code: def _read_callback(connection_id, data_buffer, data_length_pointer):
      
      wrapped_socket = None
      try:
          wrapped_socket = _connection_refs.get(connection_id)
          if wrapped_socket is None:
              return SecurityConst.errSSLInternal
          base_socket = wrapped_socket.socket
  
          requested_length = data_length_pointer[0]
  
          timeout = wrapped_socket.gettimeout()
          error = None
          read_count = 0
  
          try:
              while read_count < requested_length:
                  if timeout is None or timeout >= 0:
                      if not util.wait_for_read(base_socket, timeout):
                          raise socket.error(errno.EAGAIN, 'timed out')
  
                  remaining = requested_length - read_count
                  buffer = (ctypes.c_char * remaining).from_address(
                      data_buffer + read_count
                  )
                  chunk_size = base_socket.recv_into(buffer, remaining)
                  read_count += chunk_size
                  if not chunk_size:
                      if not read_count:
                          return SecurityConst.errSSLClosedGraceful
                      break
          except (socket.error) as e:
              error = e.errno
  
              if error is not None and error != errno.EAGAIN:
                  data_length_pointer[0] = read_count
                  if error == errno.ECONNRESET or error == errno.EPIPE:
                      return SecurityConst.errSSLClosedAbort
                  raise
  
          data_length_pointer[0] = read_count
  
          if read_count != requested_length:
              return SecurityConst.errSSLWouldBlock
  
          return 0
      except Exception as e:
          if wrapped_socket is not None:
              wrapped_socket._exception = e
          return SecurityConst.errSSLInternal
docs: SecureTransport read callback. This is called by ST to request that data
      be returned from the socket.