Shell: Race during init w CONFIG_SHELL_BACKEND_SERIAL_API_INTERRUPT_DRIVEN=y

[aescolar]

Describe the bug

There seems to be a race in the shell if CONFIG_SHELL_BACKEND_SERIAL_API_INTERRUPT_DRIVEN is enabled ,
where the UART Rx interrupt is enabled
https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/shell/backends/shell_uart.c#L221
before shell_thread is spawned.
With the UART Rx interrupt triggering (before that thread is processing), an SHELL_SIGNAL_RXRDY event
https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/shell/backends/shell_uart.c#L130
will be queued, but it will never be dequeued.

Note that as part of shell_start(), z_shell_backend_rx_buffer_flush() is called.
https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/shell/shell.c#L1444
Removing the already received data. But the SHELL_SIGNAL_RXRDY event is left pending, so this is inconsistent.

In any case, this k_event_wait() https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/shell/shell.c#L1338
would seem to wait for new events (not return if one was already pended), so it will block there.

Regression

• This is a regression.

Steps to reproduce

The easiest way to repro it may be by taking this #93957,
and run
echo -e "help\n" | zephyr/zephyr.exe -uart_stdinout | head -n 80
You can see that the shell does not process the help message and does not print anything.
While with CONFIG_SHELL_BACKEND_SERIAL_API_INTERRUPT_DRIVEN=n it does.

Relevant log output

N/A

Expected behavior

The shell enables the UART reception when it can start processing the data.

Impact

Functional Limitation – Some features not working as expected, but system usable.

Environment

• OS Linux
• host gcc 14.2
• Main as of now (8843fd9 , but with patch to trigger issue)

Additional Context

Issue was discovered while trying #93957

[henrikbrixandersen]

The easiest way to repro it may be by taking this #93957,

For those who wants to follow these instructions please note that #93957 was reverted in #94426 due to the issues reported in #94425.

[tpambor]

This can now also be reproduced on main:

1. cd samples/subsys/shell/shell_module/
2. west build -b native_sim -- -DCONFIG_SHELL_BACKEND_SERIAL_INTERRUPT_DRIVEN=y
3. echo -e "help\n" | build/zephyr/zephyr.exe -uart_stdinout | head -n 80