feat: reverse the ping-pong mechanism

The ping packets will now be sent by the server, because the timers set
in the browsers are not reliable enough. We suspect that a lot of
timeout problems came from timers being delayed on the client-side.

Breaking change: v3.x clients will not be able to connect anymore (they
will send a ping packet and timeout while waiting for a pong packet).

Related: socketio/engine.io#312

README.md

@@ -172,9 +172,9 @@ Exposed as `eio` in the browser standalone build.
 - `upgrade`
   - Fired upon upgrade success, after the new transport is set
 - `ping`
-  - Fired upon _flushing_ a ping packet (ie: actual packet write out)
+  - Fired upon receiving a ping packet.
 - `pong`
-  - Fired upon receiving a pong packet.
+  - Fired upon _flushing_ a pong packet (ie: actual packet write out)
 
 #### Methods
 

lib/socket.js

@@ -116,7 +116,6 @@ class Socket extends Emitter {
     this.pingTimeout = null;
 
     // set on heartbeat
-    this.pingIntervalTimer = null;
     this.pingTimeoutTimer = null;
 
     this.open();
@@ -415,8 +414,9 @@ class Socket extends Emitter {
           this.onHandshake(JSON.parse(packet.data));
           break;
 
-        case "pong":
-          this.setPing();
+        case "ping":
+          this.resetPingTimeout();
+          this.sendPacket("pong");
           this.emit("pong");
           break;
 
@@ -452,56 +452,19 @@ class Socket extends Emitter {
     this.onOpen();
     // In case open handler closes socket
     if ("closed" === this.readyState) return;
-    this.setPing();
-
-    // Prolong liveness of socket on heartbeat
-    this.removeListener("heartbeat", this.onHeartbeat);
-    this.on("heartbeat", this.onHeartbeat);
+    this.resetPingTimeout();
   }
 
   /**
-   * Resets ping timeout.
+   * Sets and resets ping timeout timer based on server pings.
    *
    * @api private
    */
-  onHeartbeat(timeout) {
+  resetPingTimeout() {
     clearTimeout(this.pingTimeoutTimer);
-    const self = this;
-    self.pingTimeoutTimer = setTimeout(function() {
-      if ("closed" === self.readyState) return;
-      self.onClose("ping timeout");
-    }, timeout || self.pingInterval + self.pingTimeout);
-  }
-
-  /**
-   * Pings server every `this.pingInterval` and expects response
-   * within `this.pingTimeout` or closes connection.
-   *
-   * @api private
-   */
-  setPing() {
-    const self = this;
-    clearTimeout(self.pingIntervalTimer);
-    self.pingIntervalTimer = setTimeout(function() {
-      debug(
-        "writing ping packet - expecting pong within %sms",
-        self.pingTimeout
-      );
-      self.ping();
-      self.onHeartbeat(self.pingTimeout);
-    }, self.pingInterval);
-  }
-
-  /**
-   * Sends a ping packet.
-   *
-   * @api private
-   */
-  ping() {
-    const self = this;
-    this.sendPacket("ping", function() {
-      self.emit("ping");
-    });
+    this.pingTimeoutTimer = setTimeout(() => {
+      this.onClose("ping timeout");
+    }, this.pingInterval + this.pingTimeout);
   }
 
   /**