feat(sessions): adding implicit cursor session support

Adds support for implicit cursor sessions. Cursors will
now lazily create sessions for their operations if no session
is supplied, and will take care of cleaning up the created
session when the cursor is exhausted.

Fixes NODE-1326

lib/command_cursor.js

@@ -89,7 +89,9 @@ var CommandCursor = function(bson, ns, cmd, options, topology, topologyOptions)
     // Topology Options
     topologyOptions: topologyOptions,
     // Promise library
-    promiseLibrary: promiseLibrary
+    promiseLibrary: promiseLibrary,
+    // Optional ClientSession
+    session: options.session
   };
 };
 
@@ -145,7 +147,9 @@ var methodsToInherit = [
   'isDead',
   'explain',
   'isNotified',
-  'isKilled'
+  'isKilled',
+  '_endSession',
+  '_initImplicitSession'
 ];
 
 // Only inherit the types we need

lib/cursor.js

@@ -194,14 +194,33 @@ var Cursor = function(bson, ns, cmd, options, topology, topologyOptions) {
 inherits(Cursor, Readable);
 
 // Map core cursor _next method so we can apply mapping
-CoreCursor.prototype._next = CoreCursor.prototype.next;
+Cursor.prototype._next = function() {
+  if (this._initImplicitSession) {
+    this._initImplicitSession();
+  }
+  return CoreCursor.prototype.next.apply(this, arguments);
+};
 
 for (var name in CoreCursor.prototype) {
   Cursor.prototype[name] = CoreCursor.prototype[name];
 }
 
 var define = (Cursor.define = new Define('Cursor', Cursor, true));
 
+Cursor.prototype._initImplicitSession = function() {
+  if (!this.s.session && this.s.topology.hasSessionSupport()) {
+    this.s.session = this.s.topology.startSession({ owner: this });
+    this.cursorState.session = this.s.session;
+  }
+};
+
+Cursor.prototype._endSession = function() {
+  const didCloseCursor = CoreCursor.prototype._endSession.apply(this, arguments);
+  if (didCloseCursor) {
+    this.s.session = undefined;
+  }
+};
+
 /**
  * Check if there is any document still available in the cursor
  * @method
@@ -929,7 +948,11 @@ var toArray = function(self, callback) {
   // Fetch all the documents
   var fetchDocs = function() {
     self._next(function(err, doc) {
-      if (err) return handleCallback(callback, err);
+      if (err) {
+        return self._endSession
+          ? self._endSession(() => handleCallback(callback, err))
+          : handleCallback(callback, err);
+      }
       if (doc == null) {
         return self.close({ skipKillCursors: true }, () => handleCallback(callback, null, items));
       }
@@ -985,17 +1008,21 @@ Cursor.prototype.count = function(applySkipLimit, opts, callback) {
   if (typeof opts === 'function') (callback = opts), (opts = {});
   opts = opts || {};
 
-  return executeOperation(this.s.topology, count, [this, applySkipLimit, opts, callback], {
-    skipSessions: true
-  });
-};
-
-var count = function(self, applySkipLimit, opts, callback) {
   if (typeof applySkipLimit === 'function') {
     callback = applySkipLimit;
     applySkipLimit = true;
   }
 
+  if (this.s.session) {
+    opts = Object.assign({}, opts, { session: this.s.session });
+  }
+
+  return executeOperation(this.s.topology, count, [this, applySkipLimit, opts, callback], {
+    skipSessions: !!this.s.session
+  });
+};
+
+var count = function(self, applySkipLimit, opts, callback) {
   if (applySkipLimit) {
     if (typeof self.cursorSkip() === 'number') opts.skip = self.cursorSkip();
     if (typeof self.cursorLimit() === 'number') opts.limit = self.cursorLimit();
@@ -1080,7 +1107,7 @@ Cursor.prototype.close = function(options, callback) {
   };
 
   if (this.s.session) {
-    return this.s.session.endSession(() => completeClose());
+    return this._endSession(() => completeClose());
   }
 
   return completeClose();

lib/mongo_client.js

@@ -495,7 +495,7 @@ define.staticMethod('connect', { callback: true, promise: true });
  * @return {ClientSession} the newly established session
  */
 MongoClient.prototype.startSession = function(options) {
-  options = options || {};
+  options = Object.assign({ explicit: true }, options);
   if (!this.topology) {
     throw new MongoError('Must connect to a server before calling this method');
   }

lib/utils.js

@@ -378,11 +378,12 @@ const executeOperation = (topology, operation, args, options) => {
 
   // The driver sessions spec mandates that we implicitly create sessions for operations
   // that are not explicitly provided with a session.
-  let session, opOptions;
+  let session, opOptions, owner;
   if (!options.skipSessions && topology.hasSessionSupport()) {
     opOptions = args[args.length - 2];
     if (opOptions == null || opOptions.session == null) {
-      session = topology.startSession();
+      owner = {};
+      session = topology.startSession({ owner });
       const optionsIndex = args.length - 2;
       args[optionsIndex] = Object.assign({}, args[optionsIndex], { session: session });
     } else if (opOptions.session && opOptions.session.hasEnded) {
@@ -392,7 +393,7 @@ const executeOperation = (topology, operation, args, options) => {
 
   const makeExecuteCallback = (resolve, reject) =>
     function executeCallback(err, result) {
-      if (session && !options.returnsCursor) {
+      if (session && session.owner === owner && !options.returnsCursor) {
         session.endSession(() => {
           delete opOptions.session;
           if (err) return reject(err);