esp_modem freezes when flow control is enabled and module is unresponsive (IDFGH-12739)

[antfarming]

esp_modem freezes when flow control is enabled and module is unresponsive (IDFGH-12739)

[david-cermak]

I think I understand the problem and see that neither the modem layer nor the uart driver accounted for this situation.
General solution would be to add a send timeout to the Tx path as well, similar to generic socket API (write only if a socket is writable). On the modem layer, we'd just try to send an AT command (with a certain timeout) and fail even if we're not able to transmit within the timeout, now we'd just get stuck.

I don't think we'd address this issue generically enough however (might be that we'd add an API to check whether the ringbuffer is full or so).
I'd suggest looking into practical solutions for now, such as:

• persist the modem settings with AT&W or similar, so the flow control is ON after power-up
• checking the RTS signal manually before starting communication, or after wakeup event