[lohmag]

PYTHON-XXXX

Changes in this PR

ZstdContext.compress called zstd.compress() for every message, which creates and frees a zstd compression context each time. For a small message, setting up the context costs more than compressing it.

It now keeps one ZstdCompressor per thread in a threading.local, and compresses each message as a complete frame with compress(data, ZstdCompressor.FLUSH_FRAME).

• Thread safety. A compressor serializes its calls, so a shared one would make threads wait on each other.
• Errors. If a call raises, the thread's compressor is dropped and the next call creates a new one.

The output is a standard zstd frame, the same as before, and decompression is unchanged.

ZstdContext.compress, document-like payloads, master → this PR. Python 3.12, backports.zstd (zstd 1.5.7), Intel Core Ultra 7 265, one pinned core:

Size	Time
50 B	1.56 → 0.54 µs
200 B	3.24 → 2.26 µs
1 KiB	4.62 → 3.49 µs
4 KiB	9.42 → 7.88 µs
16 KiB	31.2 → 27.8 µs
64 KiB	126.3 → 119.5 µs

Compressed sizes are unchanged.

Test Plan

• test_zstd_compressor_reused_across_messages: 64 messages of different sizes through the same thread's compressor. Each one decompresses with zstd.decompress and with pymongo.compression_support.decompress.
• test_zstd_compress_from_many_threads: 8 threads compress and decompress 200 messages each.
• test/test_compression_support.py: 36 passed, 4 skipped (python-snappy not installed).
• Against a MongoDB 8.0.20 standalone with COMPRESSORS=zstd: test/test_client.py -k compress (2 passed), and test/test_collection.py, test/test_cursor.py and test/test_bulk.py (187 passed, 15 skipped because they need a replica set).
• ruff check and ruff format --check pass on the changed files.

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?

_{With Claude's help.}