PYTHON-5809 - Send afterClusterTime on writes in causally-consistent sessions

[NoahStapp]

PYTHON-5809

Changes in this PR

Send afterClusterTime on writes in causally-consistent sessions to prevent possible stale primary writes.

Test Plan

Synced the new unified tests.

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

This PR updates PyMongo’s session command decoration so that write commands executed in causally consistent sessions (and not in a transaction) include readConcern.afterClusterTime once the session has an operationTime, preventing potentially stale primary writes.

Changes:

• Add a shared allowlist of write command names and use it in ClientSession._apply_to / AsyncClientSession._apply_to to attach afterClusterTime on eligible writes in causally consistent sessions.
• Extend the unified test runner to support the dropDatabase client operation and sync new causal-consistency unified spec tests.
• Update existing unified transaction/retryable-write expectations and remove the obsolete Evergreen spec patch.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pymongo/operations.py	Adds _WRITES_WITH_CLUSTER_TIME allowlist for command names that should carry afterClusterTime on causal writes.
pymongo/synchronous/client_session.py	Applies afterClusterTime to eligible write commands in causally consistent sessions (non-transaction).
pymongo/asynchronous/client_session.py	Async equivalent of the session write afterClusterTime behavior.
test/unified_format.py	Adds unified runner support for client.dropDatabase.
test/asynchronous/unified_format.py	Async unified runner support for client.dropDatabase.
test/test_session.py	Removes the write “no readConcern” assertion; keeps coverage for explain/getMore behavior.
test/asynchronous/test_session.py	Async equivalent adjustment to the session tests.
test/causal_consistency/causal-consistency-write-commands.json	New unified spec coverage asserting afterClusterTime on causal write commands.
test/causal_consistency/causal-consistency-clientBulkWrite.json	New unified spec coverage for bulkWrite (clientBulkWrite) including afterClusterTime.
test/test_causal_consistency.py	Sync unified spec test generator entry point for causal consistency scenarios.
test/asynchronous/test_causal_consistency.py	Async unified spec test generator entry point for causal consistency scenarios.
test/transactions/unified/retryable-writes.json	Updates expected command monitoring events to include afterClusterTime in readConcern on relevant writes.
test/transactions/unified/commit.json	Updates expected events to include afterClusterTime where applicable.
test/transactions-convenient-api/unified/callback-commits.json	Updates expected transaction start events to include afterClusterTime.
test/transactions-convenient-api/unified/callback-aborts.json	Updates expected transaction start events to include afterClusterTime.
tools/synchro.py	Includes the new causal consistency test module in the converted test list.
.evergreen/resync-specs.sh	Adds a causal_consistency resync target for pulling spec tests.
.evergreen/spec-patch/PYTHON-5809.patch	Removes the now-unneeded patch that previously adjusted spec expectations.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[blink1073]

Why are these tests removed?

[NoahStapp]

As a result of these changes, causally-consistent writes (the default for an explicit session) now include an empty read concern to communicate afterClusterTime.

[blink1073]

LGTM