From 457d9f51b8998684d0620b76dee081bdb8a73ae3 Mon Sep 17 00:00:00 2001
From: Todd Harris <todd@coda.io>
Date: Wed, 2 May 2018 11:41:50 -0700
Subject: [PATCH] Add the "loopback" flag.

The "loopback" flag is used to emit messages on the local endpoint of
a socket; e.g., allowing the server to send a message to the server's
handler directly.

This is especially useful in conjunction with rooms, as it allows the
server to proactively trigger behavior for each socket in a room without
having to maintain a separate list of said sockets or relying on the
client endpoint to trigger the behavior.
---
 lib/client.js     | 12 ++++++++++++
 lib/namespace.js  |  1 +
 lib/socket.js     |  7 ++++++-
 test/socket.io.js | 33 +++++++++++++++++++++++++++++++--
 4 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/lib/client.js b/lib/client.js
index 32d179f971..98fb015644 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -163,6 +163,18 @@ Client.prototype.packet = function(packet, opts){
   opts = opts || {};
   var self = this;
 
+  // handle loopback packets as though they came in off the wire.
+  if (opts.loopback) {
+    if (opts.preEncoded) { // a broadcast pre-encodes a packet
+      for (var i = 0; i < packet.length; i++) {
+        this.decoder.add(packet[i]);
+      }
+    } else {
+      this.ondecoded(packet);
+    }
+    return;
+  }
+
   // this writes to the actual connection
   function writeToEngine(encodedPackets) {
     if (opts.volatile && !self.conn.transport.writable) return;
diff --git a/lib/namespace.js b/lib/namespace.js
index 6d0ac17065..85b1090b18 100644
--- a/lib/namespace.js
+++ b/lib/namespace.js
@@ -30,6 +30,7 @@ exports.events = [
  */
 
 exports.flags = [
+  'loopback',
   'json',
   'volatile',
   'local'
diff --git a/lib/socket.js b/lib/socket.js
index 9a96929728..59f48d3757 100644
--- a/lib/socket.js
+++ b/lib/socket.js
@@ -37,6 +37,7 @@ exports.events = [
  */
 
 var flags = [
+  'loopback',
   'json',
   'volatile',
   'broadcast',
@@ -155,6 +156,10 @@ Socket.prototype.emit = function(ev){
       throw new Error('Callbacks are not supported when broadcasting');
     }
 
+    if (this.flags.loopback) {
+      throw new Error('Callbacks are not supported when emitting loopback messages');
+    }
+
     debug('emitting packet with ack id %d', this.nsp.ids);
     this.acks[this.nsp.ids] = args.pop();
     packet.id = this.nsp.ids++;
@@ -311,7 +316,7 @@ Socket.prototype.onconnect = function(){
 };
 
 /**
- * Called with each packet. Called by `Client`.
+ * Called with each packet. Called by `Client` and `emit` with `loopback` flag.
  *
  * @param {Object} packet
  * @api private
diff --git a/test/socket.io.js b/test/socket.io.js
index aec781455e..ac0c947562 100644
--- a/test/socket.io.js
+++ b/test/socket.io.js
@@ -677,7 +677,7 @@ describe('socket.io', function(){
         });
       });
     });
-    
+
     it('should not reuse same-namespace connections', function(done){
       var srv = http();
       var sio = io(srv);
@@ -837,6 +837,35 @@ describe('socket.io', function(){
       }, 500);
     });
 
+    it('should emit loopback messages locally', function(done) {
+      var srv = http();
+      var sio = io(srv);
+
+      var local_counter = 0;
+      var remote_counter = 0;
+      srv.listen(function(){
+        sio.of('/chat').on('connection', function(s){
+          setTimeout(function() {
+            sio.of('/chat').loopback.emit('ev', 'data');
+          }, 50);
+          s.on('ev', function() {
+            local_counter++;
+          });
+        });
+
+        var socket = client(srv, '/chat');
+        socket.on('ev', function() {
+          remote_counter++;
+        });
+      });
+
+      setTimeout(function() {
+        expect(local_counter).to.be(1);
+        expect(remote_counter).to.be(0);
+        done();
+      }, 500);
+    });
+
     it('should emit volatile event', function(done) {
       var srv = http();
       var sio = io(srv);
@@ -1608,7 +1637,7 @@ describe('socket.io', function(){
         });
       });
     });
-    
+
     it('should see query parameters sent from secondary namespace connections in handshake object', function(done){
       var srv = http();
       var sio = io(srv);