"""Forged-request (CSRF) protection for the browser side of Steward.

A page on another site can make a logged-in browser POST to Steward, and the
browser sends Steward's session cookie with it. Every state-changing request
from a browser therefore has to carry a token that only Steward's own pages
know: a random value kept in the session, written into each form by
`csrf_field()` and sent by HTMX and fetch() as the `X-CSRF-Token` header (set
once on <body> in base.html).

One check in a before_request covers every route, so a new route cannot
forget it. Callers that authenticate with a bearer token instead of the
session cookie (the host agent's ingest; /mcp never reaches Quart) are exempt:
a forged browser request cannot carry their token.
"""
from __future__ import annotations

import hmac
import secrets

from markupsafe import Markup, escape
from quart import Response, request, session

SESSION_KEY = "_csrf"
FORM_FIELD = "csrf_token"
HEADER = "X-CSRF-Token"
UNSAFE_METHODS = ("POST", "PUT", "PATCH", "DELETE")
# Bearer-authenticated endpoints: no session cookie, nothing to forge.
EXEMPT_PATHS = frozenset({"/plugins/host_agent/ingest"})


def csrf_token() -> str:
    """This session's token, created on first use."""
    token = session.get(SESSION_KEY)
    if not token:
        token = session[SESSION_KEY] = secrets.token_urlsafe(32)
    return token


def csrf_field() -> Markup:
    """The hidden input every POST form carries."""
    return Markup(f'<input type="hidden" name="{FORM_FIELD}" value="{escape(csrf_token())}">')


def rotate() -> None:
    """Drop the token, so a new one is issued — at login, where the session
    changes hands."""
    session.pop(SESSION_KEY, None)


async def check_request() -> Response | None:
    """before_request hook: refuse an unsafe request without the session's token."""
    if request.method not in UNSAFE_METHODS or request.path in EXEMPT_PATHS:
        return None
    expected = session.get(SESSION_KEY)
    sent = request.headers.get(HEADER, "")
    if not sent:
        form = await request.form
        sent = form.get(FORM_FIELD, "")
    if expected and sent and hmac.compare_digest(expected, sent):
        return None
    return Response(
        "This form has expired or did not come from Steward. Reload the page and try again.",
        status=400, content_type="text/plain; charset=utf-8")