Do not block writes to USBSerial if CDC interface is not connected.

Author: ghent360

cores/arduino/USBSerial.cpp

@@ -83,6 +83,8 @@ size_t USBSerial::write(const uint8_t *buffer, size_t size)
       buffer += portion;
       // After storing data, start transmitting process
       CDC_continue_transmit();
+    } else if (!CDC_connected()) {
+      return size;
     }
   }
   return size;

cores/arduino/stm32/usb/cdc/usbd_cdc_if.c

@@ -245,6 +245,11 @@ void CDC_deInit(void)
   }
 }
 
+uint8_t CDC_connected()
+{
+  return hUSBD_Device_CDC.dev_state == USBD_STATE_CONFIGURED;
+}
+
 void CDC_continue_transmit(void)
 {
   uint16_t size;

cores/arduino/stm32/usb/cdc/usbd_cdc_if.h

@@ -50,6 +50,7 @@ void CDC_continue_transmit(void);
 void CDC_resume_receive(void);
 void CDC_init(void);
 void CDC_deInit(void);
+uint8_t CDC_connected();
 
 #ifdef __cplusplus
 }