"""
Cross Site Request Forgery Middleware.

This module provides a middleware that implements protection
against request forgeries from other sites.
"""
from __future__ import unicode_literals

import logging
import re

from django.conf import settings
from django.core.urlresolvers import get_callable
from django.utils.cache import patch_vary_headers
from django.utils.crypto import constant_time_compare, get_random_string
from django.utils.encoding import force_text
from django.utils.http import same_origin

logger = logging.getLogger('django.request')

REASON_NO_REFERER = "Referer checking failed - no Referer."
REASON_BAD_REFERER = "Referer checking failed - %s does not match any trusted origins."
REASON_NO_CSRF_COOKIE = "CSRF cookie not set."
REASON_BAD_TOKEN = "CSRF token missing or incorrect."

CSRF_KEY_LENGTH = 32


def _get_failure_view():
    """
    Returns the view to be used for CSRF rejections
    """
    return get_callable(settings.CSRF_FAILURE_VIEW)


def _get_new_csrf_key():
    return get_random_string(CSRF_KEY_LENGTH)


def get_token(request):
    """
    Returns the CSRF token required for a POST form. The token is an
    alphanumeric value. A new token is created if one is not already set.

    A side effect of calling this function is to make the csrf_protect
    decorator and the CsrfViewMiddleware add a CSRF cookie and a 'Vary: Cookie'
    header to the outgoing response.  For this reason, you may need to use this
    function lazily, as is done by the csrf context processor.
    """
    if "CSRF_COOKIE" not in request.META:
        request.META["CSRF_COOKIE"] = _get_new_csrf_key()
    request.META["CSRF_COOKIE_USED"] = True
    return request.META["CSRF_COOKIE"]


def rotate_token(request):
    """
    Changes the CSRF token in use for a request - should be done on login
    for security purposes.
    """
    request.META.update({
        "CSRF_COOKIE_USED": True,
        "CSRF_COOKIE": _get_new_csrf_key(),
    })


def _sanitize_token(token):
    # Allow only alphanum
    if len(token) > CSRF_KEY_LENGTH:
        return _get_new_csrf_key()
    token = re.sub('[^a-zA-Z0-9]+', '', force_text(token))
    if token == "":
        # In case the cookie has been truncated to nothing at some point.
        return _get_new_csrf_key()
    return token


class CsrfViewMiddleware(object):
    """
    Middleware that requires a present and correct csrfmiddlewaretoken
    for POST requests that have a CSRF cookie, and sets an outgoing
    CSRF cookie.

    This middleware should be used in conjunction with the csrf_token template
    tag.
    """
    # The _accept and _reject methods currently only exist for the sake of the
    # requires_csrf_token decorator.
    def _accept(self, request):
        # Avoid checking the request twice by adding a custom attribute to
        # request.  This will be relevant when both decorator and middleware
        # are used.
        request.csrf_processing_done = True
        return None

    def _reject(self, request, reason):
        logger.warning('Forbidden (%s): %s', reason, request.path,
            extra={
                'status_code': 403,
                'request': request,
            }
        )
        return _get_failure_view()(request, reason=reason)

    def process_view(self, request, callback, callback_args, callback_kwargs):

        if getattr(request, 'csrf_processing_done', False):
            return None

        try:
            csrf_token = _sanitize_token(
                request.COOKIES[settings.CSRF_COOKIE_NAME])
            # Use same token next time
            request.META['CSRF_COOKIE'] = csrf_token
        except KeyError:
            csrf_token = None

        # Wait until request.META["CSRF_COOKIE"] has been manipulated before
        # bailing out, so that get_token still works
        if getattr(callback, 'csrf_exempt', False):
            return None

        # Assume that anything not defined as 'safe' by RFC2616 needs protection
        if request.method not in ('GET', 'HEAD', 'OPTIONS', 'TRACE'):
            if getattr(request, '_dont_enforce_csrf_checks', False):
                # Mechanism to turn off CSRF checks for test suite.
                # It comes after the creation of CSRF cookies, so that
                # everything else continues to work exactly the same
                # (e.g. cookies are sent, etc.), but before any
                # branches that call reject().
                return self._accept(request)