✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b165_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Avoid stalling if uart buffer becomes full #260
5. Select one of the boards under SAMD Pull Request Avoid stalling if uart buffer becomes full #260 in Tools->Board menu
6. Compile/Upload as usual

Why it stalls? Are you trying to write from inside an ISR?

On the AVR side the IRQ handler is called only if the global interrupts are disabled, and only if the UART already emptied the internal shift register:

  // If the output buffer is full, there's nothing for it other than to 
  // wait for the interrupt handler to empty it a bit
  while (i == _tx_buffer_tail) {
    if (bit_is_clear(SREG, SREG_I)) {      <----- here
      // Interrupts are disabled, so we'll have to poll the data
      // register empty flag ourselves. If it is set, pretend an
      // interrupt has happened and call the handler to free up
      // space for us.
      if(bit_is_set(*_ucsra, UDRE0))    <------ and here
	_tx_udr_empty_irq();
    } else {
      // nop, the interrupt handler will free up space for us
    }
  }

I think that we should implement a similar check here

Yes, you caught me 😄 Indeed I was printing from an ISR; the additional check doesn't seem to fix the situation, nor forcefully rearming the DataRegisterEmptyInterruptUART() interrupt.

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b174_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Avoid stalling if uart buffer becomes full #260
5. Select one of the boards under SAMD Pull Request Avoid stalling if uart buffer becomes full #260 in Tools->Board menu
6. Compile/Upload as usual

Closing in favour of #262.