✨ Add ping/pong

This change adds the ability for a client to send a "ping" to the server
and receive a "pong" response.

The motivation for this is that sometimes the socket underlying the
`Connection` may not reliably report its connection state (for example
if using a wrapper around a "vanilla" websocket that handles
reconnection).

The most bullet-proof way for a client to determine its connection state
is to actually make a request to the server and assert that it receives
a timely response.

The implementation of ping/pong is arguably a websocket concern,
especially since it's already [defined by the spec][1]. However, the
browser JavaScript [`WebSocket`][2] does not expose this functionality,
so we have to add our own ping/pong layer on top anyway.

It's also worth noting that consumers of this library can't easily
send their own ping messages down the socket, since ShareDB will
[error][3] for unknown messages.

Note that this change only adds the ability for a client to ping the
server, and not the other way around. This is because:

 - the `Agent` is not an `Emitter`, and emitting a `pong` on the
   `Backend` is pretty meaningless
 - server-side implementations of WebSockets, such as `ws`, _do_
   [expose a `ping` API][4]

[1]: https://www.rfc-editor.org/rfc/rfc6455#section-5.5.2
[2]: https://developer.mozilla.org/en-US/docs/Web/API/WebSocket
[3]: https://github.com/share/sharedb/blob/8b531bedf19860ffe0b37a3e1c8da7a32b5005bd/lib/agent.js#L451
[4]: https://github.com/websockets/ws/blob/966f9d47cd0ff5aa9db0b2aa262f9819d3f4d414/lib/websocket.js#L351

lib/agent.js

@@ -447,6 +447,8 @@ Agent.prototype._handleMessage = function(request, callback) {
         return this._subscribePresence(request.ch, request.seq, callback);
       case ACTIONS.presenceUnsubscribe:
         return this._unsubscribePresence(request.ch, request.seq, callback);
+      case ACTIONS.pingPong:
+        return this._pingPong(callback);
       default:
         callback(new ShareDBError(ERROR_CODE.ERR_MESSAGE_BADLY_FORMED, 'Invalid or unknown message'));
     }
@@ -540,6 +542,14 @@ Agent.prototype._querySubscribe = function(queryId, collection, query, options,
   });
 };
 
+Agent.prototype._pingPong = function(callback) {
+  var error = null;
+  var message = {
+    a: ACTIONS.pingPong
+  };
+  callback(error, message);
+};
+
 function getResultsData(results) {
   var items = [];
   for (var i = 0; i < results.length; i++) {

lib/client/connection.js

@@ -253,6 +253,8 @@ Connection.prototype.handleMessage = function(message) {
       return this._handlePresenceUnsubscribe(err, message);
     case ACTIONS.presenceRequest:
       return this._handlePresenceRequest(err, message);
+    case ACTIONS.pingPong:
+      return this._handlePingPong(err);
 
     default:
       logger.warn('Ignoring unrecognized message', message);
@@ -476,6 +478,12 @@ Connection.prototype.send = function(message) {
   this.socket.send(JSON.stringify(message));
 };
 
+Connection.prototype.ping = function() {
+  var message = {
+    a: ACTIONS.pingPong
+  };
+  this.send(message);
+};
 
 /**
  * Closes the socket and emits 'closed'
@@ -725,6 +733,11 @@ Connection.prototype._handleHandshake = function(error, message) {
   this._initialize(message);
 };
 
+Connection.prototype._handlePingPong = function(error) {
+  if (error) return this.emit('error', error);
+  this.emit('pong');
+};
+
 Connection.prototype._initialize = function(message) {
   if (this.state !== 'connecting') return;
 

lib/message-actions.js

@@ -14,6 +14,7 @@ exports.ACTIONS = {
   op: 'op',
   snapshotFetch: 'nf',
   snapshotFetchByTimestamp: 'nt',
+  pingPong: 'pp',
   presence: 'p',
   presenceSubscribe: 'ps',
   presenceUnsubscribe: 'pu',

test/client/connection.js

@@ -124,6 +124,36 @@ describe('client connection', function() {
     });
   });
 
+  describe('ping/pong', function() {
+    it('pings the backend', function(done) {
+      var connection = this.backend.connect();
+
+      connection.on('pong', function() {
+        done();
+      });
+
+      connection.on('connected', function() {
+        connection.ping();
+      });
+    });
+
+    it('handles errors', function(done) {
+      this.backend.use('receive', function(request, next) {
+        if (request.data.a !== 'pp') return;
+        next(new Error('bad'));
+      });
+
+      var connection = this.backend.connect();
+
+      connection.on('error', function(error) {
+        expect(error.message).to.equal('bad');
+        done();
+      });
+
+      connection.ping();
+    });
+  });
+
   describe('backend.agentsCount', function() {
     it('updates after connect and connection.close()', function(done) {
       var backend = this.backend;