Do you have a full reproducer?

Here is example it causes issue like 50% of the time, look for immediate socket close after open

import { App } from 'uWebSockets.js';

const port = 80;

App()

.get('/', r => r.end(`<script>
   ws = new WebSocket(location.origin+'/ws');
   ws.onopen = e => console.log('open');
   ws.onmessage = e => console.log('message', e.data);
   ws.onclose = e => console.log('close');
</script>`))

.ws('/ws', {
   upgrade: async (res, req, context) => {
      const wsHeaders = [ req.getHeader('sec-websocket-key'), req.getHeader('sec-websocket-protocol'), req.getHeader('sec-websocket-extensions') ];
      res.onAborted(() => res.aborted = true);
      await new Promise(r => setTimeout(r, 100));
      res.cork(() => res.upgrade({}, ...wsHeaders, context));
   },
   open: ws => console.log('open') || ws.send('w'.repeat(56)),
   message: ws => console.log('message'),
   close: ws => console.log('close'),
})

.listen(port, s => s && console.log(`http://localhost:${port}`));

I believe I am observing a similar issue with my application. I was also able to reproduce it with the provided script. Here are my observations regarding the tests I made with the provided script.

The behavior is not consistent. Most of the time it works fine. To reproduce it I have to restart the script multiple times until the issue was observed (which happens around 1 time out of 5 to 10 tries). When it is observed, the issue occurs on all the requests (each time I refresh the page) repeatedly. However, I have observed that the server may revert back to the correct behavior after a bit of time (few minutes).

The server is closing the socket with the code 1006 and an empty message (also what I observe in my application).

• I was able to reproduce the issue without sending anything in the open handler. So that does not seem to be a condition for the issue.
• I was not able to reproduce the issue when removing the corking in the upgrade handler.
• I was not able to reproduce the issue when removing the await instruction in the upgrade handler.

Here's a full packet capture of a single connection that observes the issue. We can see the server closing the connection right after the client acknowledged the first websocket packet:

I tested using uWebSockets.js v20.51.0 (current latest).

I tested with latest source build, confirmed issue with 1006 closed connection after async corked upgrade with ws.send in open handler. For me the error requires send in open. I do setTimeout(() => ws.send(), 100) to fix