Keep the user transaction active after a failed statement

[Nandan108]

Summary

Part of #473 (the "Explicit transactions" section).

When a statement fails inside a transaction, the driver rolls back the whole transaction and ends it. MySQL undoes only the failed statement and keeps the transaction active:

CREATE TABLE t (id INT PRIMARY KEY);
START TRANSACTION;
INSERT INTO t VALUES (1);
INSERT INTO t VALUES (1);  -- fails with a duplicate key error
INSERT INTO t VALUES (2);
COMMIT;
SELECT id FROM t;
-- Driver: 2     MySQL: 1, 2

The work before the error is lost, and the work after it runs in autocommit mode. A savepoint created before the error is gone too, so ROLLBACK TO SAVEPOINT fails with error 1305.

Fix

Each statement already runs in a wrapper: a transaction when none is active, or a savepoint inside a user transaction. On failure, only that wrapper is now rolled back. This:

• keeps the user transaction and its savepoints active, and still undoes all changes of the failed statement, including the earlier rows of a multi-row INSERT;
• keeps the transaction opened by LOCK TABLES, so UNLOCK TABLES commits the rows written before the error, as in MySQL;
• replaces the special case for error 1305 (missing savepoint), since savepoint statements are not wrapped and have nothing to roll back.

Without a user transaction, the behavior is unchanged: the wrapper transaction is rolled back.

Some SQLite errors roll back the whole transaction automatically (SQLite docs). Then rolling back to the savepoint fails, and the driver ends the transaction as before. This path has no test, because those errors (such as SQLITE_FULL) are hard to trigger in a unit test.

The rest of #473 (error modes and diagnostics) is not part of this PR.

Tests

• testFailedStatementKeepsTransactionActive checks that a failed write and a failed read keep the transaction active, and that COMMIT keeps the work before and after them.
• testFailedStatementIsRolledBackInTransaction checks that a failed multi-row INSERT leaves none of its rows.
• testRollbackToSavepointAfterFailedStatement checks that a savepoint survives a failed statement.
• testFailedStatementKeepsTableLock checks that UNLOCK TABLES keeps the rows written before a failed statement.
• test_failed_query_keeps_transaction_active checks the same through PDO::beginTransaction() and PDO::commit().

All five fail on trunk. The existing testMissingSavepointDoesNotRollbackTransaction still passes.

packages/mysql-on-sqlite: 1048 tests pass (PHP 8.3, SQLite 3.45.1); PHPCS is clean.

Use of AI

This change was prepared with Claude Code (Claude Opus 5.5). I reviewed the code, the tests, and this description, and I take responsibility for the change.

Note: this PR and #535 both add tests at the same spot of WP_MySQL_On_SQLite_PDO_API_Tests.php, so whichever merges second needs a trivial rebase.

🤖 Generated with Claude Code

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• packages/mysql-on-sqlite/src/sqlite/class-wp-mysql-on-sqlite.php
• packages/mysql-on-sqlite/tests/WP_MySQL_On_SQLite_PDO_API_Tests.php
• packages/mysql-on-sqlite/tests/WP_MySQL_On_SQLite_Tests.php

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