PYTHON-2059 Do not send readPreference with OP_MSG getMore commands

Author: ShaneHarvey

pymongo/message.py

@@ -422,7 +422,7 @@ def get_message(self, dummy0, sock_info, use_cmd=False):
             spec = self.as_command(sock_info)[0]
             if sock_info.op_msg_enabled:
                 request_id, msg, size, _ = _op_msg(
-                    0, spec, self.db, ReadPreference.PRIMARY,
+                    0, spec, self.db, None,
                     False, False, self.codec_options,
                     ctx=sock_info.compression_context)
                 return request_id, msg, size
@@ -685,7 +685,8 @@ def _op_msg(flags, command, dbname, read_preference, slave_ok, check_keys,
             opts, ctx=None):
     """Get a OP_MSG message."""
     command['$db'] = dbname
-    if "$readPreference" not in command:
+    # getMore commands do not send $readPreference.
+    if read_preference is not None and "$readPreference" not in command:
         if slave_ok and not read_preference.mode:
             command["$readPreference"] = (
                 ReadPreference.PRIMARY_PREFERRED.document)

test/test_cursor.py

@@ -40,6 +40,7 @@
                             InvalidOperation,
                             OperationFailure)
 from pymongo.read_concern import ReadConcern
+from pymongo.read_preferences import ReadPreference
 from test import (client_context,
                   unittest,
                   IntegrationTest)
@@ -1407,6 +1408,26 @@ def test_delete_not_initialized(self):
         cursor = Cursor.__new__(Cursor)  # Skip calling __init__
         cursor.__del__()  # no error
 
+    @client_context.require_version_min(3, 6)
+    def test_getMore_does_not_send_readPreference(self):
+        listener = WhiteListEventListener('find', 'getMore')
+        client = rs_or_single_client(
+            event_listeners=[listener])
+        self.addCleanup(client.close)
+        coll = client[self.db.name].test
+
+        coll.delete_many({})
+        coll.insert_many([{} for _ in range(5)])
+        self.addCleanup(coll.drop)
+
+        list(coll.find(batch_size=3))
+        started = listener.results['started']
+        self.assertEqual(2, len(started))
+        self.assertEqual('find', started[0].command_name)
+        self.assertIn('$readPreference', started[0].command)
+        self.assertEqual('getMore', started[1].command_name)
+        self.assertNotIn('$readPreference', started[1].command)
+
 
 class TestRawBatchCursor(IntegrationTest):
     def test_find_raw(self):