Send dummy request before closing serial port

This avoids losing the last sent data with some USB/serial drivers.

src/main/python/main/ayab/engine/engine_fsm.py

@@ -51,6 +51,7 @@ class State(Enum):
     REQUEST_TEST = auto()
     CONFIRM_TEST = auto()
     RUN_TEST = auto()
+    DISCONNECT = auto()
     FINISHED = auto()
 
 
@@ -196,8 +197,18 @@ def _API6_run_knit(control: Control, operation: Operation) -> Output:
         if token == Token.reqLine:
             pattern_finished = control.cnf_line_API6(param)
             if pattern_finished:
-                control.state = State.FINISHED
-                return Output.KNITTING_FINISHED
+                # When closing the serial port, the final bytes written
+                # may be dropped by the driver
+                # (see https://github.com/serialport/serialport-rs/issues/117).
+                # This may cause the final `cnfLine` response to get lost and the
+                # firmware to get stuck knitting the previous row
+                # (see https://github.com/AllYarnsAreBeautiful/ayab-desktop/issues/662).
+                # To avoid this, before closing the port, we send a `reqInfo` message
+                # to the firmware and wait for the response.
+                control.com.req_info()
+                control.state = State.DISCONNECT
+                control.logger.debug("State DISCONNECT")
+                return Output.DISCONNECTING_FROM_MACHINE
             else:
                 return Output.NEXT_LINE
         # else
@@ -237,6 +248,17 @@ def _API6_run_test(control: Control, operation: Operation) -> Output:
         control.check_serial_API6()
         return Output.NONE
 
+    @staticmethod
+    def _API6_disconnect(control: Control, operation: Operation) -> Output:
+        token, _ = control.check_serial_API6()
+        if token == Token.cnfInfo:
+            # We received a response to our final `reqInfo` request,
+            # it is now safe to close the port.
+            control.state = State.FINISHED
+            return Output.KNITTING_FINISHED
+        # else
+        return Output.NONE
+
     @staticmethod
     def _API6_finished(control: Control, operation: Operation) -> Output:
         control.logger.debug("State FINISHED")

src/main/python/main/ayab/engine/output.py

@@ -33,6 +33,7 @@ class Output(Enum):
     ERROR_INVALID_SETTINGS = auto()
     ERROR_SERIAL_PORT = auto()
     CONNECTING_TO_MACHINE = auto()
+    DISCONNECTING_FROM_MACHINE = auto()
     INITIALIZING_FIRMWARE = auto()
     WAIT_FOR_INIT = auto()
     ERROR_WRONG_API = auto()
@@ -65,6 +66,9 @@ def _none(self) -> None:
     def _connecting_to_machine(self) -> None:
         self.emit_notification("Connecting to machine...", False)
 
+    def _disconnecting_from_machine(self) -> None:
+        self.emit_notification("Disconnecting from machine...")
+
     def _initializing_firmware(self) -> None:
         self.emit_notification("Initializing firmware")