new_contents: from synapse.storage.pdu import PduStore
  from synapse.storage.signatures import SignatureStore
  from synapse.storage._base import SQLBaseStore
  from synapse.federation.units import Pdu
  from synapse.crypto.event_signing import (
      add_event_pdu_content_hash, compute_pdu_event_reference_hash
  )
  from synapse.api.events.utils import prune_pdu
  from syutil.base64util import encode_base64, decode_base64
  from syutil.jsonutil import encode_canonical_json
  import sqlite3
  import sys
  
  class Store(object):
      _get_pdu_tuples = PduStore.__dict__["_get_pdu_tuples"]
      _get_pdu_content_hashes_txn = SignatureStore.__dict__["_get_pdu_content_hashes_txn"]
      _get_prev_pdu_hashes_txn = SignatureStore.__dict__["_get_prev_pdu_hashes_txn"]
      _get_pdu_origin_signatures_txn = SignatureStore.__dict__["_get_pdu_origin_signatures_txn"]
      _store_pdu_content_hash_txn = SignatureStore.__dict__["_store_pdu_content_hash_txn"]
      _store_pdu_reference_hash_txn = SignatureStore.__dict__["_store_pdu_reference_hash_txn"]
      _store_prev_pdu_hash_txn = SignatureStore.__dict__["_store_prev_pdu_hash_txn"]
      _simple_insert_txn = SQLBaseStore.__dict__["_simple_insert_txn"]
  
  
  store = Store()
  
  
  def select_pdus(cursor):
      cursor.execute(
          "SELECT pdu_id, origin FROM pdus ORDER BY depth ASC"
      )
  
      ids = cursor.fetchall()
  
      pdu_tuples = store._get_pdu_tuples(cursor, ids)
  
      pdus = [Pdu.from_pdu_tuple(p) for p in pdu_tuples]
  
      reference_hashes = {}
  
      for pdu in pdus:
          try:
              if pdu.prev_pdus:
                  print "PROCESS", pdu.pdu_id, pdu.origin, pdu.prev_pdus
                  for pdu_id, origin, hashes in pdu.prev_pdus:
                      ref_alg, ref_hsh = reference_hashes[(pdu_id, origin)]
                      hashes[ref_alg] = encode_base64(ref_hsh)
                      store._store_prev_pdu_hash_txn(cursor,  pdu.pdu_id, pdu.origin, pdu_id, origin, ref_alg, ref_hsh)
                  print "SUCCESS", pdu.pdu_id, pdu.origin, pdu.prev_pdus
              pdu = add_event_pdu_content_hash(pdu)
              ref_alg, ref_hsh = compute_pdu_event_reference_hash(pdu)
              reference_hashes[(pdu.pdu_id, pdu.origin)] = (ref_alg, ref_hsh)
              store._store_pdu_reference_hash_txn(cursor, pdu.pdu_id, pdu.origin, ref_alg, ref_hsh)
  
              for alg, hsh_base64 in pdu.hashes.items():
                  print alg, hsh_base64
                  store._store_pdu_content_hash_txn(cursor, pdu.pdu_id, pdu.origin, alg, decode_base64(hsh_base64))
  
          except:
              print "FAILED_", pdu.pdu_id, pdu.origin, pdu.prev_pdus
  
  def main():
      conn = sqlite3.connect(sys.argv[1])
      cursor = conn.cursor()
      select_pdus(cursor)
      conn.commit()
  
  if __name__=='__main__':
      main()
  
subject: Add script to hash exisitng history