stdio_cdc_acm causing corruption of first shell input

[keestux]

Description

Test programs that use shell and have stdio_cdc_acm enabled show corruption of the first input to shell. So far I have just seen this on my sodaq-sara-sff boards, but I assume this is true for all sam0 (arduino style boards).

Steps to reproduce the issue

• build one of the test programs
• connect a "term"
• look at the output
• type help and ENTER

Expected results

The banner should be printed, the test program's introduction and a prompt.

Actual results

With make ... term you'd see something like this:

$ make CC_NOCOLOR=1 BOARD=sodaq-sara-sff -C tests/periph/gpio flash term
make: Entering directory '/home/kees/src/RIOT/tests/periph/gpio'
Building application "tests_gpio" for "sodaq-sara-sff" with MCU "samd21".

"make" -C /home/kees/src/RIOT/pkg/cmsis/ 
...
/home/kees/src/RIOT/dist/tools/pyterm/pyterm -p "/dev/ttyACM0" -b "115200"  
Twisted not available, please install it if you want to use pyterm's JSON capabilities
2024-01-27 22:24:12,586 # Connect to serial port /dev/ttyACM0
Welcome to pyterm!
Type '/exit' to exit.
2024-01-27 22:24:13,590 # shell: command not found: c.
> 2024-01-27 22:24:13,590 # 
> 2024-01-27 22:24:13,590 # 
> 2024-01-27 22:24:13,591 # 
> help
2024-01-27 22:24:38,079 # NOTE: make suc.help
2024-01-27 22:24:38,079 # shell: command not found: NOTE:
> help
2024-01-27 22:28:12,523 # help
2024-01-27 22:28:12,523 # Command              Description
2024-01-27 22:28:12,524 # ---------------------------------------
2024-01-27 22:28:12,524 # init_out             init as output (push-pull mode)
...
> 

Sometimes I see part of the banner on the shell input.

Welcome to pyterm!
Type '/exit' to exit.
help
2024-01-27 22:34:26,163 # in(): This is Rmahelp
2024-01-27 22:34:26,163 # shell: command not found: main():
>