code: def smart_truncate(self):
      to_be_read = total_len(self)
      already_read = self._get_end() - to_be_read
  
      if already_read >= to_be_read:
          old_bytes = self.read()
          self.seek(0, 0)
          self.truncate()
          self.write(old_bytes)
          self.seek(0, 0)
  
imports: import contextlib
  import io
  import os
  from uuid import uuid4
  import requests
  from .._compat import fields
label_desc: Use image node_id 5 for calling the CustomBytesIO obj's underlying member method code with example usage: obj.smart_truncate() without return types