Patched code sample

import sys

# Mock base classes and exceptions for a self-contained example,
# representing the structure of the Pillow library.

class DecompressionBombError(Exception):
    """Mock exception for demonstration."""
    pass

class Image:
    """Mock base Image class."""
    # A default limit, similar to Pillow's Image.MAX_IMAGE_PIXELS
    MAX_IMAGE_PIXELS = 178956970

    def __init__(self):
        self._size = (0, 0)
        self.fp = None  # Mock file pointer

    def _decompression_bomb_check(self):
        """
        Checks if the image is a potential decompression bomb.
        This is a simplified version of the real check in Pillow.
        """
        if self._size[0] * self._size[1] > self.MAX_IMAGE_PIXELS:
            raise DecompressionBombError(
                f"Image size ({self._size[0]}x{self._size[1]}) "
                f"exceeds pixel limit of {self.MAX_IMAGE_PIXELS}"
            )

# The following class represents the fixed version of PIL/GdImageFile.py
class GdImageFile(Image):
    """
    Mock GdImageFile class demonstrating the fix for the vulnerability.
    """
    def _open(self):
        """
        This method simulates opening a .gd file and reading its header.
        """
        # Simulate reading image dimensions from a malicious file header.
        # In a real scenario, these would be read from self.fp.
        width_from_header = 65535
        height_from_header = 65535

        # The dimensions are assigned to self._size.
        self._size = width_from_header, height_from_header

        # THE FIX:
        # _decompression_bomb_check() is called immediately after setting self._size.
        # This prevents excessive memory allocation by validating the dimensions
        # before any pixel data is processed or allocated.
        self._decompression_bomb_check()

        # If the check passes, the rest of the image loading proceeds.
        self.mode = "P"