fix: properly call send callbacks during upgrade

lib/socket.ts

@@ -221,18 +221,21 @@ export class Socket extends EventEmitter {
    */
   private setTransport(transport) {
     const onError = this.onError.bind(this);
+    const onReady = () => this.flush();
     const onPacket = this.onPacket.bind(this);
     const onDrain = this.onDrain.bind(this);
     const onClose = this.onClose.bind(this, "transport close");
 
     this.transport = transport;
     this.transport.once("error", onError);
+    this.transport.on("ready", onReady);
     this.transport.on("packet", onPacket);
     this.transport.on("drain", onDrain);
     this.transport.once("close", onClose);
 
     this.cleanupFn.push(function () {
       transport.removeListener("error", onError);
+      transport.removeListener("ready", onReady);
       transport.removeListener("packet", onPacket);
       transport.removeListener("drain", onDrain);
       transport.removeListener("close", onClose);
@@ -245,8 +248,6 @@ export class Socket extends EventEmitter {
    * @private
    */
   private onDrain() {
-    this.flush();
-
     if (this.sentCallbackFn.length > 0) {
       debug("executing batch send callback");
       const seqFn = this.sentCallbackFn.shift();

lib/transports/polling.ts

@@ -96,7 +96,7 @@ export class Polling extends Transport {
     req.on("close", onClose);
 
     this.writable = true;
-    this.emit("drain");
+    this.emit("ready");
 
     // if we're still writable but had a pending close, trigger an empty send
     if (this.writable && this.shouldClose) {
@@ -258,6 +258,7 @@ export class Polling extends Transport {
     debug('writing "%s"', data);
     this.doWrite(data, options, () => {
       this.req.cleanup();
+      this.emit("drain");
     });
   }
 

lib/transports/websocket.ts

@@ -103,8 +103,9 @@ export class WebSocket extends Transport {
     if (err) {
       this.onError("write error", err.stack);
     } else {
-      this.writable = true;
       this.emit("drain");
+      this.writable = true;
+      this.emit("ready");
     }
   };
 

lib/transports/webtransport.ts

@@ -56,8 +56,9 @@ export class WebTransport extends Transport {
       debug("error while writing: %s", e.message);
     }
 
-    this.writable = true;
     this.emit("drain");
+    this.writable = true;
+    this.emit("ready");
   }
 
   doClose(fn) {

test/server.js

@@ -2701,6 +2701,29 @@ describe("server", () => {
         });
       });
 
+      it("should execute when message sent during polling upgrade window", (done) => {
+        const engine = listen((port) => {
+          const socket = new ClientSocket(`ws://localhost:${port}`, {
+            transports: ["polling", "websocket"],
+          });
+
+          const partialDone = createPartialDone(() => {
+            engine.httpServer.close();
+            socket.close();
+            done();
+          }, 2);
+
+          engine.on("connection", (conn) => {
+            conn.on("upgrading", () => {
+              conn.send("a", partialDone);
+            });
+          });
+          socket.on("open", () => {
+            socket.on("message", partialDone);
+          });
+        });
+      });
+
       it("should execute when message sent (websocket)", (done) => {
         const engine = listen({ allowUpgrades: false }, (port) => {
           const socket = new ClientSocket(`ws://localhost:${port}`, {