PYTHON-5904 RTT Thread Flips Between isMaster and hello

pymongo/asynchronous/pool.py

@@ -287,7 +287,9 @@ async def _hello(
             hello.logical_session_timeout_minutes is not None and hello.is_readable
         )
         self.logical_session_timeout_minutes: Optional[int] = hello.logical_session_timeout_minutes
-        self.hello_ok = hello.hello_ok
+        # hello_ok is set from helloOk, which is only returned from ismaster
+        # don't overwrite this when connection switches to hello
+        self.hello_ok = self.hello_ok or hello.hello_ok
         self.is_repl = hello.server_type in (
             SERVER_TYPE.RSPrimary,
             SERVER_TYPE.RSSecondary,

pymongo/synchronous/pool.py

@@ -287,7 +287,9 @@ def _hello(
             hello.logical_session_timeout_minutes is not None and hello.is_readable
         )
         self.logical_session_timeout_minutes: Optional[int] = hello.logical_session_timeout_minutes
-        self.hello_ok = hello.hello_ok
+        # hello_ok is set from helloOk, which is only returned from ismaster
+        # don't overwrite this when connection switches to hello
+        self.hello_ok = self.hello_ok or hello.hello_ok
         self.is_repl = hello.server_type in (
             SERVER_TYPE.RSPrimary,
             SERVER_TYPE.RSSecondary,

test/asynchronous/test_hello_latched.py

@@ -0,0 +1,82 @@
+# Copyright 2026-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test that connection continues to use hello after ismaster returns hello_ok"""
+
+from __future__ import annotations
+
+import unittest
+from types import SimpleNamespace
+
+from pymongo.asynchronous.pool import AsyncConnection
+from test.asynchronous import AsyncUnitTest
+
+_IS_SYNC = False
+
+
+class TestHelloLatched(AsyncUnitTest):
+    def setUp(self):
+        self._sent = []
+
+    def create_connection(self) -> AsyncConnection:
+        """Returns a minimal connection object for _hello"""
+        conn = object.__new__(AsyncConnection)
+        conn.hello_ok = False
+        conn.performed_handshake = True
+        conn.opts = SimpleNamespace(server_api=None, load_balanced=False, _credentials=None)
+
+        return conn
+
+    async def test_hello_is_latched(self):
+        """
+        Regression Test for PYTHON-5904
+        Tests for connection hello_ok persistence when connection
+        Switches from ismaster to hello
+        """
+
+        async def mock_conn_command(db, cmd, **kwargs):
+            """Returns mocked hello and ismaster results for conn.command"""
+            self._sent.append(cmd.copy())
+            if cmd.get("ismaster") == 1:
+                return {"ok": 1, "helloOk": True, "ismaster": True, "maxWireVersion": 25}
+            return {"ok": 1, "isWritablePrimary": True, "maxWireVersion": 25}
+
+        conn = self.create_connection()
+        conn.command = mock_conn_command
+
+        # First hello
+        await conn._hello(None, None)
+        # Verify hello_ok is True
+        self.assertTrue(conn.hello_ok)
+        # Verify command sent is ismaster
+        self.assertEqual(self._sent[0].get("ismaster"), 1)
+        self.assertEqual(self._sent[0].get("helloOk"), True)
+
+        # Second hello
+        await conn._hello(None, None)
+        # Verify hello_ok has not changed
+        self.assertTrue(conn.hello_ok)
+        # Verify command sent is hello
+        self.assertEqual(self._sent[1].get("hello"), 1)
+        self.assertIsNone(self._sent[1].get("ismaster", None))
+
+        # Third hello
+        await conn._hello(None, None)
+        # Verify connection continues to use hello
+        self.assertEqual(self._sent[2].get("hello"), 1)
+        self.assertIsNone(self._sent[2].get("ismaster", None))
+
+
+if __name__ == "__main__":
+    unittest.main()

test/test_hello_latched.py

@@ -0,0 +1,82 @@
+# Copyright 2026-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test that connection continues to use hello after ismaster returns hello_ok"""
+
+from __future__ import annotations
+
+import unittest
+from types import SimpleNamespace
+
+from pymongo.synchronous.pool import Connection
+from test import UnitTest
+
+_IS_SYNC = True
+
+
+class TestHelloLatched(UnitTest):
+    def setUp(self):
+        self._sent = []
+
+    def create_connection(self) -> Connection:
+        """Returns a minimal connection object for _hello"""
+        conn = object.__new__(Connection)
+        conn.hello_ok = False
+        conn.performed_handshake = True
+        conn.opts = SimpleNamespace(server_api=None, load_balanced=False, _credentials=None)
+
+        return conn
+
+    def test_hello_is_latched(self):
+        """
+        Regression Test for PYTHON-5904
+        Tests for connection hello_ok persistence when connection
+        Switches from ismaster to hello
+        """
+
+        def mock_conn_command(db, cmd, **kwargs):
+            """Returns mocked hello and ismaster results for conn.command"""
+            self._sent.append(cmd.copy())
+            if cmd.get("ismaster") == 1:
+                return {"ok": 1, "helloOk": True, "ismaster": True, "maxWireVersion": 25}
+            return {"ok": 1, "isWritablePrimary": True, "maxWireVersion": 25}
+
+        conn = self.create_connection()
+        conn.command = mock_conn_command
+
+        # First hello
+        conn._hello(None, None)
+        # Verify hello_ok is True
+        self.assertTrue(conn.hello_ok)
+        # Verify command sent is ismaster
+        self.assertEqual(self._sent[0].get("ismaster"), 1)
+        self.assertEqual(self._sent[0].get("helloOk"), True)
+
+        # Second hello
+        conn._hello(None, None)
+        # Verify hello_ok has not changed
+        self.assertTrue(conn.hello_ok)
+        # Verify command sent is hello
+        self.assertEqual(self._sent[1].get("hello"), 1)
+        self.assertIsNone(self._sent[1].get("ismaster", None))
+
+        # Third hello
+        conn._hello(None, None)
+        # Verify connection continues to use hello
+        self.assertEqual(self._sent[2].get("hello"), 1)
+        self.assertIsNone(self._sent[2].get("ismaster", None))
+
+
+if __name__ == "__main__":
+    unittest.main()