PYTHON-3349 Don't clear entire load balanced pool when serviceId is unknown (#1001)

Author: blink1073

CONTRIBUTING.rst

@@ -83,9 +83,9 @@ Running Load Balancer Tests Locally
 -----------------------------------
 - Install ``haproxy`` (available as ``brew install haproxy`` on macOS).
 - Clone ``drivers-evergreen-tools``: ``git clone git@github.com:mongodb-labs/drivers-evergreen-tools.git``.
-- Start the servers using ``LOAD_BALANCER=true TOPOLOGY=sharded_cluster AUTH=noauth SSL=nossl MONGODB_VERSION=6.0 DRIVERS_TOOLS=./drivers-evergreen-tools MONGO_ORCHESTRATION_HOME=./drivers-evergreen-tools/.evergreen/orchestration ./drivers-evergreen-tools/.evergreen/run-orchestration.sh``.
-- Start the load balancer using: ``MONGODB_URI='mongodb://localhost:27017,localhost:27018/' .evergreen/run-load-balancer.sh start``.
-- Run the tests using: ``LOADBALANCER=1 TEST_LOADBALANCER=1 SINGLE_MONGOS_LB_URI='mongodb://127.0.0.1:8000/?loadBalanced=true' MULTI_MONGOS_LB_URI='mongodb://127.0.0.1:8001/?loadBalanced=true' MONGODB_URI='mongodb://localhost:27017,localhost:27018/' python setup.py test -s test.test_load_balancer``.
+- Start the servers using ``LOAD_BALANCER=true TOPOLOGY=sharded_cluster AUTH=noauth SSL=nossl MONGODB_VERSION=6.0 DRIVERS_TOOLS=$PWD/drivers-evergreen-tools MONGO_ORCHESTRATION_HOME=$PWD/drivers-evergreen-tools/.evergreen/orchestration $PWD/drivers-evergreen-tools/.evergreen/run-orchestration.sh``.
+- Start the load balancer using: ``MONGODB_URI='mongodb://localhost:27017,localhost:27018/' $PWD/drivers-evergreen-tools/.evergreen/run-load-balancer.sh start``.
+- Run the tests from the ``pymongo`` checkout directory using: ``LOADBALANCER=1 TEST_LOADBALANCER=1 SINGLE_MONGOS_LB_URI='mongodb://127.0.0.1:8000/?loadBalanced=true' MULTI_MONGOS_LB_URI='mongodb://127.0.0.1:8001/?loadBalanced=true' MONGODB_URI='mongodb://localhost:27017,localhost:27018/' python setup.py test -s test.test_load_balancer``.
 
 Re-sync Spec Tests
 ------------------

pymongo/topology.py

@@ -644,6 +644,14 @@ def _handle_error(self, address, err_ctx):
         error = err_ctx.error
         exc_type = type(error)
         service_id = err_ctx.service_id
+
+        # Ignore a handshake error if the server is behind a load balancer but
+        # the service ID is unknown. This indicates that the error happened
+        # when dialing the connection or during the MongoDB  handshake, so we
+        # don't know the service ID to use for clearing the pool.
+        if self._settings.load_balanced and not service_id and not err_ctx.completed_handshake:
+            return
+
         if issubclass(exc_type, NetworkTimeout) and err_ctx.completed_handshake:
             # The socket has been closed. Don't reset the server.
             # Server Discovery And Monitoring Spec: "When an application