fix: fix race condition in dynamic namespaces (#4137)

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

lib/client.ts

@@ -116,7 +116,6 @@ export class Client<
           | false
       ) => {
         if (dynamicNspName) {
-          debug("dynamic namespace %s was created", dynamicNspName);
           this.doConnect(name, auth);
         } else {
           debug("creation of namespace %s was denied", name);

lib/index.ts

@@ -202,15 +202,18 @@ export class Server<
       }
       nextFn.value(name, auth, (err, allow) => {
         if (err || !allow) {
-          run();
-        } else {
-          const namespace = this.parentNsps
-            .get(nextFn.value)!
-            .createChild(name);
-          // @ts-ignore
-          this.sockets.emitReserved("new_namespace", namespace);
-          fn(namespace);
+          return run();
         }
+        if (this._nsps.has(name)) {
+          // the namespace was created in the meantime
+          debug("dynamic namespace %s already exists", name);
+          return fn(this._nsps.get(name) as Namespace);
+        }
+        const namespace = this.parentNsps.get(nextFn.value)!.createChild(name);
+        debug("dynamic namespace %s was created", name);
+        // @ts-ignore
+        this.sockets.emitReserved("new_namespace", namespace);
+        fn(namespace);
       });
     };
 

test/socket.io.ts

@@ -15,6 +15,8 @@ import { io as ioc, Socket as ClientSocket } from "socket.io-client";
 import "./support/util";
 import "./utility-methods";
 
+type callback = (err: Error | null, success: boolean) => void;
+
 // Creates a socket.io client for the given server
 function client(srv, nsp?: string | object, opts?: object): ClientSocket {
   if ("object" == typeof nsp) {
@@ -998,6 +1000,46 @@ describe("socket.io", () => {
           const socket = client(srv, "/dynamic-101");
         });
       });
+
+      it("should handle race conditions with dynamic namespaces (#4136)", (done) => {
+        const srv = createServer();
+        const sio = new Server(srv);
+        const counters = {
+          connected: 0,
+          created: 0,
+          events: 0,
+        };
+        const buffer: callback[] = [];
+        sio.on("new_namespace", (namespace) => {
+          counters.created++;
+        });
+        srv.listen(() => {
+          const handler = () => {
+            if (++counters.events === 2) {
+              expect(counters.created).to.equal(1);
+              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);
+        });
+      });
     });
   });