TIFF: greyscale with one extra sample fails to open when ExtraSamples is 0 (unspecified) or 1 (associated alpha) #10139

import io
import struct

import PIL
from PIL import Image


def grey_with_extra_sample(extra_samples: int, compression: str | None) -> bytes:
    """A 2-sample greyscale TIFF whose ExtraSamples tag (338) is set to the given value."""
    buffer = io.BytesIO()
    Image.new("LA", (64, 48), (128, 255)).save(buffer, "TIFF", compression=compression)
    data = bytearray(buffer.getvalue())
    order = {b"II": "<", b"MM": ">"}[bytes(data[:2])]
    (ifd,) = struct.unpack(order + "I", data[4:8])
    (count,) = struct.unpack(order + "H", data[ifd : ifd + 2])
    for index in range(count):
        entry = ifd + 2 + 12 * index
        tag, kind, _ = struct.unpack(order + "HHI", data[entry : entry + 8])
        if tag == 338:
            assert kind == 3  # SHORT, stored inline
            data[entry + 8 : entry + 10] = struct.pack(order + "H", extra_samples)
            return bytes(data)
    raise AssertionError("no ExtraSamples tag written")


print("Pillow", PIL.__version__)
for compression in (None, "tiff_lzw"):
    for extra in (0, 1, 2):
        try:
            with Image.open(io.BytesIO(grey_with_extra_sample(extra, compression))) as image:
                image.load()
                result = f"ok, mode {image.mode}"
        except Exception as error:
            result = f"{type(error).__name__}: {error}"
        print(f"compression={compression or 'raw'} ExtraSamples={extra}: {result}")