serial: introduce "has receive data" query

Add a serial_has_receive_data() routine to the serial layer's API which
returns the number of (known to be) available RX data bytes. Implement
support in the libserialport specific code.

src/libsigrok-internal.h

@@ -1079,6 +1079,7 @@ SR_PRIV int serial_open(struct sr_serial_dev_inst *serial, int flags);
 SR_PRIV int serial_close(struct sr_serial_dev_inst *serial);
 SR_PRIV int serial_flush(struct sr_serial_dev_inst *serial);
 SR_PRIV int serial_drain(struct sr_serial_dev_inst *serial);
+SR_PRIV size_t serial_has_receive_data(struct sr_serial_dev_inst *serial);
 SR_PRIV int serial_write_blocking(struct sr_serial_dev_inst *serial,
 		const void *buf, size_t count, unsigned int timeout_ms);
 SR_PRIV int serial_write_nonblocking(struct sr_serial_dev_inst *serial,
@@ -1132,6 +1133,7 @@ SR_PRIV GSList *sr_ser_libsp_find_usb(GSList *list, sr_ser_find_append_t append,
 		uint16_t vendor_id, uint16_t product_id);
 SR_PRIV int sr_ser_libsp_get_frame_format(struct sr_serial_dev_inst *serial,
 		int *baud, int *bits);
+SR_PRIV size_t sr_ser_libsp_get_rx_avail(struct sr_serial_dev_inst *serial);
 #endif
 
 /*--- ezusb.c ---------------------------------------------------------------*/

src/serial.c

@@ -152,6 +152,21 @@ SR_PRIV int serial_drain(struct sr_serial_dev_inst *serial)
 	return sr_ser_libsp_drain(serial);
 }
 
+/**
+ * Check for available receive data.
+ *
+ * @param[in] serial Previously opened serial port instance.
+ *
+ * @returns The number of (known) available RX data bytes.
+ *
+ * Returns 0 if no receive data is available, or if the amount of
+ * available receive data cannot get determined.
+ */
+SR_PRIV size_t serial_has_receive_data(struct sr_serial_dev_inst *serial)
+{
+	return sr_ser_libsp_get_rx_avail(serial);
+}
+
 static int _serial_write(struct sr_serial_dev_inst *serial,
 	const void *buf, size_t count,
 	int nonblocking, unsigned int timeout_ms)

src/serial_libsp.c

@@ -473,3 +473,17 @@ SR_PRIV int sr_ser_libsp_get_frame_format(struct sr_serial_dev_inst *serial,
 
 	return ret;
 }
+
+SR_PRIV size_t sr_ser_libsp_get_rx_avail(struct sr_serial_dev_inst *serial)
+{
+	int rc;
+
+	if (!serial)
+		return 0;
+
+	rc = sp_input_waiting(serial->sp_data);
+	if (rc < 0)
+		return 0;
+
+	return rc;
+}