fix: limit growth of server sessions through lazy acquisition

Server sessions are no longer acquired at `ClientSession`
instantiation, but rather lazily created at the first access of the
`serverSession` property. This has the effect of deferring the
acquisition to command construction, after a connection has been
checked out during operation execution.

NODE-2552

lib/core/sessions.js

@@ -46,6 +46,8 @@ function assertAlive(session, callback) {
  * @typedef {Object} SessionId
  */
 
+const kServerSession = Symbol('serverSession');
+
 /**
  * A class representing a client session on the server
  * WARNING: not meant to be instantiated directly.
@@ -79,8 +81,8 @@ class ClientSession extends EventEmitter {
     this.topology = topology;
     this.sessionPool = sessionPool;
     this.hasEnded = false;
-    this.serverSession = sessionPool.acquire();
     this.clientOptions = clientOptions;
+    this[kServerSession] = undefined;
 
     this.supports = {
       causalConsistency:
@@ -104,6 +106,14 @@ class ClientSession extends EventEmitter {
     return this.serverSession.id;
   }
 
+  get serverSession() {
+    if (this[kServerSession] == null) {
+      this[kServerSession] = this.sessionPool.acquire();
+    }
+
+    return this[kServerSession];
+  }
+
   /**
    * Ends this session on the server
    *
@@ -125,7 +135,7 @@ class ClientSession extends EventEmitter {
 
     // release the server session back to the pool
     this.sessionPool.release(this.serverSession);
-    this.serverSession = null;
+    this[kServerSession] = undefined;
 
     // mark the session as ended, and emit a signal
     this.hasEnded = true;
@@ -692,13 +702,12 @@ function commandSupportsReadConcern(command, options) {
  * @return {MongoError|null} An error, if some error condition was met
  */
 function applySession(session, command, options) {
-  const serverSession = session.serverSession;
-  if (serverSession == null) {
+  if (session.hasEnded) {
     // TODO: merge this with `assertAlive`, did not want to throw a try/catch here
     return new MongoError('Cannot use a session that has ended');
   }
 
-  // mark the last use of this session, and apply the `lsid`
+  const serverSession = session.serverSession;
   serverSession.lastUse = Date.now();
   command.lsid = serverSession.id;