PYTHON-2970 SDAM should give priority to electionId over setVersion when updating topology

Author: ShaneHarvey

pymongo/topology_description.py

@@ -531,23 +531,29 @@ def _update_rs_from_primary(
         sds.pop(server_description.address)
         return (_check_has_primary(sds), replica_set_name, max_set_version, max_election_id)
 
-    max_election_tuple = max_set_version, max_election_id
-    if None not in server_description.election_tuple:
-        if (
-            None not in max_election_tuple
-            and max_election_tuple > server_description.election_tuple
-        ):
-
-            # Stale primary, set to type Unknown.
-            sds[server_description.address] = server_description.to_unknown()
-            return (_check_has_primary(sds), replica_set_name, max_set_version, max_election_id)
+    max_election_tuple = max_election_id, max_set_version
+    new_election_tuple = tuple(reversed(server_description.election_tuple))
+    new_election_id = server_description.election_id
+    new_set_version = server_description.set_version
+    if None not in new_election_tuple and (
+        None not in max_election_tuple
+        and (
+            max_election_id > new_election_id
+            or (max_election_id == new_election_id and max_set_version > new_set_version)
+        )
+    ):
+        # Stale primary, set to type Unknown.
+        sds[server_description.address] = server_description.to_unknown()
+        return _check_has_primary(sds), replica_set_name, max_set_version, max_election_id
 
+    if new_election_id is not None and (
+        max_election_id is None or new_election_id > max_election_id
+    ):
         max_election_id = server_description.election_id
 
-    if server_description.set_version is not None and (
-        max_set_version is None or server_description.set_version > max_set_version
+    if new_set_version is not None and (
+        max_set_version is None or new_set_version > max_set_version
     ):
-
         max_set_version = server_description.set_version
 
     # We've heard from the primary. Is it the same primary as before?

test/discovery_and_monitoring/rs/electionId_setVersion.json

@@ -0,0 +1,92 @@
+{
+  "description": "ElectionId is considered higher precedence than setVersion",
+  "uri": "mongodb://a/?replicaSet=rs",
+  "phases": [
+    {
+      "responses": [
+        [
+          "a:27017",
+          {
+            "ok": 1,
+            "helloOk": true,
+            "isWritablePrimary": true,
+            "hosts": [
+              "a:27017",
+              "b:27017"
+            ],
+            "setName": "rs",
+            "setVersion": 1,
+            "electionId": {
+              "$oid": "000000000000000000000001"
+            },
+            "minWireVersion": 0,
+            "maxWireVersion": 6
+          }
+        ],
+        [
+          "b:27017",
+          {
+            "ok": 1,
+            "helloOk": true,
+            "isWritablePrimary": true,
+            "hosts": [
+              "a:27017",
+              "b:27017"
+            ],
+            "setName": "rs",
+            "setVersion": 2,
+            "electionId": {
+              "$oid": "000000000000000000000001"
+            },
+            "minWireVersion": 0,
+            "maxWireVersion": 6
+          }
+        ],
+        [
+          "a:27017",
+          {
+            "ok": 1,
+            "helloOk": true,
+            "isWritablePrimary": true,
+            "hosts": [
+              "a:27017",
+              "b:27017"
+            ],
+            "setName": "rs",
+            "setVersion": 1,
+            "electionId": {
+              "$oid": "000000000000000000000002"
+            },
+            "minWireVersion": 0,
+            "maxWireVersion": 6
+          }
+        ]
+      ],
+      "outcome": {
+        "servers": {
+          "a:27017": {
+            "type": "RSPrimary",
+            "setName": "rs",
+            "setVersion": 1,
+            "electionId": {
+              "$oid": "000000000000000000000002"
+            }
+          },
+          "b:27017": {
+            "type": "Unknown",
+            "setName": null,
+            "setVersion": null,
+            "electionId": null
+          }
+        },
+        "topologyType": "ReplicaSetWithPrimary",
+        "logicalSessionTimeoutMinutes": null,
+        "setName": "rs",
+        "maxSetVersion": 2,
+        "maxElectionId": {
+          "$oid": "000000000000000000000002"
+        }
+      }
+    }
+  ]
+}

test/discovery_and_monitoring/rs/use_setversion_without_electionid.json

@@ -115,22 +115,25 @@
       "outcome": {
         "servers": {
           "a:27017": {
+            "type": "RSPrimary",
+            "setName": "rs",
+            "setVersion": 1,
+            "electionId": {
+              "$oid": "000000000000000000000002"
+            }
+          },
+          "b:27017": {
             "type": "Unknown",
             "setName": null,
             "electionId": null
-          },
-          "b:27017": {
-            "type": "RSPrimary",
-            "setName": "rs",
-            "setVersion": 2
           }
         },
         "topologyType": "ReplicaSetWithPrimary",
         "logicalSessionTimeoutMinutes": null,
         "setName": "rs",
         "maxSetVersion": 2,
         "maxElectionId": {
-          "$oid": "000000000000000000000001"
+          "$oid": "000000000000000000000002"
         }
       }
     }