Check out an implicit session only after checking out a connection (#865)

This is accomplished by delaying allocation of a server session until actually required.

JAVA-4365

Author: jyemin

driver-core/src/main/com/mongodb/internal/session/BaseClientSessionImpl.java

@@ -33,7 +33,7 @@ public class BaseClientSessionImpl implements ClientSession {
     private static final String CLUSTER_TIME_KEY = "clusterTime";
 
     private final ServerSessionPool serverSessionPool;
-    private final ServerSession serverSession;
+    private ServerSession serverSession;
     private final Object originator;
     private final ClientSessionOptions options;
     private BsonDocument clusterTime;
@@ -46,7 +46,6 @@ public class BaseClientSessionImpl implements ClientSession {
 
     public BaseClientSessionImpl(final ServerSessionPool serverSessionPool, final Object originator, final ClientSessionOptions options) {
         this.serverSessionPool = serverSessionPool;
-        this.serverSession = serverSessionPool.get();
         this.originator = originator;
         this.options = options;
         this.pinnedServerAddress = null;
@@ -120,6 +119,9 @@ public BsonTimestamp getOperationTime() {
     @Override
     public ServerSession getServerSession() {
         isTrue("open", !closed);
+        if (serverSession == null) {
+            serverSession = serverSessionPool.get();
+        }
         return serverSession;
     }
 
@@ -179,7 +181,9 @@ private BsonTimestamp greaterOf(final BsonTimestamp newOperationTime) {
     public void close() {
         if (!closed) {
             closed = true;
-            serverSessionPool.release(serverSession);
+            if (serverSession != null) {
+                serverSessionPool.release(serverSession);
+            }
             clearTransactionContext();
         }
     }

driver-core/src/test/unit/com/mongodb/internal/session/BaseClientSessionImplTest.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2008-present MongoDB, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.mongodb.internal.session;
+
+import com.mongodb.ClientSessionOptions;
+import com.mongodb.session.ClientSession;
+import org.junit.jupiter.api.Test;
+
+import static com.mongodb.ClusterFixture.getCluster;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class BaseClientSessionImplTest {
+
+    @Test
+    void shouldNotCheckoutServerSessionIfNeverRequested() {
+        ServerSessionPool serverSessionPool = new ServerSessionPool(getCluster(), null);
+        ClientSession clientSession = new BaseClientSessionImpl(serverSessionPool, new Object(), ClientSessionOptions.builder().build());
+
+        assertEquals(0, serverSessionPool.getInUseCount());
+
+        clientSession.close();
+
+        assertEquals(0, serverSessionPool.getInUseCount());
+    }
+
+    @Test
+    void shouldDelayServerSessionCheckoutUntilRequested() {
+        ServerSessionPool serverSessionPool = new ServerSessionPool(getCluster(), null);
+        ClientSession clientSession = new BaseClientSessionImpl(serverSessionPool, new Object(), ClientSessionOptions.builder().build());
+
+        assertEquals(0, serverSessionPool.getInUseCount());
+
+        clientSession.getServerSession();
+
+        assertEquals(1, serverSessionPool.getInUseCount());
+
+        clientSession.close();
+
+        assertEquals(0, serverSessionPool.getInUseCount());
+    }
+}