Root cause of the fresh-DB multi-connection write failure: economy writes ran in
a DEFERRED transaction (pool.begin() = BEGIN) that read then wrote; SQLite returns
SQLITE_BUSY (code 5, 'database is locked') *immediately* when a deferred tx
upgrades to a write while another holds the write lock, bypassing busy_timeout to
avoid deadlock. Fix: open each write op with BEGIN IMMEDIATE on a dedicated pooled
connection (finish() commits/rolls back), taking the write lock up front so
busy_timeout serializes writers. Reproduction test: 100 fresh DBs x 8 concurrent
grant_reward — was 644/800 failures, now 800/800 succeed with correct final
balance (no lost update). Reads unchanged.
OpenFUT Agent