feat: add timeout feature

Usage:

```js
socket.timeout(5000).emit("my-event", (err) => {
  if (err) {
    // the client did not acknowledge the event in the given delay
  }
});
```

lib/socket.ts

@@ -137,7 +137,7 @@ export class Socket<
   private readonly adapter: Adapter;
   private acks: Map<number, () => void> = new Map();
   private fns: Array<(event: Event, next: (err?: Error) => void) => void> = [];
-  private flags: BroadcastFlags = {};
+  private flags: BroadcastFlags & { timeout?: number } = {};
   private _anyListeners?: Array<(...args: any[]) => void>;
 
   /**
@@ -207,9 +207,11 @@ export class Socket<
 
     // access last argument to see if it's an ACK callback
     if (typeof data[data.length - 1] === "function") {
-      debug("emitting packet with ack id %d", this.nsp._ids);
-      this.acks.set(this.nsp._ids, data.pop());
-      packet.id = this.nsp._ids++;
+      const id = this.nsp._ids++;
+      debug("emitting packet with ack id %d", id);
+
+      this.registerAckCallback(id, data.pop());
+      packet.id = id;
     }
 
     const flags = Object.assign({}, this.flags);
@@ -220,6 +222,27 @@ export class Socket<
     return true;
   }
 
+  /**
+   * @private
+   */
+  private registerAckCallback(id: number, ack: (...args: any[]) => void): void {
+    const timeout = this.flags.timeout;
+    if (timeout === undefined) {
+      this.acks.set(id, ack);
+      return;
+    }
+
+    const timer = setTimeout(() => {
+      debug("event with ack id %d has timed out after %d ms", id, timeout);
+      ack.call(this, new Error("operation has timed out"));
+    }, timeout);
+
+    this.acks.set(id, (...args) => {
+      clearTimeout(timer);
+      ack.apply(this, [null, ...args]);
+    });
+  }
+
   /**
    * Targets a room when broadcasting.
    *
@@ -567,6 +590,26 @@ export class Socket<
     return this.newBroadcastOperator().local;
   }
 
+  /**
+   * Sets a modifier for a subsequent event emission that the callback will be called with an error when the
+   * given number of milliseconds have elapsed without an acknowledgement from the client:
+   *
+   * ```
+   * socket.timeout(5000).emit("my-event", (err) => {
+   *   if (err) {
+   *     // the client did not acknowledge the event in the given delay
+   *   }
+   * });
+   * ```
+   *
+   * @returns self
+   * @public
+   */
+  public timeout(timeout: number): this {
+    this.flags.timeout = timeout;
+    return this;
+  }
+
   /**
    * Dispatch incoming event to socket listeners.
    *

test/socket-timeout.ts

@@ -0,0 +1,34 @@
+import { Server } from "..";
+import { createClient, success } from "./support/util";
+import expect from "expect.js";
+
+describe("timeout", () => {
+  it("should timeout if the client does not acknowledge the event", (done) => {
+    const io = new Server(0);
+    const client = createClient(io, "/");
+
+    io.on("connection", (socket) => {
+      socket.timeout(50).emit("unknown", (err) => {
+        expect(err).to.be.an(Error);
+        success(done, io, client);
+      });
+    });
+  });
+
+  it("should not timeout if the client does acknowledge the event", (done) => {
+    const io = new Server(0);
+    const client = createClient(io, "/");
+
+    client.on("echo", (arg, cb) => {
+      cb(arg);
+    });
+
+    io.on("connection", (socket) => {
+      socket.timeout(50).emit("echo", 42, (err, value) => {
+        expect(err).to.be(null);
+        expect(value).to.be(42);
+        success(done, io, client);
+      });
+    });
+  });
+});

test/socket.io.ts

@@ -2888,4 +2888,6 @@ describe("socket.io", () => {
       });
     });
   });
+
+  require("./socket-timeout");
 });

test/support/util.ts

@@ -1,3 +1,11 @@
+import type { Server } from "../..";
+import {
+  io as ioc,
+  ManagerOptions,
+  Socket as ClientSocket,
+  SocketOptions,
+} from "socket.io-client";
+
 const expect = require("expect.js");
 const i = expect.stringify;
 
@@ -20,3 +28,19 @@ expect.Assertion.prototype.contain = function (...args) {
   }
   return contain.apply(this, args);
 };
+
+export function createClient(
+  io: Server,
+  nsp: string,
+  opts?: ManagerOptions & SocketOptions
+): ClientSocket {
+  // @ts-ignore
+  const port = io.httpServer.address().port;
+  return ioc(`http://localhost:${port}${nsp}`, opts);
+}
+
+export function success(done: Function, io: Server, client: ClientSocket) {
+  io.close();
+  client.disconnect();
+  done();
+}