stdio_tinyusb_cdc_acm hangs with picolibc

[benpicco]

Description

When picolibc is used instead of newlib, the output of stdio_tinyusb_cdc_acm hangs. This does not happen with stdio_cdc_acm (usbus).

Steps to reproduce the issue

Combine

USEMODULE += stdio_tinyusb_cdc_acm

with

FEATURES_REQUIRED += picolibc

Expected results

You can connect to the /dev/ttyACMx interface and receive stdio in/output.

Actual results

The terminal hangs when connecting to /dev/ttyACMx, even disconnect is only possible after a timeout.

Versions

[maribu]

What was the board you used? (I'm not saying I was unable to reproduce. But it might be easier to look into the issue if I use a board with an MCU of the same family.)

[benpicco]

I think I used a weact-f411ce