Patched code sample

from thrift.protocol.TProtocol import TProtocolException
from thrift.transport import TTransport

# Simplified protocol implementation for demonstration of a logical flaw.
class TExampleProtocol:
    def __init__(self, trans: TTransport.TTransportBase):
        self.trans = trans

    def readI32(self) -> int:
        # In a real scenario, this reads a signed 32-bit integer
        # from the transport. We simulate receiving a negative value
        # from a malicious or malformed payload.
        # val, = struct.unpack('!i', self.trans.readAll(4))
        # return val
        return -1

    def skip_element(self):
        # In a real scenario, this would skip one element of a given type.
        pass

    def read_list_data(self):
        """
        Reads a list by first reading its size, then processing elements.
        """
        list_size = self.readI32()

        # FIX: Validate container sizes to prevent negative values before looping.
        if list_size < 0:
            raise TProtocolException(
                TProtocolException.NEGATIVE_SIZE,
                "Negative size: %d" % list_size)

        while True:
            if list_size == 0:
                break
            self.skip_element()
            list_size -= 1