fix: prevent the socket from joining a room after disconnection

Calling `socket.join()` after disconnection would lead to a memory
leak, because the room was never removed from the memory:

```js
io.on("connection", (socket) => {
  socket.disconnect();
  socket.join("room1"); // leak
});
```

Related:

- #4067
- #4380

Backported from 18f3fda

lib/namespace.js

@@ -163,25 +163,31 @@ Namespace.prototype.add = function(client, query, fn){
   var self = this;
   this.run(socket, function(err){
     process.nextTick(function(){
-      if ('open' == client.conn.readyState) {
-        if (err) return socket.error(err.data || err.message);
-
-        // track socket
-        self.sockets[socket.id] = socket;
-
-        // it's paramount that the internal `onconnect` logic
-        // fires before user-set events to prevent state order
-        // violations (such as a disconnection before the connection
-        // logic is complete)
-        socket.onconnect();
-        if (fn) fn();
-
-        // fire user-set events
-        self.emit('connect', socket);
-        self.emit('connection', socket);
-      } else {
-        debug('next called after client was closed - ignoring socket');
+      if ("open" !== client.conn.readyState) {
+        debug("next called after client was closed - ignoring socket");
+        socket._cleanup();
+        return;
       }
+
+      if (err) {
+        debug("middleware error, sending CONNECT_ERROR packet to the client");
+        socket._cleanup();
+        return socket.error(err.data || err.message);
+      }
+
+      // track socket
+      self.sockets[socket.id] = socket;
+
+      // it's paramount that the internal `onconnect` logic
+      // fires before user-set events to prevent state order
+      // violations (such as a disconnection before the connection
+      // logic is complete)
+      socket.onconnect();
+      if (fn) fn();
+
+      // fire user-set events
+      self.emit('connect', socket);
+      self.emit('connection', socket);
     });
   });
   return socket;

lib/socket.js

@@ -447,7 +447,7 @@ Socket.prototype.onclose = function(reason){
   if (!this.connected) return this;
   debug('closing socket - reason %s', reason);
   this.emit('disconnecting', reason);
-  this.leaveAll();
+  this._cleanup();
   this.nsp.remove(this);
   this.client.remove(this);
   this.connected = false;
@@ -576,3 +576,8 @@ Socket.prototype.run = function(event, fn){
 
   run(0);
 };
+
+Socket.prototype._cleanup = function () {
+  this.leaveAll();
+  this.join = function noop() {};
+}

test/socket.io.js

@@ -1901,6 +1901,43 @@ describe('socket.io', function(){
       });
     });
 
+    it("should leave all rooms joined after a middleware failure", (done) => {
+      const srv = http().listen(0);
+      const sio = io(srv);
+      const clientSocket = client(srv, "/");
+
+      sio.use((socket, next) => {
+        socket.join("room1");
+        next(new Error("nope"));
+      });
+
+      clientSocket.on("error", () => {
+        expect(sio.of("/").adapter.rooms).to.eql(0);
+
+        clientSocket.disconnect();
+        sio.close();
+        done();
+      });
+    });
+
+    it("should not join rooms after disconnection", (done) => {
+      const srv = http().listen(0);
+      const sio = io(srv);
+      const clientSocket = client(srv, "/");
+
+      sio.on("connection", (socket) => {
+        socket.disconnect();
+        socket.join("room1");
+      });
+
+      clientSocket.on("disconnect", () => {
+        expect(sio.of("/").adapter.rooms).to.eql(0);
+
+        sio.close();
+        done();
+      });
+    });
+
     it('should always trigger the callback (if provided) when joining a room', function(done){
       var srv = http();
       var sio = io(srv);