feat(sessions): MongoClient will now track sessions and release

As initially implemented, sessions must be explicitly ended by the
user.  This is likely to lead to cases where users forget to do
this, and therefore leak sessions.  Instead, MongoClient now tracks
all sessions created on the client, and explicitly cleans them up
upon client close.

NODE-1106

Author: mbroadst

lib/mongo_client.js

@@ -194,7 +194,8 @@ function MongoClient(url, options) {
     url: url,
     options: options || {},
     promiseLibrary: null,
-    dbCache: {}
+    dbCache: {},
+    sessions: []
   };
 
   // Get the promiseLibrary
@@ -308,6 +309,14 @@ MongoClient.prototype.close = function(force, callback) {
   // Remove listeners after emit
   self.removeAllListeners('close');
 
+  // If we have sessions, we want to send a single `endSessions` command for them,
+  // and then individually clean them up.  They will be removed from the internal state
+  // when they emit their `ended` events.
+  if (this.s.sessions.length) {
+    this.topology.endSessions(this.s.sessions);
+    this.s.sessions.forEach(session => session.endSession({ skipCommand: true }));
+  }
+
   // Callback after next event loop tick
   if (typeof callback === 'function')
     return process.nextTick(function() {
@@ -481,7 +490,13 @@ MongoClient.prototype.startSession = function(options) {
     throw new MongoError('Current topology does not support sessions');
   }
 
-  return this.topology.startSession(options);
+  const session = this.topology.startSession(options);
+  session.once('ended', () => {
+    this.s.sessions = this.s.sessions.filter(s => s.equals(session));
+  });
+
+  this.s.sessions.push(session);
+  return session;
 };
 
 var mergeOptions = function(target, source, flatten) {

lib/topologies/topology_base.js

@@ -368,6 +368,10 @@ class TopologyBase extends EventEmitter {
     return this.s.coreTopology.getServer(options);
   }
 
+  endSessions(sessions, callback) {
+    return this.s.coreTopology.endSessions(sessions, callback);
+  }
+
   /**
    * Unref all sockets
    * @method

package.json

@@ -13,7 +13,7 @@
     "official"
   ],
   "dependencies": {
-    "mongodb-core": "mongodb-js/mongodb-core#8aaac8bcc7bdc02bf8bf4b03337a0af71ddc513e"
+    "mongodb-core": "mongodb-js/mongodb-core#6510d7d3d82d84cc0811a853355deaa918b96941"
   },
   "devDependencies": {
     "betterbenchmarks": "^0.1.0",

test/functional/sessions_tests.js

@@ -0,0 +1,48 @@
+'use strict';
+const expect = require('chai').expect,
+  mongo = require('../..'),
+  setupDatabase = require('./shared').setupDatabase;
+
+const ignoredCommands = ['ismaster'];
+const test = { commands: { started: [], succeeded: [] } };
+describe('Sessions', function() {
+  before(function() {
+    return setupDatabase(this.configuration);
+  });
+
+  afterEach(() => test.listener.uninstrument());
+  beforeEach(function() {
+    test.commands = { started: [], succeeded: [] };
+    test.listener = mongo.instrument(err => expect(err).to.be.null);
+    test.listener.on('started', event => {
+      if (ignoredCommands.indexOf(event.commandName) === -1) test.commands.started.push(event);
+    });
+
+    test.listener.on('succeeded', event => {
+      if (ignoredCommands.indexOf(event.commandName) === -1) test.commands.succeeded.push(event);
+    });
+
+    test.client = this.configuration.newClient({ w: 1 }, { poolSize: 1, auto_reconnect: false });
+    return test.client.connect();
+  });
+
+  it('should send endSessions for multiple sessions', {
+    metadata: { requires: { topology: ['single'] } },
+    test: function(done) {
+      var client = this.configuration.newClient({ w: 1 }, { poolSize: 1, auto_reconnect: false });
+      client.connect((err, client) => {
+        let sessions = [client.startSession(), client.startSession()].map(s => s.id);
+
+        client.close(err => {
+          expect(err).to.not.exist;
+          expect(test.commands.started).to.have.length(1);
+          expect(test.commands.started[0].commandName).to.equal('endSessions');
+          expect(test.commands.started[0].command.endSessions).to.include.deep.members(sessions);
+
+          expect(client.s.sessions).to.have.length(0);
+          done();
+        });
+      });
+    }
+  });
+});

yarn.lock

@@ -2115,9 +2115,9 @@ mongodb-core@mongodb-js/mongodb-core#3.0.0:
     bson "~1.0.4"
     require_optional "^1.0.1"
 
-mongodb-core@mongodb-js/mongodb-core#8aaac8bcc7bdc02bf8bf4b03337a0af71ddc513e:
+mongodb-core@mongodb-js/mongodb-core#6510d7d3d82d84cc0811a853355deaa918b96941:
   version "3.0.0-beta2"
-  resolved "https://codeload.github.com/mongodb-js/mongodb-core/tar.gz/8aaac8bcc7bdc02bf8bf4b03337a0af71ddc513e"
+  resolved "https://codeload.github.com/mongodb-js/mongodb-core/tar.gz/6510d7d3d82d84cc0811a853355deaa918b96941"
   dependencies:
     bson "~1.0.4"
     require_optional "^1.0.1"