Don't trip the WDT if interrupts are disabled during write() or flush().

Prior to this change, if interrupts were disabled during a call to
HardwareSerial::write() when the circular buffer was full, or
HardwareSerial::flush() when the circular buffer was non-empty,
we would loop forever and trip a watchdog timeout.

Author: alltheblinkythings

cores/esp8266/HardwareSerial.cpp

@@ -636,6 +636,9 @@ void HardwareSerial::flush() {
             if(_tx_buffer->getSize() == 0 &&
                UART_GET_TX_FIFO_ROOM(uart_nr) >= UART_TX_FIFO_SIZE) {
                 break;
+            } else if(il.savedInterruptLevel() > 0) {
+                _tx_empty_irq();
+                continue;
             }
         }
         yield();
@@ -663,6 +666,9 @@ size_t HardwareSerial::write(uint8_t c) {
                 break;
             } else if(_tx_buffer->write(c)) {
                 break;
+            } else if(il.savedInterruptLevel() > 0) {
+                _tx_empty_irq();
+                continue;
             }
         }
         yield();

cores/esp8266/interrupts.h

@@ -31,6 +31,10 @@ class InterruptLock {
         xt_wsr_ps(_state);
     }
 
+    uint32_t savedInterruptLevel() const {
+        return _state & 0x0f;
+    }
+
 protected:
     uint32_t _state;
 };