PYTHON-4780 Implement fast path for server selection with Primary

.github/workflows/zizmor.yml

@@ -18,4 +18,4 @@ jobs:
         with:
           persist-credentials: false
       - name: Run zizmor 🌈
-        uses: zizmorcore/zizmor-action@0f0557ab4a0b31211d42435e42df31cbd63fdd59
+        uses: zizmorcore/zizmor-action@1c7106082dbc1753372e3924b7da1b9417011a21

doc/changelog.rst

@@ -10,6 +10,7 @@ PyMongo 4.14 brings a number of changes including:
 - Added :meth:`pymongo.asynchronous.mongo_client.AsyncMongoClient.append_metadata` and
   :meth:`pymongo.mongo_client.MongoClient.append_metadata` to allow instantiated MongoClients to send client metadata
   on-demand
+- Improved performance of selecting a server with the Primary selector.
 
 - Introduces a minor breaking change. When encoding :class:`bson.binary.BinaryVector`, a ``ValueError`` will be raised
   if the 'padding' metadata field is < 0 or > 7, or non-zero for any type other than PACKED_BIT.

pymongo/topology_description.py

@@ -34,7 +34,7 @@
 from bson.objectid import ObjectId
 from pymongo import common
 from pymongo.errors import ConfigurationError, PyMongoError
-from pymongo.read_preferences import ReadPreference, _AggWritePref, _ServerMode
+from pymongo.read_preferences import Primary, ReadPreference, _AggWritePref, _ServerMode
 from pymongo.server_description import ServerDescription
 from pymongo.server_selectors import Selection
 from pymongo.server_type import SERVER_TYPE
@@ -324,6 +324,14 @@ def apply_selector(
             description = self.server_descriptions().get(address)
             return [description] if description else []
 
+        # Primary selection fast path.
+        if self.topology_type == TOPOLOGY_TYPE.ReplicaSetWithPrimary and type(selector) is Primary:
+            for sd in self._server_descriptions.values():
+                if sd.server_type == SERVER_TYPE.RSPrimary:
+                    return [sd]
+            # No primary found, return an empty list.
+            return []
+
         selection = Selection.from_topology_description(self)
         # Ignore read preference for sharded clusters.
         if self.topology_type != TOPOLOGY_TYPE.Sharded:

test/asynchronous/test_server_selection.py

@@ -130,12 +130,12 @@ async def test_selector_called(self):
         test_collection = mongo_client.testdb.test_collection
         self.addAsyncCleanup(mongo_client.drop_database, "testdb")
 
-        # Do N operations and test selector is called at least N times.
+        # Do N operations and test selector is called at least N-1 times due to fast path.
         await test_collection.insert_one({"age": 20, "name": "John"})
         await test_collection.insert_one({"age": 31, "name": "Jane"})
         await test_collection.update_one({"name": "Jane"}, {"$set": {"age": 21}})
         await test_collection.find_one({"name": "Roe"})
-        self.assertGreaterEqual(selector.call_count, 4)
+        self.assertGreaterEqual(selector.call_count, 3)
 
     @async_client_context.require_replica_set
     async def test_latency_threshold_application(self):

test/test_server_selection.py

@@ -130,12 +130,12 @@ def test_selector_called(self):
         test_collection = mongo_client.testdb.test_collection
         self.addCleanup(mongo_client.drop_database, "testdb")
 
-        # Do N operations and test selector is called at least N times.
+        # Do N operations and test selector is called at least N-1 times due to fast path.
         test_collection.insert_one({"age": 20, "name": "John"})
         test_collection.insert_one({"age": 31, "name": "Jane"})
         test_collection.update_one({"name": "Jane"}, {"$set": {"age": 21}})
         test_collection.find_one({"name": "Roe"})
-        self.assertGreaterEqual(selector.call_count, 4)
+        self.assertGreaterEqual(selector.call_count, 3)
 
     @client_context.require_replica_set
     def test_latency_threshold_application(self):