[jnsdls]

Summary

A pool hook that runs a statement through the driver (a SQLAlchemy checkout listener, a psycopg-pool check) hands the connection out INTRANS: in non-autocommit mode the driver opened a transaction for that statement. Found upgrading Nebula to 0.0.4 (agent-labs-dev/nebula#11193), whose _on_checkout runs SELECT 1; SET statement_timeout ….

What broke on such a connection, by path:

path	sqlalchemy (asyncpg)	sqlalchemy (psycopg)	psycopg pool
bare read (ZQLDatabase.run, 0.0.4)	hook's tx silently rolled back: its SETs lost	ProgrammingError: can't change 'autocommit' now	same ProgrammingError
transaction / push session	session: AssertionError in _send_begin	ok	session: transaction() is only a savepoint, pool rolls the writes back: mutation lost
any transaction with isolation_level	SET TRANSACTION ISOLATION LEVEL must be called before any query (after the hook's SELECT 1)	same	same

Fix: every adapter path that takes a connection first commits a transaction the pool left open, keeping what the hook set, then carries on unchanged. An IDLE connection costs a local status check, no round trip, so bare reads still go out with no BEGIN/COMMIT.

 SQLAlchemyConnection.query / transaction / _transaction (session)
   engine.connect()
+  _settle(connection)      # driver INTRANS? commit it
   AUTOCOMMIT | begin()
 PsycopgConnection.query / _transaction (session)
   pool.connection() | getconn()
+  _settle(connection)
   set_autocommit(True) | connection.transaction()

asyncpg direct: unaffected. asyncpg opens no implicit transactions, so a pool setup/init leaves one open only by sending BEGIN itself.

Evidence

New differential tests in tests/wire/test_mutate.py. The hooked engine or pool runs SELECT 1 then SET statement_timeout = '4321ms' on every checkout:

test_run_matches_upstream_on_a_connection_checked_out_in_a_transaction[sqlalchemy|sqlalchemy-psycopg|psycopg]
  recordedRun (bare) == upstream's, and SHOW statement_timeout == 4321ms
test_transactions_match_upstream_on_a_connection_checked_out_in_a_transaction[None|serializable × 3 adapters]
  transaction alone and in a session: isolation == upstream's, statement_timeout == 4321ms, both writes committed

• Before: 8 failed, 1 passed (sqlalchemy-psycopg transaction with no isolation, the 0.0.3 path)
• After: 9 passed. Full pytest 795 passed, tests.compat complete, ruff and ty clean.

Merge Danger

Door: two-way

Blast Radius: adapters

Only connections handed out INTRANS behave differently: the hook's transaction now commits up front. Before, it crashed, rolled back, or dropped writes. One COMMIT round trip per checkout in that case.

🤖 Generated with Claude Code

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[codspeed]

Merging this PR will not alter performance

✅ 46 untouched benchmarks

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_{Comparing fix/sqlalchemy-intrans-checkout (2a244c2) with main (1e2b8da)}