v2-sockets cannot connect to Socket.io 4.3.0 with allowEIO3 enabled.

[spasma]

Describe the bug
Version 4.3.0 breaks allowEIO3 support. I know the package is not officially released yet, but I accidentally updated tot 4.3.0 due to my package file with socket.io": "^4.2.0".

I saw that allowEIO3 was removed from the EngineOptions-interface in 4.2.0...4.3.0#diff-d5a4b3a0309f2337144861084c1014523cced561eb722cb9684c63f5e41e0567L119, maybe that's related to this issue.

To Reproduce
Connect to a server with a 2.0-client, with allowEIO3 on true while having version 4.3.0 installed.

Expected behavior
I expect to successfully connect to a 4.3.0-server with allowEIO3 on.

[spasma]

Just saw the reference by @chriskrycho.

It is possibly not only 2.0-clients, I'm moving from 2.0 to 4.* and all clients are currently still on the 2.0-client.

At this point I'm not sure if the 3.0-client was connecting to the 4.3 dev server or the production server on 4.2.

Excuses for the confusion.

[darrachequesne]

Hi! I cannot reproduce: https://github.com/socketio/socket.io-fiddle/tree/issues/socket.io/4120

Could you please check?

The allowEIO3 option is still supported, it was only moved to the engine.io repository: https://github.com/socketio/engine.io/blob/fe5d97fc3d7a26d34bce786a97962fae3d7ce17f/lib/server.ts#L113

[spasma]

Issue #4121 is related, it's not only 2.0-clients.