Allow writes to any server when in direct connection mode

JAVA-3309

Author: jyemin

driver-core/src/main/com/mongodb/selector/WritableServerSelector.java

@@ -16,6 +16,7 @@
 
 package com.mongodb.selector;
 
+import com.mongodb.connection.ClusterConnectionMode;
 import com.mongodb.connection.ClusterDescription;
 import com.mongodb.connection.ServerDescription;
 
@@ -32,6 +33,9 @@ public final class WritableServerSelector implements ServerSelector {
     @Override
     @SuppressWarnings("deprecation")
     public List<ServerDescription> select(final ClusterDescription clusterDescription) {
+        if (clusterDescription.getConnectionMode() == ClusterConnectionMode.SINGLE) {
+            return clusterDescription.getAny();
+        }
         return clusterDescription.getPrimaries();
     }
 

driver-core/src/test/unit/com/mongodb/selector/WritableServerSelectorSpecification.groovy

@@ -24,6 +24,7 @@ import spock.lang.Specification
 import spock.lang.Unroll
 
 import static com.mongodb.connection.ClusterConnectionMode.MULTIPLE
+import static com.mongodb.connection.ClusterConnectionMode.SINGLE
 import static com.mongodb.connection.ServerConnectionState.CONNECTED
 import static com.mongodb.connection.ServerType.REPLICA_SET_PRIMARY
 import static com.mongodb.connection.ServerType.REPLICA_SET_SECONDARY
@@ -49,4 +50,15 @@ class WritableServerSelectorSpecification extends Specification {
         []                 | new ClusterDescription(MULTIPLE, ClusterType.REPLICA_SET, [SECONDARY_SERVER])
     }
 
+    @Unroll
+    def 'WritableServerSelector will choose secondary server in single mode for #clusterDescription'() throws UnknownHostException {
+        expect:
+        WritableServerSelector selector = new WritableServerSelector()
+        expectedServerList == selector.select(clusterDescription)
+
+        where:
+        expectedServerList | clusterDescription
+        [SECONDARY_SERVER] | new ClusterDescription(SINGLE, ClusterType.REPLICA_SET, [SECONDARY_SERVER])
+    }
+
 }