fix(web): issue2551356. Add etag header for not-modified (304) request. When a 304 is returned to a conditional request for a static file, print an ETag for the response. ETag was always sent with a 200 response. This also adds initial support for if-none-match conditional requests for static files. Changes: Refactors the if-modified-since code out to a method. It moves a file stat call from serve_static_file to _serve_file so that an etag can be generated by both serve_static_file and serve_file which call _serve_file. Tests added. This does not test the codepath where serve_file pulls content from the database rather than from a local file on disk. Test mocking _serve_file changed to account for 5th argument to serve_file BREAKING CHANGE: function signature for client.py-Client::_serve_file() now has 5 not 4 parameters (added etag param). Since this is a "hidden" method I am not too worried about it.

import time, struct, base64
from roundup.cgi.actions import RegisterAction
from roundup.cgi.exceptions import *

def timestamp():
    return base64.encodestring(struct.pack("i", time.time())).strip()

def unpack_timestamp(s):
    return struct.unpack("i",base64.decodestring(s))[0]

class Timestamped:
    def check(self):
        try:
            created = unpack_timestamp(self.form['opaque'].value)
        except KeyError:
            raise FormError("somebody tampered with the form")
        if time.time() - created < 4:
            raise FormError("responding to the form too quickly")
        return True

class TimestampedRegister(Timestamped, RegisterAction):
    def permission(self):
        self.check()
        RegisterAction.permission(self)

def init(instance):
    instance.registerUtil('timestamp', timestamp)
    instance.registerAction('register', TimestampedRegister)