# -*- coding: utf-8 -*-

# copyright 2014 Logilab, PARIS

"""A set of utility functions to handle CORS requests

Unless specified, all references in this file are related to:

  http://www.w3.org/TR/cors

The provided implementation roughly follows:

  http://www.html5rocks.com/static/images/cors_server_flowchart.png

See also:

  https://developer.mozilla.org/en-US/docs/HTTP/Access_control_CORS

"""

import urlparse

from cubicweb.web import LOGGER

info = LOGGER.info

class CORSFailed(Exception):

    """Raised when cross origin resource sharing checks failed"""

class CORSPreflight(Exception):

    """Raised when cross origin resource sharing checks detects the

    request as a valid preflight request"""

def process_request(req, config):

    """

    Process a request to apply CORS specification algorithms

    Check whether the CORS specification is respected and set corresponding

    headers to ensure response complies with the specification.

    In case of non-compliance, no CORS-related header is set.

    """

    base_url = urlparse.urlsplit(req.base_url())

    expected_host = '://'.join((base_url.scheme, base_url.netloc))

    if not req.get_header('Origin') or req.get_header('Origin') == expected_host:

        # not a CORS request, nothing to do

        return

    try:

        # handle cross origin resource sharing (CORS)

        if req.http_method() == 'OPTIONS':

            if req.get_header('Access-Control-Request-Method'):

                # preflight CORS request

                process_preflight(req, config)

        else: # Simple CORS or actual request

            process_simple(req, config)

    except CORSFailed, exc:

        info('Cross origin resource sharing failed: %s' % exc)

    except CORSPreflight:

        info('Cross origin resource sharing: valid Preflight request %s')

        raise

def process_preflight(req, config):

    """cross origin resource sharing (preflight)

    Cf http://www.w3.org/TR/cors/#resource-preflight-requests

    """

    origin = check_origin(req, config)

    allowed_methods = set(config['access-control-allow-methods'])

    allowed_headers = set(config['access-control-allow-headers'])

    try:

        method = req.get_header('Access-Control-Request-Method')

    except ValueError:

        raise CORSFailed('Access-Control-Request-Method is incorrect')

    if method not in allowed_methods:

        raise CORSFailed('Method is not allowed')

    try:

        req.get_header('Access-Control-Request-Headers', ())

    except ValueError:

        raise CORSFailed('Access-Control-Request-Headers is incorrect')

    req.set_header('Access-Control-Allow-Methods', allowed_methods, raw=False)

    req.set_header('Access-Control-Allow-Headers', allowed_headers, raw=False)

    process_common(req, config, origin)

    raise CORSPreflight()

def process_simple(req, config):

    """Handle the Simple Cross-Origin Request case

    """

    origin = check_origin(req, config)

    exposed_headers = config['access-control-expose-headers']

    if exposed_headers:

        req.set_header('Access-Control-Expose-Headers', exposed_headers, raw=False)

    process_common(req, config, origin)

def process_common(req, config, origin):

    req.set_header('Access-Control-Allow-Origin', origin)

    # in CW, we always support credential/authentication

    req.set_header('Access-Control-Allow-Credentials', 'true')

def check_origin(req, config):

    origin = req.get_header('Origin').lower()

    allowed_origins = config.get('access-control-allow-origin')

    if not allowed_origins:

        raise CORSFailed('access-control-allow-origin is not configured')

    if '*' not in allowed_origins and origin not in allowed_origins:

        raise CORSFailed('Origin is not allowed')

    # bit of sanity check; see "6.3 Security"

    myhost = urlparse.urlsplit(req.base_url()).netloc

    host = req.get_header('Host')

    if host != myhost:

        info('cross origin resource sharing detected possible '

             'DNS rebinding attack Host header != host of base_url: '

             '%s != %s' % (host, myhost))

        raise CORSFailed('Host header and hostname do not match')

    # include "Vary: Origin" header (see 6.4)

    req.headers_out.addHeader('Vary', 'Origin')

    return origin