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app/db/session.py

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+from collections.abc import AsyncIterator
+import logging
+
+from sqlalchemy import text
+from sqlalchemy.ext.asyncio import (
+    AsyncEngine,
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+from sqlalchemy.pool import NullPool
+
+from app.settings import settings
+
+logger = logging.getLogger(__name__)
+
+_engine: AsyncEngine | None = None
+_session_factory: async_sessionmaker[AsyncSession] | None = None
+
+
+def get_engine() -> AsyncEngine:
+    global _engine
+    if _engine is None:
+        # NullPool avoids cross-event-loop connection reuse in tests and dev tools.
+        _engine = create_async_engine(
+            settings.database_url,
+            pool_pre_ping=True,
+            poolclass=NullPool,
+        )
+        logger.info("db.engine_initialized", extra={"event": "db.engine_initialized"})
+    return _engine
+
+
+def get_session_factory() -> async_sessionmaker[AsyncSession]:
+    global _session_factory
+    if _session_factory is None:
+        _session_factory = async_sessionmaker(get_engine(), expire_on_commit=False)
+    return _session_factory
+
+
+async def get_db_session() -> AsyncIterator[AsyncSession]:
+    session_factory = get_session_factory()
+    async with session_factory() as session:
+        yield session
+
+
+async def check_database() -> bool:
+    engine = get_engine()
+    try:
+        async with engine.connect() as conn:
+            result = await conn.execute(text("SELECT 1"))
+            return result.scalar_one() == 1
+    except Exception:
+        logger.exception("db.healthcheck_failed", extra={"event": "db.healthcheck_failed"})
+        return False
+
+
+async def close_engine() -> None:
+    global _engine, _session_factory
+    if _engine is not None:
+        await _engine.dispose()
+        logger.info("db.engine_disposed", extra={"event": "db.engine_disposed"})
+        _engine = None
+        _session_factory = None