feat: allow excluding all sockets in a room (#92)

Syntax:

```js
io.to("room1").except("room2").emit(/* ... */);
```

Related: https://socket.io/docs/v4/migrating-from-3-x-to-4-0/#Allow-excluding-specific-rooms-when-broadcasting

index.js

@@ -75,6 +75,7 @@ function Emitter(redis, prefix, nsp){
 
   this._rooms = [];
   this._flags = {};
+  this._except = [];
 }
 
 /**
@@ -104,6 +105,14 @@ Emitter.prototype.to = function(room){
   return this;
 };
 
+Emitter.prototype.except = function(room) {
+  if (!~this._except.indexOf(room)) {
+    debug('except %s', room);
+    this._except.push(room);
+  }
+  return this;
+};
+
 /**
  * Return a new emitter for the given namespace.
  *
@@ -127,7 +136,8 @@ Emitter.prototype.emit = function(){
 
   var opts = {
     rooms: this._rooms,
-    flags: this._flags
+    flags: this._flags,
+    except: this._except
   };
 
   var msg = msgpack.encode([uid, packet, opts]);
@@ -141,6 +151,7 @@ Emitter.prototype.emit = function(){
   // reset state
   this._rooms = [];
   this._flags = {};
+  this._except = [];
 
   return this;
 };

test/index.js

@@ -214,5 +214,46 @@ describe('emitter', function() {
         });
       });
     });
+
+    it('should be able to exclude a socket by id', function(done) {
+      var pub = redis.createClient();
+      var sub = redis.createClient(null, null, {return_buffers: true});
+      srv = http();
+      var sio = io(srv, {adapter: redisAdapter({pubClient: pub, subClient: sub})});
+
+      var firstId = false;
+      srv.listen(function() {
+        sio.on('connection', function(socket) {
+          if (firstId === false) {
+            firstId = socket.id;
+          }
+        });
+      });
+
+      var a = client(srv, { forceNew: true });
+      var b;
+      a.on('connect', function() {
+        b = client(srv, { forceNew: true });
+        b.on('connect', function() {
+
+          var calls = 0;
+          a.on('except event', function() {
+            calls++;
+            expect().fail();
+          });
+          b.on('except event', function() {
+            calls++;
+            setTimeout(function() {
+              expect(calls).to.be(1);
+              done();
+            }, 1);
+          });
+
+          var emitter = ioe({ host: 'localhost', port: '6379' });
+          emitter.except(firstId).emit('except event');
+
+        });
+      });
+    });
   });
 });