import json
from django.core.exceptions import SuspiciousOperation
from django.http import HttpResponseRedirect
from django.utils.http import is_safe_url, urlsafe_base64_decode

TARGET_HOSTNAME="www.example.com"  # You expect all redirects to go here

def legacy_mandrill_click_tracking_redirect(request, mandrill_account=None, target_host=None):
    """Handle a Mandrill click-tracking redirect link"""

    try:
        b64payload = request.GET['p']
        # (Django's urlsafe_base64_decode handles missing '=' padding)
        payload = json.loads(urlsafe_base64_decode(b64payload))
        assert payload['v'] == 1  # we've only seen v:1 signed payloads
        params = json.loads(payload['p'])
        assert params['v'] == 1  # we've only seen v:1 params
        target = params['url']
    except (AssertionError, KeyError, TypeError, ValueError):
        # Missing/unparseable query params/payload format
        raise SuspiciousOperation("tried to redirect with garbled payload")

    # Verify we're only redirecting to our own site (don't be an open redirect server):
    if not is_safe_url(target, TARGET_HOSTNAME):
        raise SuspiciousOperation("tried to redirect to unsafe url '%s'" % target)
        
    # If you want to be extra-paranoid, you could also check that:
    #   mandrill_account == params['u']
    #   target_host == urlparse(target).netloc

    # Want to actually log the click for your own metrics?
    # This would be a good place to do it.
    
    return HttpResponseRedirect(target)