"""
Correlation ID middleware.
Reads X-Request-ID from incoming headers (or generates one) and makes it
available via a ContextVar so it can be injected into logs and error responses
without threading issues.
"""
import uuid
from contextvars import ContextVar

from starlette.middleware.base import BaseHTTPMiddleware
from starlette.requests import Request

_request_id_ctx: ContextVar[str] = ContextVar("request_id", default="")


def get_request_id() -> str:
    return _request_id_ctx.get()


class CorrelationIdMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        request_id = request.headers.get("X-Request-ID") or str(uuid.uuid4())
        token = _request_id_ctx.set(request_id)
        try:
            response = await call_next(request)
            response.headers["X-Request-ID"] = request_id
            return response
        finally:
            _request_id_ctx.reset(token)