"""
Request ID middleware — attaches a unique request ID for tracing.

Adds `request.request_id` and sets the `X-Request-ID` response header.
"""
import uuid


class RequestIDMiddleware:
    """
    Attach a unique request ID to every request for observability and tracing.

    If the client sends an `X-Request-ID` header, it is reused.
    Otherwise, a new UUID4 is generated.
    """

    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        request_id = request.headers.get("X-Request-ID") or str(uuid.uuid4())
        request.request_id = request_id

        response = self.get_response(request)
        response["X-Request-ID"] = request_id
        return response