HardwareSerial/Uart write and flush lock up when interrupts are disabled #472

[fpistm]

Looking good. There is one worry I have still - if this code is called from inside an ISR (or another context where interrupts are disabled) and Serial has TX ongoing, I think this will result in an infinite loop, right? I know that logging/serial printing from inside an ISR is often considered unsupported and should not be done, but it is often very useful so if at all possible, we should support it (or at least not lock up - dropping output might be preferable then). For example AVR HardwareSerial does support it, and there's an issue about SAMD here (which also has some details on how it works on AVR).

Originally posted by @matthijskooijman in https://github.com/stm32duino/Arduino_Core_STM32/pull/1826#pullrequestreview-1116691995