[Plugin] Enhancements for serial number validation (#8311)

* Error handling for plugin.increment_serial_number

* Improve error handling for plugin validation functions

* Add Part.get_next_serial_number method

- Simplify call for incrementing

* ValidationMixin: add "part" to increment_serial_number func

* Pass part information through to serial number functions

* Fix circular imports

* Allow "get_latest_serial_number" to use plugin system

* Better working example for plugin sample

* Update SampleValidatorPlugin

* Fix indent

* Add code comment

* Cleanup code logic

* Revert previous commit

* Update unit tests

src/backend/InvenTree/plugin/samples/integration/validation_sample.py

@@ -135,6 +135,34 @@ class SampleValidatorPlugin(SettingsMixin, ValidationMixin, InvenTreePlugin):
             if serial[0] != part.name[0]:
                 self.raise_error('Serial number must start with same letter as part')
 
+        # Prevent serial numbers which are a multiple of 5
+        try:
+            sn = int(serial)
+            if sn % 5 == 0:
+                self.raise_error('Serial number cannot be a multiple of 5')
+        except ValueError:
+            pass
+
+    def increment_serial_number(self, serial: str, part=None, **kwargs):
+        """Increment a serial number.
+
+        These examples are silly, but serve to demonstrate how the feature could be used
+        """
+        try:
+            sn = int(serial)
+            sn += 1
+
+            # Skip any serial number which is a multiple of 5
+            if sn % 5 == 0:
+                sn += 1
+
+            return str(sn)
+        except ValueError:
+            pass
+
+        # Return "None" to defer to the next plugin or builtin functionality
+        return None
+
     def validate_batch_code(self, batch_code: str, item):
         """Ensure that a particular batch code meets specification.