feat: add support for Socket.IO v2 clients

In order to ease the migration to Socket.IO v3, the Socket.IO server
can now communicate with v2 clients.

```js
const io = require("socket.io")({
  allowEIO3: true
});
```

This feature is disabled by default.

lib/index.ts

@@ -100,6 +100,11 @@ interface EngineOptions {
    * the options that will be forwarded to the cors module
    */
   cors: CorsOptions;
+  /**
+   * whether to enable compatibility with Socket.IO v2 clients
+   * @default false
+   */
+  allowEIO3: boolean;
 }
 
 interface AttachOptions {
@@ -522,7 +527,11 @@ export class Server extends EventEmitter {
    */
   private onconnection(conn): Server {
     debug("incoming connection with id %s", conn.id);
-    new Client(this, conn);
+    const client = new Client(this, conn);
+    if (conn.protocol === 3) {
+      // @ts-ignore
+      client.connect("/");
+    }
     return this;
   }
 

lib/namespace.ts

@@ -135,11 +135,16 @@ export class Namespace extends EventEmitter {
     this.run(socket, (err) => {
       process.nextTick(() => {
         if ("open" == client.conn.readyState) {
-          if (err)
-            return socket._error({
-              message: err.message,
-              data: err.data,
-            });
+          if (err) {
+            if (client.conn.protocol === 3) {
+              return socket._error(err.data || err.message);
+            } else {
+              return socket._error({
+                message: err.message,
+                data: err.data,
+              });
+            }
+          }
 
           // track socket
           this.sockets.set(socket.id, socket);

lib/socket.ts

@@ -105,7 +105,12 @@ export class Socket extends EventEmitter {
     super();
     this.server = nsp.server;
     this.adapter = this.nsp.adapter;
-    this.id = base64id.generateId(); // don't reuse the Engine.IO id because it's sensitive information
+    if (client.conn.protocol === 3) {
+      // @ts-ignore
+      this.id = nsp.name !== "/" ? nsp.name + "#" + client.id : client.id;
+    } else {
+      this.id = base64id.generateId(); // don't reuse the Engine.IO id because it's sensitive information
+    }
     this.connected = true;
     this.disconnected = false;
     this.handshake = this.buildHandshake(auth);
@@ -286,7 +291,11 @@ export class Socket extends EventEmitter {
   _onconnect(): void {
     debug("socket connected - writing packet");
     this.join(this.id);
-    this.packet({ type: PacketType.CONNECT, data: { sid: this.id } });
+    if (this.conn.protocol === 3) {
+      this.packet({ type: PacketType.CONNECT });
+    } else {
+      this.packet({ type: PacketType.CONNECT, data: { sid: this.id } });
+    }
   }
 
   /**

package.json

@@ -49,7 +49,7 @@
     "accepts": "~1.3.4",
     "base64id": "~2.0.0",
     "debug": "~4.3.1",
-    "engine.io": "~4.0.6",
+    "engine.io": "~4.1.0",
     "socket.io-adapter": "~2.0.3",
     "socket.io-parser": "~4.0.3"
   },
@@ -64,6 +64,7 @@
     "prettier": "^2.2.0",
     "rimraf": "^3.0.2",
     "socket.io-client": "3.0.5",
+    "socket.io-client-v2": "npm:socket.io-client@^2.4.0",
     "superagent": "^6.1.0",
     "supertest": "^6.0.1",
     "ts-node": "^9.0.0",

test/socket.io.ts

@@ -8,11 +8,11 @@ import { exec } from "child_process";
 import request from "supertest";
 import expect from "expect.js";
 import type { AddressInfo } from "net";
+import * as io_v2 from "socket.io-client-v2";
 
 const ioc = require("socket.io-client");
 
 import "./support/util";
-import exp = require("constants");
 
 // Creates a socket.io client for the given server
 function client(srv, nsp?: string | object, opts?: object) {
@@ -26,6 +26,18 @@ function client(srv, nsp?: string | object, opts?: object) {
   return ioc(url, opts);
 }
 
+const success = (sio, clientSocket, done) => {
+  sio.close();
+  clientSocket.close();
+  done();
+};
+
+const waitFor = (emitter, event) => {
+  return new Promise((resolve) => {
+    emitter.once(event, resolve);
+  });
+};
+
 describe("socket.io", () => {
   it("should be the same version as client", () => {
     const version = require("../package").version;
@@ -2430,4 +2442,48 @@ describe("socket.io", () => {
       });
     });
   });
+
+  describe("v2 compatibility", () => {
+    it("should connect if `allowEIO3` is true", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv, {
+        allowEIO3: true,
+      });
+
+      srv.listen(async () => {
+        const port = (srv.address() as AddressInfo).port;
+        const clientSocket = io_v2.connect(`http://localhost:${port}`, {
+          multiplex: false,
+        });
+
+        const [socket]: Array<any> = await Promise.all([
+          waitFor(sio, "connection"),
+          waitFor(clientSocket, "connect"),
+        ]);
+
+        expect(socket.id).to.eql(clientSocket.id);
+        success(sio, clientSocket, done);
+      });
+    });
+
+    it("should not connect if `allowEIO3` is false (default)", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      srv.listen(() => {
+        const port = (srv.address() as AddressInfo).port;
+        const clientSocket = io_v2.connect(`http://localhost:${port}`, {
+          multiplex: false,
+        });
+
+        clientSocket.on("connect", () => {
+          done(new Error("should not happen"));
+        });
+
+        clientSocket.on("connect_error", () => {
+          success(sio, clientSocket, done);
+        });
+      });
+    });
+  });
 });