Prioritize electionId over setVersion on 6.0+ servers

driver-core/src/main/com/mongodb/internal/connection/AbstractMultiServerCluster.java

@@ -45,6 +45,7 @@
 import static com.mongodb.connection.ServerType.REPLICA_SET_GHOST;
 import static com.mongodb.connection.ServerType.SHARD_ROUTER;
 import static com.mongodb.connection.ServerType.STANDALONE;
+import static com.mongodb.internal.operation.ServerVersionHelper.SIX_DOT_ZERO_WIRE_VERSION;
 import static java.lang.String.format;
 
 public abstract class AbstractMultiServerCluster extends BaseCluster {
@@ -224,9 +225,7 @@ private boolean handleReplicaSetMemberChanged(final ServerDescription newDescrip
         }
 
         if (newDescription.getType() == REPLICA_SET_GHOST) {
-            if (LOGGER.isInfoEnabled()) {
-                LOGGER.info(format("Server %s does not appear to be a member of an initiated replica set.", newDescription.getAddress()));
-            }
+            LOGGER.info(format("Server %s does not appear to be a member of an initiated replica set.", newDescription.getAddress()));
             return true;
         }
 
@@ -247,64 +246,78 @@ private boolean handleReplicaSetMemberChanged(final ServerDescription newDescrip
         if (newDescription.getCanonicalAddress() != null
                 && !newDescription.getAddress().equals(new ServerAddress(newDescription.getCanonicalAddress()))
                 && !newDescription.isPrimary()) {
-            if (LOGGER.isInfoEnabled()) {
-                LOGGER.info(format("Canonical address %s does not match server address.  Removing %s from client view of cluster",
-                                   newDescription.getCanonicalAddress(), newDescription.getAddress()));
-            }
+            LOGGER.info(format("Canonical address %s does not match server address.  Removing %s from client view of cluster",
+                    newDescription.getCanonicalAddress(), newDescription.getAddress()));
             removeServer(newDescription.getAddress());
             return true;
         }
 
-        if (newDescription.isPrimary()) {
-            ObjectId electionId = newDescription.getElectionId();
-            Integer setVersion = newDescription.getSetVersion();
-            if (setVersion != null && electionId != null) {
-                if (isStalePrimary(newDescription)) {
-                    if (LOGGER.isInfoEnabled()) {
-                        LOGGER.info(format("Invalidating potential primary %s whose (set version, election id) tuple of (%d, %s) "
-                                + "is less than one already seen of (%d, %s)",
-                                newDescription.getAddress(),
-                                setVersion, electionId,
-                                maxSetVersion, maxElectionId));
-                    }
-                    addressToServerTupleMap.get(newDescription.getAddress()).server.resetToConnecting();
-                    return false;
-                }
+        if (!newDescription.isPrimary()) {
+            return true;
+        }
 
-                if (!electionId.equals(maxElectionId)) {
-                    if (LOGGER.isInfoEnabled()) {
-                        LOGGER.info(format("Setting max election id to %s from replica set primary %s", electionId,
-                                newDescription.getAddress()));
-                    }
-                    maxElectionId = electionId;
-                }
-            }
+        if (isStalePrimary(newDescription)) {
+            invalidatePotentialPrimary(newDescription);
+            return false;
+        }
 
-            if (setVersion != null
-                    && (maxSetVersion == null || setVersion.compareTo(maxSetVersion) > 0)) {
-                if (LOGGER.isInfoEnabled()) {
-                    LOGGER.info(format("Setting max set version to %d from replica set primary %s", setVersion,
-                            newDescription.getAddress()));
-                }
-                maxSetVersion = setVersion;
-            }
+        maxElectionId = nullSafeMax(newDescription.getElectionId(), maxElectionId);
+        maxSetVersion = nullSafeMax(newDescription.getSetVersion(), maxSetVersion);
 
-            if (isNotAlreadyPrimary(newDescription.getAddress())) {
-                LOGGER.info(format("Discovered replica set primary %s", newDescription.getAddress()));
-            }
-            invalidateOldPrimaries(newDescription.getAddress());
+        invalidateOldPrimaries(newDescription.getAddress());
+
+        if (isNotAlreadyPrimary(newDescription.getAddress())) {
+            LOGGER.info(format("Discovered replica set primary %s with max election id %s and max set version %d",
+                    newDescription.getAddress(), newDescription.getElectionId(), newDescription.getSetVersion()));
         }
+
         return true;
     }
 
-    private boolean isStalePrimary(final ServerDescription newDescription) {
-        if (maxSetVersion == null || maxElectionId == null) {
-            return false;
+    private boolean isStalePrimary(final ServerDescription description) {
+        ObjectId electionId = description.getElectionId();
+        Integer setVersion = description.getSetVersion();
+        if (description.getMaxWireVersion() >= SIX_DOT_ZERO_WIRE_VERSION) {
+            return nullSafeCompareTo(electionId, maxElectionId) < 0
+                    || (nullSafeCompareTo(electionId, maxElectionId) == 0 && nullSafeCompareTo(setVersion, maxSetVersion) < 0);
+        } else {
+            return setVersion != null && electionId != null
+                    && (nullSafeCompareTo(setVersion, maxSetVersion) < 0
+                    || (nullSafeCompareTo(setVersion, maxSetVersion) == 0
+                    && nullSafeCompareTo(electionId, maxElectionId) < 0));
         }
+     }
+
+    private void invalidatePotentialPrimary(final ServerDescription newDescription) {
+        LOGGER.info(format("Invalidating potential primary %s whose (set version, election id) tuple of (%d, %s) "
+                        + "is less than one already seen of (%d, %s)",
+                newDescription.getAddress(), newDescription.getSetVersion(), newDescription.getElectionId(),
+                maxSetVersion, maxElectionId));
+        addressToServerTupleMap.get(newDescription.getAddress()).server.resetToConnecting();
+    }
 
-        Integer setVersion = newDescription.getSetVersion();
-        return (setVersion == null || maxSetVersion.compareTo(setVersion) > 0
-                || (maxSetVersion.equals(setVersion) && maxElectionId.compareTo(newDescription.getElectionId()) > 0));
+    /**
+     * Implements the same contract as {@link Comparable#compareTo(Object)}, except that a null value is always considers less-than any
+     * other value (except null, which it considers as equal-to).
+     */
+    private static <T extends Comparable<T>> int nullSafeCompareTo(final T first, final T second) {
+        if (first == null) {
+            return second == null ? 0 : -1;
+        }
+        if (second == null) {
+            return 1;
+        }
+        return first.compareTo(second);
+    }
+
+    private static <T extends Comparable<T>> T nullSafeMax(final T first, final T second) {
+        if (first == null) {
+            return second;
+        }
+        if (second == null) {
+            return first;
+        }
+        return first.compareTo(second) >= 0 ? first : second;
     }
 
     private boolean isNotAlreadyPrimary(final ServerAddress address) {
@@ -315,7 +328,7 @@ private boolean isNotAlreadyPrimary(final ServerAddress address) {
     private boolean handleShardRouterChanged(final ServerDescription newDescription) {
         if (!newDescription.isShardRouter()) {
             LOGGER.error(format("Expecting a %s, but found a %s.  Removing %s from client view of cluster.",
-                                 SHARD_ROUTER, newDescription.getType(), newDescription.getAddress()));
+                    SHARD_ROUTER, newDescription.getType(), newDescription.getAddress()));
             removeServer(newDescription.getAddress());
         }
         return true;

driver-core/src/main/com/mongodb/internal/operation/ServerVersionHelper.java

@@ -30,6 +30,7 @@ public final class ServerVersionHelper {
     public static final int FOUR_DOT_TWO_WIRE_VERSION = 8;
     public static final int FOUR_DOT_FOUR_WIRE_VERSION = 9;
     public static final int FIVE_DOT_ZERO_WIRE_VERSION = 12;
+    public static final int SIX_DOT_ZERO_WIRE_VERSION = 17;
 
     public static boolean serverIsAtLeastVersionFourDotZero(final ConnectionDescription description) {
         return description.getMaxWireVersion() >= FOUR_DOT_ZERO_WIRE_VERSION;

driver-core/src/test/resources/server-discovery-and-monitoring/rs/electionId_precedence_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" : {
+              "$oid" : "000000000000000000000001"
+            },
+            "minWireVersion" : 0,
+            "maxWireVersion" : 17
+          }
+        ],
+        [
+          "b:27017",
+          {
+            "ok" : 1,
+            "helloOk" : true,
+            "isWritablePrimary" : true,
+            "hosts" : [
+              "a:27017",
+              "b:27017"
+            ],
+            "setName" : "rs",
+            "setVersion" : 2,
+            "electionId" : {
+              "$oid" : "000000000000000000000001"
+            },
+            "minWireVersion" : 0,
+            "maxWireVersion" : 17
+          }
+        ],
+        [
+          "a:27017",
+          {
+            "ok" : 1,
+            "helloOk" : true,
+            "isWritablePrimary" : true,
+            "hosts" : [
+              "a:27017",
+              "b:27017"
+            ],
+            "setName" : "rs",
+            "setVersion" : 1,
+            "electionId" : {
+              "$oid" : "000000000000000000000002"
+            },
+            "minWireVersion" : 0,
+            "maxWireVersion" : 17
+          }
+        ]
+      ],
+      "outcome" : {
+        "servers" : {
+          "a:27017" : {
+            "type" : "RSPrimary",
+            "setName" : "rs",
+            "setVersion" : 1,
+            "electionId" : {
+              "$oid" : "000000000000000000000002"
+            }
+          },
+          "b:27017" : {
+            "type" : "Unknown",
+            "setName" : null,
+            "setVersion" : null,
+            "electionId" : null
+          }
+        },
+        "topologyType" : "ReplicaSetWithPrimary",
+        "logicalSessionTimeoutMinutes" : null,
+        "setName" : "rs",
+        "maxSetVersion" : 1,
+        "maxElectionId" : {
+          "$oid" : "000000000000000000000002"
+        }
+      }
+    }
+  ]
+}