writeRecomputeChanges' purchase_task/return_match rechecks (added in the
previous phase 3 commit) used plain SELECT COUNT(*). On MySQL 8.4 under
REPEATABLE-READ, a plain read inside a transaction reuses the snapshot
taken at that transaction's first read (recomputeChanges' own SELECT), so
a purchase_task or return_match row committed by another connection AFTER
that snapshot was invisible to the recheck even after the syb_product row's
FOR UPDATE lock was granted — the row lock only serializes writers against
each other, it doesn't force a later plain read to see newer committed
data. Confirmed against a real local MySQL 8.4 server with two connections:
after the other transaction committed a task, plain COUNT(*) returned 0
while COUNT(*) ... FOR SHARE correctly returned 1. Every existing test
passed anyway because this package's tests run on SQLite, which has no
multi-connection snapshot isolation to reproduce the race at all.
Fix: both rechecks now use clause.Locking{Strength: "SHARE"} (a locking/
"current" read is enough since they only need to observe committed rows,
not lock them further). Split the three per-row queries (row FOR UPDATE,
purchase_task FOR SHARE, return_match FOR SHARE) into small query-builder
helpers (sybProductRowLockQuery / purchaseTaskLockedQuery /
returnMatchLockedQuery) so writeRecomputeChanges consumes them and a test
can independently assert their generated SQL.
New test: TestRecheckQueriesUseLockingReads opens a DryRun gorm session
against the MySQL dialector (mysql.New with SkipInitializeWithVersion,
DisableAutomaticPing — no real network connection is ever made) and pins
the exact SQL shape via db.ToSQL:
SELECT * FROM `syb_product` WHERE id = 1 FOR UPDATE
SELECT count(*) FROM `purchase_task` WHERE syb_product_id = 1 FOR SHARE
SELECT count(*) FROM `return_match` WHERE syb_product_id = 1
AND active_syb_product_id IS NOT NULL FOR SHARE
The test's own comment documents why SQLite cannot reproduce this race
(gorm's SQLite driver drops clause.Locking entirely; SQLite also has no
multi-connection REPEATABLE-READ snapshot semantics to begin with).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NTDbDcwbDw1TSAcE6wfh2F
