code: def read_oplog_progress(self):
          
  
          if self.oplog_checkpoint is None:
              return None
  
          # Check for empty file
          try:
              if os.stat(self.oplog_checkpoint).st_size == 0:
                  LOG.info("MongoConnector: Empty oplog progress file.")
                  return None
          except OSError:
              return None
  
          with open(self.oplog_checkpoint, "r") as progress_file:
              try:
                  data = json.load(progress_file)
              except ValueError:
                  LOG.exception(
                      'Cannot read oplog progress file "%s". '
                      "It may be corrupt after Mongo Connector was shut down"
                      "uncleanly. You can try to recover from a backup file "
                      '(may be called "%s.backup") or create a new progress file '
                      "starting at the current moment in time by running "
                      "mongo-connector --no-dump <other options>. "
                      "You may also be trying to read an oplog progress file "
                      "created with the old format for sharded clusters. "
                      "See https://github.com/10gen-labs/mongo-connector/wiki"
                      "/Oplog-Progress-File for complete documentation."
                      % (self.oplog_checkpoint, self.oplog_checkpoint)
                  )
                  return
              # data format:
              # [name, timestamp] = replica set
              # [[name, timestamp], [name, timestamp], ...] = sharded cluster
              if not isinstance(data[0], list):
                  data = [data]
              with self.oplog_progress:
                  self.oplog_progress.dict = dict(
                      (name, util.long_to_bson_ts(timestamp)) for name, timestamp in data
                  )
docs: Reads oplog progress from file provided by user.
          This method is only called once before any threads are spanwed.