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Type: Bug
Resolution: Unresolved
Priority: Minor - P4
Fix Version/s: None
Affects Version/s: None
Component/s: None
Labels:
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The sessions spec discusses several races that could occur when checking if a cluster supports sessions: https://github.com/mongodb/specifications/blob/a9096ad/source/sessions/driver-sessions.rst#possible-race-conditions-when-checking-whether-a-deployment-supports-sessions

Pymongo does not implement this requirement:

To handle these race conditions, the driver MUST ignore any implicit session if at the point it is sending a command to a specific server it turns out that that particular server doesn't support sessions after all.

Additionally after the changes in ~~PYTHON-2956~~, pymongo checks the topology for session support twice: first when creating the implicit session and a second time when "materializing" the server session object. The second check should be removed as it causes a ConfigurationError: Sessions are not supported by this MongoDB deployment error if the topology no longer supports sessions. As seen in HELP-41313

Edit: This is the error in question:

Traceback (most recent call last):
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/src/buildscripts/resmokelib/testing/hooks/tenant_migration.py", line 628, in _wait_for_reroute_or_test_completion
    doc = donor_primary_client["testTenantMigration"]["rerouted"].find_one(
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/collection.py", line 1459, in find_one
    for result in cursor.limit(-1):
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/cursor.py", line 1248, in next
    if len(self.__data) or self._refresh():
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/cursor.py", line 1165, in _refresh
    self.__send_message(q)
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/cursor.py", line 1052, in __send_message
    response = client._run_operation(
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/_csot.py", line 105, in csot_wrapper
    return func(self, *args, **kwargs)
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/mongo_client.py", line 1330, in _run_operation
    return self._retryable_read(
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/_csot.py", line 105, in csot_wrapper
    return func(self, *args, **kwargs)
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/mongo_client.py", line 1448, in _retryable_read
    return func(session, server, sock_info, read_pref)
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/mongo_client.py", line 1326, in _cmd
    return server.run_operation(
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/server.py", line 100, in run_operation
    message = operation.get_message(read_preference, sock_info, use_cmd)
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/message.py", line 387, in get_message
    spec = self.as_command(sock_info, apply_timeout=True)[0]
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/message.py", line 358, in as_command
    session._apply_to(cmd, False, self.read_preference, sock_info)
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/client_session.py", line 959, in _apply_to
    self._materialize()
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/client_session.py", line 953, in _materialize
    self._server_session = self._client._topology.get_server_session()
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/topology.py", line 566, in get_server_session
    session_timeout = self._check_session_support()
  File "/data/mci/ba7a8bb79aa8310ff27545993d5601d7/venv/lib/python3.10/site-packages/pymongo/topology.py", line 560, in _check_session_support
    raise ConfigurationError("Sessions are not supported by this MongoDB deployment")
pymongo.errors.ConfigurationError: Sessions are not supported by this MongoDB deployment

related to

PYTHON-2956 Drivers should check out an implicit session only after checking out a connection

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