migrate() and fix_flow_versioning_migration re-acquired a second connection from the pool while already holding one (the migrator's checked-out, advisory-locked connection). That deadlocks any backend limited to one connection at a time — connection-constrained managed Postgres, PgBouncer transaction pooling, or an embedded single-connection dev database. Route those housekeeping queries onto the already-held connection via a new CustomMigrator::connection() accessor. Fewer connections during migration and, for fix_flow_versioning, the existence check and write now run on the same advisory-locked connection. Default multi-connection behavior is unchanged.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Ruben Fiszel
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