PYTHON-5885 - Client Backpressure with retryAfterMS

[NoahStapp]

PYTHON-5885

Changes in this PR

Adds retryAfterMS support for backoff calculation on retryable overload errors.

Test Plan

Adds a new prose test test_05_overload_errors_with_retryafterms_override_backoff to verify the retryAfterMS behavior. Note that this new test requires a server with retryAfterMS support.

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?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[NoahStapp]

Note: SERVER-130142 will rename retryAfterMS to baseBackoffMS. That change will require the spec to update as well as this PR.

[Copilot]

Pull request overview

Adds support for using retryAfterMS from retryable overload errors to influence the client’s adaptive retry backoff, plus updates handshake backpressure metadata and related tests/docs.

Changes:

• Parse and propagate retryAfterMS from server error documents into the overload backoff calculation.
• Update hello/handshake backpressure field value (and metadata tests) to "2".
• Add a new prose-style timing test validating that retryAfterMS reduces overload backoff.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
pymongo/errors.py	Parses retryAfterMS from error details into PyMongoError.
pymongo/synchronous/mongo_client.py	Uses retryAfterMS override when computing overload retry backoff (sync).
pymongo/asynchronous/mongo_client.py	Uses retryAfterMS override when computing overload retry backoff (async).
pymongo/synchronous/helpers.py	Adjusts backoff helper to accept an override base backoff (sync).
pymongo/asynchronous/helpers.py	Adjusts backoff helper to accept an override base backoff (async).
pymongo/synchronous/pool.py	Updates handshake hello backpressure value (sync).
pymongo/asynchronous/pool.py	Updates handshake hello backpressure value (async).
test/test_client_backpressure.py	Adds a new test for retryAfterMS-driven backoff behavior (sync).
test/asynchronous/test_client_backpressure.py	Adds a new test for retryAfterMS-driven backoff behavior (async).
test/test_client_metadata.py	Updates handshake backpressure assertion (sync).
test/asynchronous/test_client_metadata.py	Updates handshake backpressure assertion (async).
doc/changelog.rst	Updates changelog entry for 4.18.0.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.