Patched code sample

import io
from typing import IO

class PdfStreamError(Exception):
    """A custom exception for PDF stream parsing errors."""
    pass

def read_inline_image_data(stream: IO[bytes]) -> bytes:
    """
    Reads data for an inline image from a stream until the 'EI' marker.

    This function represents the logic used to fix an infinite loop
    vulnerability. The vulnerability occurs when a PDF stream for an inline
    image is not properly terminated with an 'EI' (End Image) marker.
    """
    data = b""
    while b"EI" not in data:
        # Read one byte at a time.
        char = stream.read(1)

        # THE FIX: Check if the end of the stream was reached.
        # If 'char' is an empty byte string, it means there's no more data to
        # read. The original vulnerable code lacked this check, causing an
        # infinite loop on a malformed stream that was missing the 'EI' marker.
        # By raising an error, we prevent the infinite loop.
        if not char:
            raise PdfStreamError(
                "Stream ended before 'EI' marker was found for an inline image."
            )

        data += char
    
    # Once the marker is found, return the data preceding it.
    image_data, _ = data.split(b"EI", 1)
    return image_data

# Example of how the fixed function prevents the infinite loop.

# 1. A malformed stream without the required 'EI' end marker.
# In a vulnerable version, this would cause an infinite loop.
malformed_stream = io.BytesIO(b"some unterminated image data")

# 2. A well-formed stream for comparison.
well_formed_stream = io.BytesIO(b"some image data EI and other content")


# Demonstrate the fix
try:
    print("Attempting to read from malformed stream...")
    # This call will not loop forever; it will raise an exception.
    read_inline_image_data(malformed_stream)
except PdfStreamError as e:
    # The exception is caught, preventing the application from hanging.
    print(f"Success: Caught expected error: {e}")

# Demonstrate normal operation
try:
    print("\nAttempting to read from well-formed stream...")
    content = read_inline_image_data(well_formed_stream)
    print(f"Success: Read content: {content}")
except PdfStreamError as e:
    print(f"Caught unexpected error: {e}")