Free-threaded CPython with PYTHON_GIL=0: first calls from several threads at once crash the process #68

This work was produced with the help of language models.

On free-threaded CPython with the GIL disabled (PYTHON_GIL=0 or -Xgil=0), several threads making their first driver calls at the same time crash the process intermittently: SIGSEGV, free(): invalid pointer or malloc(): corrupted top size. The driver fills its import cache (the Python objects it loads on first use: datetime, uuid, pandas, ...) lazily with no synchronization and relies on the GIL, so concurrent first loads race on a shared vector. Making one driver call from a single thread before starting the others avoids it.

The module does not declare free-threading support, so by default importing it re-enables the GIL (with a RuntimeWarning) and nothing crashes; the crash needs the GIL disabled explicitly.

import threading, ladybug as lb

db = lb.Database(":memory:")
conns = [lb.Connection(db) for _ in range(8)]
start = threading.Barrier(len(conns))

def run(conn, t):
    start.wait()
    for i in range(300):
        conn.execute(f"RETURN $x + {t * 100000 + i}", {"x": 1}).get_next()

threads = [threading.Thread(target=run, args=(c, t)) for t, c in enumerate(conns)]
for th in threads: th.start()
for th in threads: th.join()

Run it as PYTHON_GIL=0 python repro.py, in a fresh interpreter each time, a few times over.

Expected: the script finishes.
Actual: it crashes in about 1 run of 7 (2 of 15).

Reproduced on main (548c183), built from source, with free-threaded CPython 3.15.0rc3.

Fix: #69