[blink1073]

PYTHON-6163

Changes in this PR

The pure-Python decoder silently accepted buffers ending in a partial document, while the C extension raises InvalidBSON. The Python version now applies the same checks and messages.

Test Plan

• Added a regression test covering 1 to 4 byte buffers, a trailing partial after a valid document, and empty input, across bytes, bytearray, and memoryview.
• BSON suite passes with and without the C extension (100 tests each); the new test fails without the fix in pure-Python mode.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? Not yet; add an entry under unreleased bug fixes before merge.
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s). PYTHON-4232

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[Copilot]

🟡 Changes recommended

Align the error message with the C implementation and add the required changelog entry.

2 open findings

• Match C decoder error message for oversized documents · New
• Document pure-Python decode_all truncation fix in changelog · New

What changed in this PR

Updates pure-Python decode_all to reject truncated BSON consistently with the C extension.

Changes:

• Adds BSON size and boundary validation.
• Adds regression tests for partial buffers and supported buffer types.

File	Summary
bson/​__init__.py	Adds pure-Python BSON size validation.
test/​test_bson.py	Adds truncated-document regression coverage.

🧠 Review effort: Lite

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[Copilot]

🟢 Approval recommended

The validation, regression tests, and changelog updates fully address the reported issue.

0 open findings

2 resolved since last review

• Match C decoder error message for oversized documents
• Document pure-Python decode_all truncation fix in changelog

🧠 Review effort: Lite

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bson/__init__.py	16.66%	3 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[aclark4life]

LGTM!