mysql: auto-reprepare prepared statements on ER_NEED_REPREPARE to keep scans correct after DDL

[ljluestc]

Title

mysql: auto-reprepare prepared statements on ER_NEED_REPREPARE to keep scans correct after DDL

Summary

This PR makes prepared statements resilient to schema changes by automatically re-preparing and retrying once when the server returns ER_NEED_REPREPARE (1615). It fixes incorrect scan results after DDL (e.g., changing a column type) without requiring an app restart. Adds integration tests reproducing and verifying the behavior.

Fixes #563

Background

Some servers invalidate prepared-statement metadata after DDL (e.g., ALTER TABLE ... MODIFY column TYPE). Applications may then see wrong values (e.g., zero timestamps) or receive ER_NEED_REPREPARE until they restart. The driver previously didn’t auto-reprepare.

Changes

• Store original SQL on statements:
  • Add queryString string to mysqlStmt, set in mysqlConn.Prepare.
• Auto-reprepare on schema changes:
  • Add mysqlStmt.reprepare() to close the current stmt ID and prepare again (same connection, same SQL).
  • Update mysqlStmt.Exec and mysqlStmt.Query to retry the operation once when readResultSetHeaderPacket() returns ER_NEED_REPREPARE (1615).
• Tests:
  • TestPreparedStmtReprepareAfterDDL: verifies scans remain correct across a type change with parseTime=true.
  • TestPreparedStmtExecReprepareAfterDDL: verifies Exec continues to work after a type change.
  • TestPreparedStmtReprepareMultipleScansAfterDDL_NullTime: repeated scans across DDL using sql.NullTime.

Behavior

• Transparent to users of database/sql.
• On ER_NEED_REPREPARE:
  • Reprepare once and retry the same Exec/Query.
  • If reprepare fails, return the error.
• No overhead on the fast path; only triggers on the error path.

Compatibility

• No public API changes.
• Metadata caching semantics preserved (respects clientCacheMetadata).
• Works regardless of whether the server emits ER_NEED_REPREPARE. If not emitted, behavior is unchanged.

Performance impact

• None on the normal path.
• On schema invalidation: one COM_STMT_CLOSE + COM_STMT_PREPARE roundtrip and a single retry.

Edge cases and safeguards

• Single retry only (avoids masking persistent failures).
• Returns driver.ErrBadConn / ErrInvalidConn as before if the statement/connection is invalid.
• Closes the old server-side stmt ID to avoid leaks.

Files touched

• statement.go: add queryString, implement reprepare, add single-retry in Exec/Query.
• connection.go: set stmt.queryString = query in Prepare.
• reprepare_test.go: new integration tests.
• README.md: note about prepared statements being resilient to DDL (ER_NEED_REPREPARE).

Testing

• Automated:
  • go test -run Reprepare -count=1 ./... passes with a live MySQL.
  • Full suite passes given environment (LOCAL INFILE may need enabling).
• Manual repro (optional):
  • Prepare a SELECT, run it, ALTER TABLE ... MODIFY state INT, scan again; values remain correct.
  • Prepare an INSERT, ALTER TABLE ... MODIFY value BIGINT, insert again; succeeds.