"Transport not open" after upgrade to v4

Describe the bug Yesterday I have upgraded an Angular application which uses socket.io-client to version 4.0.0. The server was also upgraded to 4.0.0. Today our logging recorded multiple “Transport not open” errors on the client side which were never observed before. The (minified) stack trace looks like this:

m.send at line 1:198353      {snip} .onClose()),this}send(e){if("open"!==this.readyState)throw new Error("Transport not open");this.write(e)}onOpen(){this.readyState="open",thi {snip}
c.flush
c.sendPacket
c.write
PVQj.t.Manager._packet
t.Socket.packet
t.Socket.emit

To Reproduce Unable to reproduce so far.

Platform:

• Device: Chrome 89 + Safari 14
• OS: Windows + Mac

Additional context socket.emit() is only called when socket.connected returns true. But somehow during the emit the Transport not open is thrown.

I have downgraded the client to 3.1.3 to see if the error persists and will update this issue tomorrow with the results.

[texonidas]

I’m having a similar issue to @RonnieRocket147, ours seems to be triggered by a user clicking on tel protocol links. The user in question also has relatively poor internet, but it only occurs as an actual error when tel protocol links are clicked.

[darrachequesne]

I could indeed reproduce, thanks.

This seems to be linked to this commit, included in engine.io-client@4.1.2 and socket.io-client@3.1.2.