From c5c95fac2b9ec90e3e8eb31512d5d738fbad8768 Mon Sep 17 00:00:00 2001
From: Mujeeb <jibulumujeeb@gmail.com>
Date: Sat, 1 Aug 2026 11:51:37 +0100
Subject: [PATCH] Fix event dispatch before disconnection

---
 packages/socket.io/lib/socket.ts  |  38 +++++++-
 packages/socket.io/test/socket.ts | 141 ++++++++++++++++++++++++++++++
 2 files changed, 175 insertions(+), 4 deletions(-)

diff --git a/packages/socket.io/lib/socket.ts b/packages/socket.io/lib/socket.ts
index 96d11a5573..02a81d6d79 100644
--- a/packages/socket.io/lib/socket.ts
+++ b/packages/socket.io/lib/socket.ts
@@ -36,6 +36,23 @@ import {
 
 const debug = debugModule("socket.io:socket");
 
+function hasBinary(data: unknown): boolean {
+  if (!data || typeof data !== "object") {
+    return false;
+  }
+  if (
+    Buffer.isBuffer(data) ||
+    data instanceof ArrayBuffer ||
+    ArrayBuffer.isView(data)
+  ) {
+    return true;
+  }
+  if (Array.isArray(data)) {
+    return data.some(hasBinary);
+  }
+  return Object.values(data).some(hasBinary);
+}
+
 const RECOVERABLE_DISCONNECT_REASONS: ReadonlySet<DisconnectReason> = new Set([
   "transport error",
   "transport close",
@@ -571,7 +588,10 @@ export class Socket<
         listener.apply(this, args);
       }
     }
-    this.dispatch(args);
+    this.dispatch(
+      args,
+      packet.type === PacketType.BINARY_EVENT || hasBinary(args),
+    );
   }
 
   /**
@@ -817,10 +837,12 @@ export class Socket<
    * @param {Array} event - event that will get emitted
    * @private
    */
-  private dispatch(event: Event): void {
+  private dispatch(event: Event, isBinary: boolean): void {
     debug("dispatching an event %j", event);
+    let isSynchronous = true;
+
     this.run(event, (err) => {
-      process.nextTick(() => {
+      const invoke = () => {
         if (err) {
           return this._onerror(err);
         }
@@ -829,8 +851,16 @@ export class Socket<
         } else {
           debug("ignore packet received after disconnection");
         }
-      });
+      };
+
+      if (isSynchronous && !isBinary && !err) {
+        invoke();
+      } else {
+        process.nextTick(invoke);
+      }
     });
+
+    isSynchronous = false;
   }
 
   /**
diff --git a/packages/socket.io/test/socket.ts b/packages/socket.io/test/socket.ts
index 41882a7987..9a93bac40f 100644
--- a/packages/socket.io/test/socket.ts
+++ b/packages/socket.io/test/socket.ts
@@ -9,6 +9,7 @@ import {
 } from "./support/util";
 import { Server } from "..";
 import expect from "expect.js";
+import { PacketType } from "socket.io-parser";
 
 describe("socket", () => {
   it("should not fire events more than once after manually reconnecting", (done) => {
@@ -907,6 +908,146 @@ describe("socket", () => {
     });
   });
 
+  it("should dispatch a non-binary event before disconnect cleanup", (done) => {
+    const io = new Server(0);
+    const clientSocket = createClient(io);
+
+    io.on("connection", (socket) => {
+      let anyCalled = false;
+      let namedCalled = false;
+
+      socket.onAny((event) => {
+        if (event === "before-disconnect") {
+          anyCalled = true;
+        }
+      });
+      socket.on("before-disconnect", () => {
+        namedCalled = true;
+      });
+
+      (socket as any).onevent({
+        type: PacketType.EVENT,
+        data: ["before-disconnect"],
+      });
+      (socket as any).ondisconnect();
+
+      expect(anyCalled).to.be(true);
+      expect(namedCalled).to.be(true);
+      success(done, io, clientSocket);
+    });
+  });
+
+  it("should dispatch a non-binary event after synchronous middleware", (done) => {
+    const io = new Server(0);
+    const clientSocket = createClient(io);
+
+    io.on("connection", (socket) => {
+      let namedCalled = false;
+
+      socket.use((_event, next) => {
+        next();
+      });
+      socket.on("before-disconnect", () => {
+        namedCalled = true;
+      });
+
+      (socket as any).onevent({
+        type: PacketType.EVENT,
+        data: ["before-disconnect"],
+      });
+      (socket as any).ondisconnect();
+
+      expect(namedCalled).to.be(true);
+      success(done, io, clientSocket);
+    });
+  });
+
+  it("should ignore an event when middleware completes after disconnect", (done) => {
+    const io = new Server(0);
+    const clientSocket = createClient(io);
+
+    io.on("connection", (socket) => {
+      let namedCalled = false;
+
+      socket.use((_event, next) => {
+        setImmediate(next);
+      });
+      socket.on("before-disconnect", () => {
+        namedCalled = true;
+      });
+
+      (socket as any).onevent({
+        type: PacketType.EVENT,
+        data: ["before-disconnect"],
+      });
+      (socket as any).ondisconnect();
+
+      setImmediate(() => {
+        expect(namedCalled).to.be(false);
+        success(done, io, clientSocket);
+      });
+    });
+  });
+
+  it("should ignore an event received after disconnection", (done) => {
+    const io = new Server(0);
+    const clientSocket = createClient(io);
+
+    io.on("connection", (socket) => {
+      let namedCalled = false;
+
+      socket.on("before-disconnect", () => {
+        namedCalled = true;
+      });
+
+      (socket as any).ondisconnect();
+      (socket as any).onevent({
+        type: PacketType.EVENT,
+        data: ["before-disconnect"],
+      });
+
+      setImmediate(() => {
+        expect(namedCalled).to.be(false);
+        success(done, io, clientSocket);
+      });
+    });
+  });
+
+  it("should dispatch non-binary events before disconnect cleanup for many clients", (done) => {
+    const io = new Server(0);
+    const clients = [];
+    let anyCalled = 0;
+    let namedCalled = 0;
+    let disconnected = 0;
+
+    io.on("connection", (socket) => {
+      socket.onAny((event) => {
+        if (event === "before-disconnect") {
+          anyCalled++;
+        }
+      });
+      socket.on("before-disconnect", () => {
+        namedCalled++;
+      });
+      socket.on("disconnect", () => {
+        if (++disconnected === 40) {
+          expect(anyCalled).to.be(40);
+          expect(namedCalled).to.be(40);
+          success(done, io, ...clients);
+        }
+      });
+    });
+
+    for (let i = 0; i < 40; i++) {
+      const clientSocket = createClient(io);
+      clients.push(clientSocket);
+      clientSocket.on("connect", () => {
+        clientSocket.emit("before-disconnect");
+        clientSocket.disconnect();
+      });
+    }
+  });
+
   it("should leave all rooms joined after a middleware failure", (done) => {
     const io = new Server(0);
     const client = createClient(io, "/");