new_contents: # -*- coding: utf-8 -*-
  # Copyright 2015 OpenMarket Ltd
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  
  from twisted.internet import defer
  
  from synapse.events.utils import prune_event, old_prune_event
  
  from syutil.jsonutil import encode_canonical_json
  
  from synapse.crypto.event_signing import check_event_content_hash
  
  from synapse.api.errors import SynapseError
  
  import logging
  
  
  logger = logging.getLogger(__name__)
  
  
  class FederationBase(object):
      @defer.inlineCallbacks
      def _check_sigs_and_hash_and_fetch(self, origin, pdus, outlier=False):
          """Takes a list of PDUs and checks the signatures and hashs of each
          one. If a PDU fails its signature check then we check if we have it in
          the database and if not then request if from the originating server of
          that PDU.
  
          If a PDU fails its content hash check then it is redacted.
  
          The given list of PDUs are not modified, instead the function returns
          a new list.
  
          Args:
              pdu (list)
              outlier (bool)
  
          Returns:
              Deferred : A list of PDUs that have valid signatures and hashes.
          """
          signed_pdus = []
          for pdu in pdus:
              try:
                  new_pdu = yield self._check_sigs_and_hash(pdu)
                  signed_pdus.append(new_pdu)
              except SynapseError:
                  # FIXME: We should handle signature failures more gracefully.
  
                  # Check local db.
                  new_pdu = yield self.store.get_event(
                      pdu.event_id,
                      allow_rejected=True
                  )
                  if new_pdu:
                      signed_pdus.append(new_pdu)
                      continue
  
                  # Check pdu.origin
                  if pdu.origin != origin:
                      new_pdu = yield self.get_pdu(
                          destinations=[pdu.origin],
                          event_id=pdu.event_id,
                          outlier=outlier,
                      )
  
                      if new_pdu:
                          signed_pdus.append(new_pdu)
                          continue
  
                  logger.warn("Failed to find copy of %s with valid signature")
  
          defer.returnValue(signed_pdus)
  
      @defer.inlineCallbacks
      def _check_sigs_and_hash(self, pdu):
          """Throws a SynapseError if the PDU does not have the correct
          signatures.
  
          Returns:
              FrozenEvent: Either the given event or it redacted if it failed the
              content hash check.
          """
          # Check signatures are correct.
          redacted_event = prune_event(pdu)
          redacted_pdu_json = redacted_event.get_pdu_json()
  
          old_redacted = old_prune_event(pdu)
          old_redacted_pdu_json = old_redacted.get_pdu_json()
  
          try:
              try:
                  yield self.keyring.verify_json_for_server(
                      pdu.origin, old_redacted_pdu_json
                  )
              except SynapseError:
                  yield self.keyring.verify_json_for_server(
                      pdu.origin, redacted_pdu_json
                  )
          except SynapseError:
              logger.warn(
                  "Signature check failed for %s redacted to %s",
                  encode_canonical_json(pdu.get_pdu_json()),
                  encode_canonical_json(redacted_pdu_json),
              )
              raise
  
          if not check_event_content_hash(pdu):
              logger.warn(
                  "Event content has been tampered, redacting %s, %s",
                  pdu.event_id, encode_canonical_json(pdu.get_dict())
              )
              defer.returnValue(redacted_event)
  
          defer.returnValue(pdu)
subject: Add new FederationBase