perf: add the wsPreEncodedFrame option

This optimization is only applied if:

- the permessage-deflate extension is disabled (which is the default)
- the "ws" package is used (which is the default)

In that case, the WebSocket frame will only be computed once, when
broadcasting to multiple clients.

Related: socketio/socket.io-adapter@5f7b47d

lib/transports/websocket.ts

@@ -93,11 +93,31 @@ export class WebSocket extends Transport {
 
     if (packet.options && typeof packet.options.wsPreEncoded === "string") {
       send(packet.options.wsPreEncoded);
+    } else if (this._canSendPreEncodedFrame(packet)) {
+      // the WebSocket frame was computed with WebSocket.Sender.frame()
+      // see https://github.com/websockets/ws/issues/617#issuecomment-283002469
+      this.socket._sender.sendFrame(packet.options.wsPreEncodedFrame, err => {
+        if (err) return this.onError("write error", err.stack);
+        this.send(packets);
+      });
     } else {
       this.parser.encodePacket(packet, this.supportsBinary, send);
     }
   }
 
+  /**
+   * Whether the encoding of the WebSocket frame can be skipped.
+   * @param packet
+   * @private
+   */
+  private _canSendPreEncodedFrame(packet) {
+    return (
+      !this.perMessageDeflate &&
+      typeof this.socket?._sender?.sendFrame === "function" &&
+      packet.options?.wsPreEncodedFrame !== undefined
+    );
+  }
+
   /**
    * Closes the transport.
    *

test/server.js

@@ -2822,6 +2822,53 @@ describe("server", () => {
           });
         });
       });
+
+      it("should use the pre-encoded frame", function(done) {
+        if (process.env.EIO_WS_ENGINE === "uws") {
+          return this.skip();
+        }
+        engine = listen(port => {
+          client = new ClientSocket(`ws://localhost:${port}`, {
+            transports: ["websocket"]
+          });
+
+          engine.on("connection", conn => {
+            conn.send("test", {
+              wsPreEncodedFrame: [
+                Buffer.from([129, 4]),
+                Buffer.from([52, 49, 50, 51])
+              ]
+            });
+          });
+
+          client.on("message", msg => {
+            expect(msg).to.be("123");
+            done();
+          });
+        });
+      });
+
+      it("should not use the pre-encoded frame when the permessage-deflate extension is enabled", done => {
+        engine = listen({ perMessageDeflate: true }, port => {
+          client = new ClientSocket(`ws://localhost:${port}`, {
+            transports: ["websocket"]
+          });
+
+          engine.on("connection", conn => {
+            conn.send("test", {
+              wsPreEncodedFrame: [
+                Buffer.from([129, 4]),
+                Buffer.from([52, 49, 50, 51])
+              ]
+            });
+          });
+
+          client.on("message", msg => {
+            expect(msg).to.be("test");
+            done();
+          });
+        });
+      });
     });
   });