Rewind file streams before rate-limit retries

[CR29-22-2805]

Summary

Rewind seekable multipart file streams before retrying a POST request after a retryable RATELIMIT response.

Without rewinding, the first request consumes the stream and a retry may upload an empty or truncated file.

Changes

• Record the initial position of seekable file streams passed to Reddit.post()
• Restore those positions before each retry
• Leave non-seekable streams unchanged
• Add a regression test covering a rate-limit retry with a partially advanced BytesIO stream

Fixes #2178

Tests

• python -m pytest tests/unit/test_reddit.py -k post_ratelimit
• python -m pytest tests/unit/test_reddit.py

[bboe]

Thanks, this is a well-scoped fix and the regression test starting the stream at a non-zero offset is a nice touch. Two things:

1. Please add a CHANGES.rst entry under Unreleased — this is a user-visible bug fix.
2. Silently skipping non-seekable streams leaves the original bug in the case that's hardest to diagnose. I'd note in the post() docstring that automatic retries only preserve seekable file objects.

[bboe]

Equivalent behavior, but the loop counter reads more directly than the attempts < max_attempts - 1 arithmetic.

Suggested change

	max_attempts = 3
	attempts = max_attempts
	last_exception: RedditAPIException | None = None
	while attempts > 0:
	attempts -= 1
	if attempts < max_attempts - 1 and file_positions:
	assert files is not None
	for name, position in file_positions.items():
	files[name].seek(position)
	last_exception: RedditAPIException | None = None
	for attempt in range(3):
	if attempt and file_positions:
	assert files is not None
	for name, position in file_positions.items():
	files[name].seek(position)

[bboe]

One more thing: if you'd like, please add yourself to AUTHORS.rst — it's listed as a step in contributing and you're not in there yet. New entries go just above the Add "Name <email (optional)> and github profile link" above this line. marker at the bottom, following the existing format:

- Your Name `@CR29-22-2805 <https://github.com/CR29-22-2805>`_

Entirely optional, but the contribution is worth the credit.