socketio / socketio/socket.io

Express Middleware Errors Do Not Stop Socket.IO Reconnections

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Description

Describe the bug
The documentation states that middleware errors should stop the client from reconnecting. However, this does not seem to be the case for middleware created using io.engine.use.

To Reproduce

Socket.IO server version: 4.7.5
Socket.IO client version: 4.7.5

Server

For example, let's create two middlewares that will trigger errors:

First:

io.use((socket, next) => {
  next(new Error('thou shall not pass and stop'))
})

Second:

io.engine.use((req, res, next) => {
  next(new Error('thou shall not pass'))
})

Client

Log the response as follows:

socket.on('connect_error', (err) => {
  console.log('websocket error:', err.message, socket.active)
})

Using the first middleware, the response is:

websocket error: thou shall not pass and stop false

Using the second middleware, however, the response is:

websocket error: xhr poll error true
websocket error: xhr poll error true
websocket error: xhr poll error true
...

Expected behavior
I'm not sure if I missed something, but I believe that this type of middleware should, in fact, stop reconnections!

Platform:

  • Device: PC
  • OS: Windows 10

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the reported server and client snippets using io.use, io.engine.use, and connect_error, reproducing the behavior with Socket.IO 4.7.5. Trace how each middleware error is handled during Engine.IO polling and add a regression test showing that the engine middleware error stops reconnections, matching the existing io.use behavior.

Written by the indexing model from the issue text.

Assessment

Tech stack
nodejs, typescript
Domain
backend, networking
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
42/100

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