, so callers using (*sql.Conn).Raw can cheaply decide whether to issue SHOW WARNINGS.

Changes:

• Add mysqlConn.warnings storage and a (*mysqlConn).Warnings() uint16 accessor, resetting the value at statement start via clearResult().
• Parse and record warning counts from OK packets and from resultset terminator packets, refactoring duplicated terminator parsing into readResultsetTerminator.
• Add unit and integration tests covering little-endian decoding, OK/EOF layouts, multi-statement behavior, and end-to-end warning counts.

Reviewed changes

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

File	Description
warnings.go	Documents and exposes (*mysqlConn).Warnings() for consumers via (*sql.Conn).Raw.
warnings_test.go	Adds unit/integration tests validating warning-count parsing and lifecycle behavior.
packets.go	Implements warning-count parsing, adds readResultsetTerminator, and resets warnings in clearResult().
connection.go	Adds warnings uint16 to mysqlConn state.

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