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Cura, serial port and lock files on linux - possible bug

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I was printing from simplify3D (on linux) via USB to UM2 when I loaded Cura (15.06.01) to check another design. At startup Cura connected to the printer (which was already printing) which lead to head crashing into the right wall and the belt skipping and then the print resumed several centimeters to the side (in the air). While there was no damage to the printer, a 4h print was ruined.

I am not sure what Cura's default behaviour is but, as far as I know, Cura should not try to connect to a printer which is in use.

Simplify3D created the lock file var/lock/LCK..ttyACM0 meaning it needs exclusive access to the tty port. Cura should have checked for this file (and for the PID of the process which created it) before trying to connect to the printer. More details on serial port locking on linux can be found here. Unless I got everything wrong, cura should be following this.


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Will post it on the github, have to make an account first.

By the way, on linux serial port is a terminal, just like text terminals. They don't get locked automatically on opening as, in some cases, it is useful to have access to a terminal from multiple programs. For example if you are using a text terminal and someone wants to send you a message. I can also see use of this for debugging devices connected via virtual serial port.

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