Octoprint: [1.3.10rc1] Timeout when connecting to printer that doesn't send start on connect

[zeroflow]

What were you doing?

1. Update OctoPrint to 1.3.10rc1
2. Restart the server as prompted
3. Click "Connect"

Hint: Going back to 1.3.9 makes it work again.

What did you expect to happen?

Connection to the printer established

What happened instead?

Nothing / Serial timeout in the console after a few seconds

Did the same happen when running OctoPrint in safe mode?

Yes, safe mode shows the same behavior.

Version of OctoPrint

1.3.10rc1

Operating System running OctoPrint

OctoPi 0.14.0

Printer model & used firmware incl. version

Anycubic Kossel, MKS sBase board, Smoothieware (~ June 2018)

Browser and version of browser, operating system running browser

Google Chrome latest, Windows 10 Fall18 Update

Link to octoprint.log

https://gist.github.com/zeroflow/1efe8597188962995618d9ca0886fa34

Link to contents of terminal tab or serial.log

Changing monitoring state from "Offline" to "Detecting serial port"
Serial port list: ['/dev/ttyACM0']
Connecting to: /dev/ttyACM0
Changing monitoring state from "Detecting serial port" to "Opening serial port"
Connected to: Serial(port='/dev/ttyACM0', baudrate=250000, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitor
Changing monitoring state from "Opening serial port" to "Connecting"
Connection closed, closing down monitor
Changing monitoring state from "Connecting" to "Offline"

Link to contents of Javascript console in the browser

Screenshot(s)/video(s) showing the problem:

No applicable / or neccesary.

I have read the FAQ.

[foosel]

Can you provide an octoprint.log and serial.log from a working connection attempt under 1.3.9?

[zeroflow]

Uploaded here: https://gist.github.com/zeroflow/dc1f63f4009600b6acbe9bf7c25b1a08

[foosel]

Do you have Settings > Serial Connection > Firmware & protocol > Wait for start on connect enabled by any chance?

[zeroflow]

No, Wait for start on connect is not enabled.

[foosel]

I can reproduce it, the missing start at connection init is the trigger. Shouldn't make stuff stall, but for some reason it does. On it.

[foosel]

Fixed by the above commit, will be part of 1.3.10rc2

[foosel]

1.3.10rc2 was just released.

[cmock]

I can confirm 1.3.10rc2 connects fine to my Klipper printer now (originally reported in #2875).