fix(server): send events once the handshake is completed

Before that change, an event which was emitted in a middleware (before
the socket is considered connected) was sent before the CONNECT packet,
which is a bit weird.

Note: this explains why the Node.js client implementation has a buffer
there [1]

[1]: https://github.com/socketio/socket.io-client/blob/2d708137298784761763fdebbd64785819527f45/lib/socket.ts#L68

packages/socket.io/lib/socket.ts

@@ -144,6 +144,8 @@ export class Socket<
   #anyIncomingListeners?: Array<(...args: Event) => void>;
   #anyOutgoingListeners?: Array<(...args: Event) => void>;
 
+  #preConnectBuffer: Packet[] = [];
+
   /* private */ readonly client: Client<
     ListenEvents,
     EmitEvents,
@@ -197,8 +199,12 @@ export class Socket<
     const flags = Object.assign({}, this.flags);
     this.flags = {};
 
-    this._notifyOutgoingListeners(packet.data);
-    this.packet(packet, flags);
+    if (this.connected) {
+      this._notifyOutgoingListeners(packet.data);
+      this.packet(packet, flags);
+    } else {
+      this.#preConnectBuffer.push(packet);
+    }
 
     return true;
   }
@@ -514,6 +520,11 @@ export class Socket<
     this.connected = true;
     this.join(this.id);
     this.packet({ type: PacketType.CONNECT, data: { sid: this.id } });
+    this.#preConnectBuffer.forEach((packet) => {
+      this._notifyOutgoingListeners(packet.data);
+      this.packet(packet);
+    });
+    this.#preConnectBuffer = [];
   }
 
   /**

packages/socket.io/test/handshake.test.ts

@@ -10,6 +10,7 @@ import {
   eioPush,
   enableLogs,
   parseSessionID,
+  runHandshake,
 } from "../../util.test.ts";
 import { setup } from "./setup.test.ts";
 
@@ -236,4 +237,30 @@ describe("handshake", () => {
       },
     );
   });
+
+  it("should complete handshake before sending any event", () => {
+    const io = new Server();
+
+    return setup(
+      io,
+      1,
+      async (port, done) => {
+        io.use((socket) => {
+          socket.emit("1");
+          io.emit("ignored"); // socket is not connected yet
+          return Promise.resolve();
+        });
+
+        io.on("connection", (socket) => {
+          socket.emit("2");
+        });
+
+        const [_, firstPacket] = await runHandshake(port);
+
+        assertEquals(firstPacket, '42["1"]\x1e42["2"]');
+
+        done();
+      },
+    );
+  });
 });