USBSerial would not block on write if the USB inteface is not connected

cores/arduino/USBSerial.cpp

@@ -83,9 +83,11 @@ 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()) {
+      break;
     }
   }
-  return size;
+  return size - rest;
 }
 
 int USBSerial::available(void)

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

@@ -32,6 +32,15 @@
 #define CDC_MAX_PACKET_SIZE USB_MAX_EP0_SIZE
 #endif
 
+/*
+ * The value USB_CDC_TRANSMIT_TIMEOUT is defined in terms of HAL_GetTick() units.
+ * Typically it is 1ms value. The timeout determines when we would consider the
+ * host "too slow" and threat the USB CDC port as disconnected.
+ */
+#ifndef USB_CDC_TRANSMIT_TIMEOUT
+#define USB_CDC_TRANSMIT_TIMEOUT 3
+#endif
+
 /* USBD_CDC Private Variables */
 /* USB Device Core CDC handle declaration */
 USBD_HandleTypeDef hUSBD_Device_CDC;
@@ -43,6 +52,7 @@ CDC_TransmitQueue_TypeDef TransmitQueue;
 CDC_ReceiveQueue_TypeDef ReceiveQueue;
 __IO uint32_t lineState = 0;
 __IO bool receivePended = true;
+static uint32_t transmitStart = 0;
 
 
 /** USBD_CDC Private Function Prototypes */
@@ -212,6 +222,7 @@ static int8_t USBD_CDC_Receive(uint8_t *Buf, uint32_t *Len)
 
 static int8_t USBD_CDC_Transferred(void)
 {
+  transmitStart = 0;
   CDC_TransmitQueue_CommitRead(&TransmitQueue);
   CDC_continue_transmit();
   return (USBD_OK);
@@ -245,6 +256,17 @@ void CDC_deInit(void)
   }
 }
 
+bool CDC_connected()
+{
+  uint32_t transmitTime = 0;
+  if (transmitStart) {
+    transmitTime = HAL_GetTick() - transmitStart;
+  }
+  return hUSBD_Device_CDC.dev_state == USBD_STATE_CONFIGURED
+         && transmitTime < USB_CDC_TRANSMIT_TIMEOUT
+         && lineState;
+}
+
 void CDC_continue_transmit(void)
 {
   uint16_t size;
@@ -261,6 +283,7 @@ void CDC_continue_transmit(void)
   if (hcdc->TxState == 0U) {
     buffer = CDC_TransmitQueue_ReadBlock(&TransmitQueue, &size);
     if (size > 0) {
+      transmitStart = HAL_GetTick();
       USBD_CDC_SetTxBuffer(&hUSBD_Device_CDC, buffer, size);
       /*
        * size never exceed PMA buffer and USBD_CDC_TransmitPacket make full

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);
+bool CDC_connected();
 
 #ifdef __cplusplus
 }