fix: properly handle manually created dynamic namespaces

Namespaces that match the regex of a parent namespace will now be added
as a child of this namespace:

```js
const parentNamespace = io.of(/^\/dynamic-\d+$/);
const childNamespace = io.of("/dynamic-101");
```

Related:

- #4615
- #4164
- #4015
- #3960

lib/index.ts

@@ -178,6 +178,18 @@ export class Server<
     ParentNspNameMatchFn,
     ParentNamespace<ListenEvents, EmitEvents, ServerSideEvents, SocketData>
   > = new Map();
+
+  /**
+   * A subset of the {@link parentNsps} map, only containing {@link ParentNamespace} which are based on a regular
+   * expression.
+   *
+   * @private
+   */
+  private parentNamespacesFromRegExp: Map<
+    RegExp,
+    ParentNamespace<ListenEvents, EmitEvents, ServerSideEvents, SocketData>
+  > = new Map();
+
   private _adapter?: AdapterConstructor;
   private _serveClient: boolean;
   private readonly opts: Partial<ServerOptions>;
@@ -314,8 +326,6 @@ export class Server<
         }
         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);
       });
     };
@@ -691,6 +701,7 @@ export class Server<
           (nsp, conn, next) => next(null, (name as RegExp).test(nsp)),
           parentNsp
         );
+        this.parentNamespacesFromRegExp.set(name, parentNsp);
       }
       if (fn) {
         // @ts-ignore
@@ -703,6 +714,13 @@ export class Server<
 
     let nsp = this._nsps.get(name);
     if (!nsp) {
+      for (const [regex, parentNamespace] of this.parentNamespacesFromRegExp) {
+        if (regex.test(name as string)) {
+          debug("attaching namespace %s to parent namespace %s", name, regex);
+          return parentNamespace.createChild(name as string);
+        }
+      }
+
       debug("initializing namespace %s", name);
       nsp = new Namespace(this, name);
       this._nsps.set(name, nsp);

lib/parent-namespace.ts

@@ -11,6 +11,21 @@ import debugModule from "debug";
 
 const debug = debugModule("socket.io:parent-namespace");
 
+/**
+ * A parent namespace is a special {@link Namespace} that holds a list of child namespaces which were created either
+ * with a regular expression or with a function.
+ *
+ * @example
+ * const parentNamespace = io.of(/\/dynamic-\d+/);
+ *
+ * parentNamespace.on("connection", (socket) => {
+ *   const childNamespace = socket.nsp;
+ * }
+ *
+ * // will reach all the clients that are in one of the child namespaces, like "/dynamic-101"
+ * parentNamespace.emit("hello", "world");
+ *
+ */
 export class ParentNamespace<
   ListenEvents extends EventsMap = DefaultEventsMap,
   EmitEvents extends EventsMap = ListenEvents,
@@ -81,6 +96,10 @@ export class ParentNamespace<
     }
 
     this.server._nsps.set(name, namespace);
+
+    // @ts-ignore
+    this.server.sockets.emitReserved("new_namespace", namespace);
+
     return namespace;
   }
 

test/namespaces.ts

@@ -652,5 +652,16 @@ describe("namespaces", () => {
 
       io.of(/^\/dynamic-\d+$/);
     });
+
+    it("should attach a child namespace to its parent upon manual creation", () => {
+      const io = new Server(0);
+      const parentNamespace = io.of(/^\/dynamic-\d+$/);
+      const childNamespace = io.of("/dynamic-101");
+
+      // @ts-ignore
+      expect(parentNamespace.children.has(childNamespace)).to.be(true);
+
+      io.close();
+    });
   });
 });