Bug report

Bug description:

Since 3.14, zipfile.is_zipfile() raises ValueError: negative seek value instead of returning False when given a file-like object such as io.BytesIO whose end of central directory record places the central directory before offset 0. A real file returns False, because the OSError from seek() is caught there; the ValueError also skips restoring the caller's file position.

import io, struct, zipfile

buf = io.BytesIO()
with zipfile.ZipFile(buf, "w") as zf:
    zf.writestr("a.txt", b"hello")
data = bytearray(buf.getvalue())
struct.pack_into("<L", data, len(data) - 22 + 12, len(data))  # inflate CD size
zipfile.is_zipfile(io.BytesIO(data))  # ValueError: negative seek value -...

Expected False, as for the same bytes on disk. This was introduced by gh-134250 (gh-72680).

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-161094: Fix zipfile.is_zipfile() raising ValueError on file-like objects #161095
• [3.15] gh-161094: Fix zipfile.is_zipfile() raising ValueError on file-like objects (GH-161095) #161120
• [3.14] gh-161094: Fix zipfile.is_zipfile() raising ValueError on file-like objects (GH-161095) #161121