PYTHON-5906 Raise ProtocolError for malformed OP_COMPRESSED messages in sync receive_message

[aclark4life]

PYTHON-5906

Changes in this PR

The sync receive_message in pymongo/network_layer.py was missing the OP_COMPRESSED minimum-length guard that exists in the async PyMongoProtocol.process_header. For a malformed OP_COMPRESSED frame with 16 < length <= 24, length - 25 is negative, so receive_data calls bytearray(negative) and raises ValueError instead of ProtocolError; for length == 25, decompress was called on 0 bytes.

Added the same guard before entering the op_code == 2012 branch so the sync path raises ProtocolError, matching async parity. No public API changes.

Test Plan

Added test_compressed_length_too_small_raises to test/test_network_layer.py, covering lengths 17, 24, and 25 and asserting ProtocolError. All tests in the file pass.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

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]

Pull request overview

Aligns the synchronous wire-protocol receive path (receive_message) with the existing async PyMongoProtocol.process_header behavior by rejecting malformed OP_COMPRESSED messages whose declared length is too small to include the compression sub-header plus payload. This ensures malformed frames raise ProtocolError (instead of bubbling up ValueError from an invalid receive_data size or attempting to decompress an empty payload).

Changes:

• Add a minimum-length guard for OP_COMPRESSED (length <= 25) in synchronous receive_message, raising ProtocolError.
• Add a sync-only regression test asserting ProtocolError for malformed compressed message lengths (17, 24, 25).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pymongo/network_layer.py	Adds an OP_COMPRESSED minimum-length check in sync receive_message to raise ProtocolError for malformed frames.
test/test_network_layer.py	Adds a regression test covering malformed OP_COMPRESSED lengths that previously could raise ValueError or attempt decompression on empty payload.

[codeowners-service-app]

Assigned sleepyStick for team dbx-python because blink1073 is out of office.