PYTHON-6013 Consolidate session/transaction helpers into a shared file

[NoahStapp]

PYTHON-6013

Changes in this PR

Consolidate code that does not need to be synchronized into a new pymongo/client_session_shared.py.

Test Plan

Existing test suite.

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 refactors PyMongo’s session/transaction implementation by extracting logic that is identical across the synchronous and asynchronous stacks into a new shared module (pymongo/client_session_shared.py), then updates call sites and tests to import from the shared location.

Changes:

• Added pymongo/client_session_shared.py to host shared session/transaction helpers (e.g., _TxnState, server session pool types, retry timing helpers, write-concern validation, and option classes).
• Updated sync/async runtime modules (client_session, topology, pool, bulk/client_bulk, mongo_client) to import the moved helpers from the shared module.
• Updated unified format and transaction tests (sync + async) to reference _TxnState and retry timeout constants from the new shared module.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pymongo/client_session_shared.py	New shared module containing session/transaction helpers used by both sync and async code paths.
pymongo/synchronous/client_session.py	Replaced locally-defined helpers/options with imports from client_session_shared and re-exported options for docs.
pymongo/asynchronous/client_session.py	Same consolidation as sync: imports from client_session_shared and re-exported options for docs.
pymongo/synchronous/topology.py	Switched server session pool imports to the shared module.
pymongo/asynchronous/topology.py	Switched server session pool imports to the shared module.
pymongo/synchronous/pool.py	Switched _validate_session_write_concern import to the shared module.
pymongo/asynchronous/pool.py	Switched _validate_session_write_concern import to the shared module.
pymongo/synchronous/bulk.py	Switched _validate_session_write_concern import to the shared module.
pymongo/asynchronous/bulk.py	Switched _validate_session_write_concern import to the shared module.
pymongo/synchronous/client_bulk.py	Switched _validate_session_write_concern import to the shared module.
pymongo/asynchronous/client_bulk.py	Switched _validate_session_write_concern import to the shared module.
pymongo/synchronous/mongo_client.py	Switched _EmptyServerSession (and typing-only _ServerSession) imports to the shared module.
pymongo/asynchronous/mongo_client.py	Switched _EmptyServerSession (and typing-only _ServerSession) imports to the shared module.
test/unified_format.py	Updated _TxnState import to come from client_session_shared.
test/asynchronous/unified_format.py	Updated _TxnState import to come from client_session_shared.
test/test_transactions.py	Updated retry-timeout patching to target client_session_shared._WITH_TRANSACTION_RETRY_TIME_LIMIT.
test/asynchronous/test_transactions.py	Updated retry-timeout patching to target client_session_shared._WITH_TRANSACTION_RETRY_TIME_LIMIT.

Suppressed comments (1)

pymongo/client_session_shared.py:124

• The TransactionOptions docstring says that when write_concern is None the client's read_preference is used, but the default is the client's write_concern; this is misleading in the rendered docs.

    :param write_concern: The
        :class:`~pymongo.write_concern.WriteConcern` to use for this
        transaction. If ``None`` (the default) the :attr:`read_preference` of
        the client is used.

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Codecov Report

❌ Patch coverage is 94.17989% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pymongo/client_session_shared.py	93.49%	5 Missing and 6 partials ⚠️

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