Incorrect behaviour of the ImageFile._safe_read function

[leon9812]

What did you do?

Randomly came across the ImageFile._safe_read fuction.

What did you expect to happen?

An "OSError: Truncated File Read" if the length of the file-like object fp is less than the value of the size parameter.

What actually happened?

Not always getting an OSError if the value for size is larger than ImageFile.SAFEBLOCK (and the length of fp is less than size).

What are your OS, Python and Pillow versions?

• OS: Windows 10 (21H1)
• Python: 3.8.10
• Pillow: 8.4.0

import io
from PIL import ImageFile

fp = io.BytesIO(b"0" * ImageFile.SAFEBLOCK)
size = ImageFile.SAFEBLOCK + 1

# no error, but expected OSError:
content = ImageFile._safe_read(fp=fp, size=size)

print(len(content))
# 1048576
print(size)
# 1048577

[hugovk]

Why are you calling _safe_read? It's a "private" function and not part of the public API, as denoted by the leading underscore.

Is there a concrete problem when calling this via the public API?

[leon9812]

I didn't find a problem via the public API. I just found this function because it raised an exception for an incompatible / truncated image and noticed, that it does not always do what its docstring describes. I just thought it is worth reporting so there are no problems in the future (regarding that function).

[radarhere]

I've figured out a test case for this using our public APIs, and created #5872 accordingly.

[hugovk]

Merged, thanks both!