Handle EINTR from tcdrain() in sp_drain().

libserialport.h.in

@@ -878,6 +878,13 @@ enum sp_return sp_flush(struct sp_port *port, enum sp_buffer buffers);
 /**
  * Wait for buffered data to be transmitted.
  *
+ * @warning If your program runs on Unix, defines its own signal handlers, and
+ *          needs to abort draining the output buffer when when these are
+ *          called, then you should not use this function. It repeats system
+ *          calls that return with EINTR. To be able to abort a drain from a
+ *          signal handler, you would need to implement your own blocking
+ *          drain by polling the result of sp_output_waiting().
+ *
  * @param port Pointer to port structure.
  *
  * @return SP_OK upon success, a negative error code otherwise.

serialport.c

@@ -799,13 +799,23 @@ enum sp_return sp_drain(struct sp_port *port)
 	/* Returns non-zero upon success, 0 upon failure. */
 	if (FlushFileBuffers(port->hdl) == 0)
 		RETURN_FAIL("FlushFileBuffers() failed");
-#else
-	/* Returns 0 upon success, -1 upon failure. */
-	if (tcdrain(port->fd) < 0)
-		RETURN_FAIL("tcdrain() failed");
-#endif
-
 	RETURN_OK();
+#else
+	int result;
+	while (1) {
+		result = tcdrain(port->fd);
+		if (result < 0) {
+			if (errno == EINTR) {
+				DEBUG("tcdrain() was interrupted");
+				continue;
+			} else {
+				RETURN_FAIL("tcdrain() failed");
+			}
+		} else {
+			RETURN_OK();
+		}
+	}
+#endif
 }
 
 enum sp_return sp_blocking_write(struct sp_port *port, const void *buf, size_t count, unsigned int timeout)