fix: properly call the send callback during upgrade

The "drain" event (added in [1]) had two different meanings:

- the transport is ready to be written
- the packets are sent over the wire

For the WebSocket and the WebTransport transports, those two events
happen at the same time, but this is not the case for the HTTP
long-polling transport:

- the transport is ready to be written when the client sends a GET request
- the packets are sent over the wire when the server responds to the GET request

Which caused an issue with send callbacks during an upgrade, since the
packets were written but the client would not open a new GET request.

There are now two distinct events: "ready" and "drain"

Related: #695

[1]: 2a93f06

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-uws/polling.ts

@@ -97,7 +97,7 @@ export class Polling extends Transport {
     res.onAborted(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) {
@@ -291,6 +291,7 @@ export class Polling extends Transport {
     debug('writing "%s"', data);
     this.doWrite(data, options, () => {
       this.req.cleanup();
+      this.emit("drain");
     });
   }
 

lib/transports-uws/websocket.ts

@@ -60,8 +60,9 @@ export class WebSocket extends Transport {
         this.socket.send(data, isBinary, compress);
 
         if (isLast) {
-          this.writable = true;
           this.emit("drain");
+          this.writable = true;
+          this.emit("ready");
         }
       };
 

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) {

lib/userver.ts

@@ -80,7 +80,7 @@ export class uServer extends BaseServer {
           const transport = ws.getUserData().transport;
           transport.socket = ws;
           transport.writable = true;
-          transport.emit("drain");
+          transport.emit("ready");
         },
         message: (ws, message, isBinary) => {
           ws.getUserData().transport.onData(

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}`, {