import psycopg


def claim_next(conn: psycopg.Connection) -> str | None:
    """Atomically claim the oldest queued job. Returns its id or None."""
    with conn.cursor() as cur:
        cur.execute(
            """
            SELECT id FROM "Job"
            WHERE state = 'requested'
            ORDER BY "createdAt" ASC
            FOR UPDATE SKIP LOCKED
            LIMIT 1
            """
        )
        row = cur.fetchone()
        if row is None:
            conn.rollback()
            return None
        job_id = row[0]
        cur.execute(
            """
            UPDATE "Job"
            SET state = 'matching',
                "currentStage" = 'match',
                "claimedAt" = now(),
                attempts = attempts + 1,
                "updatedAt" = now()
            WHERE id = %s
            """,
            (job_id,),
        )
    conn.commit()
    return job_id