PYTHON-4780 Implement fast path for server selection with Primary

.github/workflows/zizmor.yml

@@ -17,16 +17,5 @@ jobs:
         uses: actions/checkout@v4
         with:
           persist-credentials: false
-      - name: Setup Rust
-        uses: actions-rust-lang/setup-rust-toolchain@fb51252c7ba57d633bc668f941da052e410add48 # v1
-      - name: Get zizmor
-        run: cargo install zizmor
       - name: Run zizmor 🌈
-        run: zizmor --format sarif . > results.sarif
-        env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-      - name: Upload SARIF file
-        uses: github/codeql-action/upload-sarif@39edc492dbe16b1465b0cafca41432d857bdb31a # v3
-        with:
-          sarif_file: results.sarif
-          category: zizmor
+        uses: zizmorcore/zizmor-action@1c7106082dbc1753372e3924b7da1b9417011a21

README.md

@@ -14,7 +14,7 @@ a native Python driver for MongoDB, offering both synchronous and asynchronous A
 [gridfs](https://github.com/mongodb/specifications/blob/master/source/gridfs/gridfs-spec.md/)
 implementation on top of `pymongo`.
 
-PyMongo supports MongoDB 4.0, 4.2, 4.4, 5.0, 6.0, 7.0, and 8.0. PyMongo follows [semantic versioning](https://semver.org/spec/v2.0.0.html) for its releases. 
+PyMongo supports MongoDB 4.0, 4.2, 4.4, 5.0, 6.0, 7.0, and 8.0. PyMongo follows [semantic versioning](https://semver.org/spec/v2.0.0.html) for its releases.
 
 ## Support / Feedback
 

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 isinstance(
+            selector, Primary
+        ):
+            for sd in self._server_descriptions.values():
+                if sd.server_type == SERVER_TYPE.RSPrimary:
+                    return [sd]
+
         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):