"""
Async SQLAlchemy engine and session factory.

Usage in FastAPI:
    async def route(session: AsyncSession = Depends(get_session)):
        ...

Usage in tests:
    async with async_session_factory() as session:
        ...
"""

from __future__ import annotations

from collections.abc import AsyncGenerator

import os

from sqlalchemy.ext.asyncio import AsyncEngine, AsyncSession, async_sessionmaker, create_async_engine
from sqlalchemy.pool import NullPool

from shared.config import settings

# ---------------------------------------------------------------------------
# Engine — one per process; shared across all requests
#
# Celery workers use asyncio.run() per task, creating a new event loop each
# time. Connection pools hold connections bound to the previous (closed) loop,
# causing "Future attached to a different loop" errors. NullPool avoids this
# by never reusing connections. FastAPI (single event loop) can safely use a
# regular pool, but NullPool works fine there too with minimal overhead.
# ---------------------------------------------------------------------------
_is_celery_worker = "celery" in os.environ.get("_", "") or "celery" in " ".join(os.sys.argv)

engine: AsyncEngine = create_async_engine(
    settings.database_url,
    echo=settings.debug,
    **({"poolclass": NullPool} if _is_celery_worker else {
        "pool_pre_ping": True,
        "pool_size": 10,
        "max_overflow": 20,
    }),
)

# ---------------------------------------------------------------------------
# Session factory
# ---------------------------------------------------------------------------
async_session_factory: async_sessionmaker[AsyncSession] = async_sessionmaker(
    engine,
    class_=AsyncSession,
    expire_on_commit=False,
)


async def get_session() -> AsyncGenerator[AsyncSession, None]:
    """
    FastAPI dependency that yields an async database session.

    The session is automatically closed (and the connection returned to the
    pool) when the request context exits, even if an exception is raised.

    Example:
        @router.get("/agents")
        async def list_agents(session: AsyncSession = Depends(get_session)):
            result = await session.execute(select(Agent))
            return result.scalars().all()
    """
    async with async_session_factory() as session:
        yield session