RUBY-912 Make sure that max_election_id is never reset to nil

lib/mongo/cluster/topology/replica_set.rb

@@ -67,7 +67,7 @@ def elect_primary(description, servers)
                   server.description.unknown!
                 end
               end
-              @max_election_id = description.election_id
+              update_max_election_id(description)
             end
           else
             log_warn([
@@ -219,6 +219,12 @@ def standalone_discovered; self; end
 
         private
 
+        def update_max_election_id(description)
+          if description.election_id && description.election_id > @max_election_id
+            @max_election_id = description.election_id
+          end
+        end
+
         def detect_stale_primary!(description)
           if description.election_id && description.election_id < @max_election_id
             description.unknown!

spec/support/sdam/rs/null_election_id.yml

@@ -1,4 +1,4 @@
-description: "Primary with no electionId, then a primary with electionId"
+description: "Primaries with and without electionIds"
 
 uri: "mongodb://a/?replicaSet=rs"
 
@@ -10,7 +10,7 @@ phases: [
             ["a:27017", {
                 ok: 1,
                 ismaster: true,
-                hosts: ["a:27017", "b:27017"],
+                hosts: ["a:27017", "b:27017", "c:27017"],
                 setName: "rs"
             }]
         ],
@@ -26,11 +26,15 @@ phases: [
                     type: "Unknown",
                     setName: ,
                     electionId:
+                },
+                "c:27017": {
+                    type: "Unknown",
+                    setName: ,
+                    electionId:
                 }
             },
             topologyType: "ReplicaSetWithPrimary",
             setName: "rs",
-            maxElectionId:
         }
     },
 
@@ -40,9 +44,9 @@ phases: [
             ["b:27017", {
                 ok: 1,
                 ismaster: true,
-                hosts: ["a:27017", "b:27017"],
+                hosts: ["a:27017", "b:27017", "c:27017"],
                 setName: "rs",
-                electionId: {"$oid": "000000000000000000000001"}
+                electionId: {"$oid": "000000000000000000000002"}
             }]
         ],
 
@@ -56,12 +60,16 @@ phases: [
                 "b:27017": {
                     type: "RSPrimary",
                     setName: "rs",
-                    electionId: {"$oid": "000000000000000000000001"}
+                    electionId: {"$oid": "000000000000000000000002"}
+                },
+                "c:27017": {
+                    type: "Unknown",
+                    setName: ,
+                    electionId:
                 }
             },
             topologyType: "ReplicaSetWithPrimary",
             setName: "rs",
-            maxElectionId: {"$oid": "000000000000000000000001"}
         }
     },
 
@@ -71,7 +79,7 @@ phases: [
             ["a:27017", {
                 ok: 1,
                 ismaster: true,
-                hosts: ["a:27017", "b:27017"],
+                hosts: ["a:27017", "b:27017", "c:27017"],
                 setName: "rs"
             }]
         ],
@@ -86,12 +94,51 @@ phases: [
                     type: "Unknown",
                     setName: ,
                     electionId:
+                },
+                "c:27017": {
+                    type: "Unknown",
+                    setName: ,
+                    electionId:
+                }
+            },
+            topologyType: "ReplicaSetWithPrimary",
+            setName: "rs",
+        }
+    },
+
+    # But we remember A's electionId, so when we finally hear from C
+    # claiming it is primary, we ignore it due to its outdated electionId
+    {
+        responses: [
+            ["c:27017", {
+                ok: 1,
+                ismaster: true,
+                hosts: ["a:27017", "b:27017", "c:27017"],
+                setName: "rs",
+                electionId: {"$oid": "000000000000000000000001"}
+            }]
+        ],
+        outcome: {
+            servers: {
+                # Still primary.
+                "a:27017": {
+                    type: "RSPrimary",
+                    setName: "rs",
+                    electionId:
+                },
+                "b:27017": {
+                    type: "Unknown",
+                    setName: ,
+                    electionId:
+                },
+                "c:27017": {
+                    type: "Unknown",
+                    setName: ,
+                    electionId:
                 }
             },
             topologyType: "ReplicaSetWithPrimary",
             setName: "rs",
-            # But we remember old electionId.
-            maxElectionId: {"$oid": "000000000000000000000001"}
         }
     }
 ]