fix: fix race condition in dynamic namespaces

Using an async operation with `io.use()` could lead to the creation of
several instances of a same namespace, each of them overriding the
previous one.

Example:

```js
io.use(async (nsp, auth, next) => {
  await anOperationThatTakesSomeTime();
  next();
});
```

Related: #4136

Backported from 9d86397

lib/client.js

@@ -68,7 +68,6 @@ Client.prototype.connect = function(name, query){
 
   this.server.checkNamespace(name, query, (dynamicNsp) => {
     if (dynamicNsp) {
-      debug('dynamic namespace %s was created', dynamicNsp.name);
       this.doConnect(name, query);
     } else {
       debug('creation of namespace %s was denied', name);

lib/index.js

@@ -182,11 +182,17 @@ Server.prototype.checkNamespace = function(name, query, fn){
       return fn(false);
     }
     nextFn.value(name, query, (err, allow) => {
-      if (err || !allow) {
-        run();
-      } else {
-        fn(this.parentNsps.get(nextFn.value).createChild(name));
+      if (err || !allow) {
+        return run();
       }
+      if (this.nsps[name]) {
+        // the namespace was created in the meantime
+        debug("dynamic namespace %s already exists", name);
+        return fn(this.nsps[name]);
+      }
+      const namespace = this.parentNsps.get(nextFn.value).createChild(name);
+      debug("dynamic namespace %s was created", name);
+      fn(namespace);
     });
   };
 

test/socket.io.js

@@ -945,6 +945,42 @@ describe('socket.io', function(){
           });
         });
       });
+
+      it("should handle race conditions with dynamic namespaces (#4136)", (done) => {
+        const srv = http();
+        const sio = io(srv);
+        const counters = {
+          connected: 0,
+          created: 0,
+          events: 0,
+        };
+        const buffer = [];
+        srv.listen(() => {
+          const handler = () => {
+            if (++counters.events === 2) {
+              done();
+            }
+          };
+
+          sio
+            .of((name, query, next) => {
+              buffer.push(next);
+              if (buffer.length === 2) {
+                buffer.forEach((next) => next(null, true));
+              }
+            })
+            .on("connection", (socket) => {
+              if (++counters.connected === 2) {
+                sio.of("/dynamic-101").emit("message");
+              }
+            });
+
+          let one = client(srv, "/dynamic-101");
+          let two = client(srv, "/dynamic-101");
+          one.on("message", handler);
+          two.on("message", handler);
+        });
+      });
     });
   });