`hashlib.file_digest` does not honor current position for `io.BytesIO` inputs

[picnixz]

Bug report

Bug description:

hashlib.file_digest() takes a getbuffer() fast path that hashes the entire buffer regardless of tell(), so the same bytes at the same position give a different digest for io.BytesIO than for a real binary file.

We never had tests to exercise that behavior and we never got report about it. I want to sync the merges of all PRs at once so that the behavior is fixed simultaneously before the next release.

cc @gpshead @hugovk

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-155419: honor the file object's current position in hashlib.file_digest #155422

[picnixz]

I decided to fix this instead of document the inconsistency because when not given a io.BytesIO object, we are actually honoring the position. So it's a bug.