Python-socketio: Seeming race condition for the synchronous client

[sigmavirus24]

Hi there old friend,

We just upgraded from 4.6.1 to 5.0.4. Prior to the upgrade, our code was working totally fine but now occasionally crashes.

Our code first callsconnect(url, transports=["websockets"], headers={"Authorization": "FakeAuthHeader"}) which succeeds and the library's logging prints Engine.IO connection established. Sometimes, it then promptly prints Namespace / is connected. Other times, our code reaches emit("auth", API_KEY) which then errors with socketio.exceptions.BadNamespaceError: / is not a connected namespace. but interleaved in the traceback is Namespace / is connected. It seems like there's a race-condition in how connect(...) -> _handle_eio_connect -> _send_packet -> _handle_connect flows and it's not entirely synchronous.

I skimmed the code but didn't immediately see the problem. Hopefully this helps.

[miguelgrinberg]

Hey Ian, great to hear from you!

This is not a new problem, but the error is indeed new. The Socket.IO connection happens in the background. When you call connect() you are initiating it, but several exchanges between the client and the server have to happen to get everything set up. You can be sure that it is safe to emit when the connect handler for your namespace is invoked.

In the 4.x and older releases if you called emit() after connect() and the connection wasn't fully set up the message could be lost silently. In the 5.x releases I attempted to "fix" this by adding the BadNamespaceError, which is triggered all the way up to the point where it is 100% safe to emit without the risk of losing a message.

You are not the first to complain about this, so I intend to improve this. The two options that I'm considering are either to delay the return from connect() until the namespaces are connected, or alternatively to cache emits if they are done before the namespace can be used. Either way this is going to be transparent, you will just stop getting BadNamespaceErrror and everything is going to work.

What I've been telling people as a workaround is to move your emit to the connect event handler, and then it'll work 100%. But for some people this is weird, they prefer to have a sequential logic in their client, so I've seen code that does a sleep(1) between the connect and the emit and that also sort of works.

It doesn't appear that I have any open issues to track this, so I might as well use this one. Thanks!

[sigmavirus24]

The emits being silently dropped makes a lot of sense now in the context of other behaviour we were seeing. That's super helpful to understand. Also I had no clue it would be safe to use a connect handler to emit those messages. I think we'll try that and see how it works. Thanks!

[miguelgrinberg]

@sigmavirus24 I have attempted to fix this by adding a wait argument to the connect() method. There is no need to change anything in the application code, the default for wait is True. There is also a wait_timeout, which is 1 second by default, but can be made larger for slow networks.

I'll run some more test in the next couple of days. For now, you can test by installing this package from git.

[jsib0]

This problem still persist even with the upgrade. In my case, I am testing the lost of wifi connection of the Raspberry Pi4 by running sudo ifconfig wlan0 down ,then sudo ifconfig wlan0 up. Once the internet is back up, I get the same error of is not connected to namespace <namespace>

[miguelgrinberg]

@jsib0 my guess is that your problem is different, but in any case, if you think this is a problem then write a new issue and include detailed logs that cover the incident.