Add upload/download of sud.json over the Sud websocket channel

Sud.download() returns the on-disk document; Sud.upload() validates,
persists, and resets the schedule, refusing mid-brew or malformed input
so the current schedule is never left half-applied.

components/sud.py

@@ -48,17 +48,12 @@ def _build_step(number, defaults, raw_step):
 class Sud(AttributeChange):
 	def __init__(self, path):
 		AttributeChange.__init__(self)
+		self.path = path
 		with open(path) as f:
 			data = json.load(f)
 
-		self.name = data.get('Name', '')
-		self.description = data.get('Description', '')
-
-		step_defaults = data.get('default', {}).get('step', {})
-		self.schedule = [_build_step(i + 1, step_defaults, raw) for i, raw in enumerate(data['steps'])]
-
-		self.pot_mass = data.get('pot_mass', 0)
-		self.pot_material = data.get('pot_material')
+		(self.name, self.description, self.schedule,
+			self.pot_mass, self.pot_material) = self._parse_data(data)
 
 		self.index = -1
 		self.hold_remaining = 0.0
@@ -66,6 +61,52 @@ class Sud(AttributeChange):
 		self.step = None
 		self.user_message = None
 
+	@staticmethod
+	def _parse_data(data):
+		"""Parses a sud.json document into the (name, description, schedule,
+		pot_mass, pot_material) tuple Sud needs. Computed up front rather
+		than assigned straight onto self, so a malformed upload() can't
+		leave a half-applied schedule in place."""
+		name = data.get('Name', '')
+		description = data.get('Description', '')
+
+		step_defaults = data.get('default', {}).get('step', {})
+		schedule = [_build_step(i + 1, step_defaults, raw) for i, raw in enumerate(data['steps'])]
+
+		pot_mass = data.get('pot_mass', 0)
+		pot_material = data.get('pot_material')
+
+		return name, description, schedule, pot_mass, pot_material
+
+	def download(self):
+		"""Returns the sud.json document currently on disk, for a client to
+		edit and hand back to upload()."""
+		with open(self.path) as f:
+			return json.load(f)
+
+	def upload(self, data):
+		"""Replaces the schedule with a new sud.json document and persists
+		it to disk, resetting to IDLE as if freshly loaded from that file.
+		Refused while a brew is in progress, or if data is malformed - in
+		either case the current schedule is left untouched."""
+		if self.state not in (SudState.IDLE, SudState.DONE):
+			return False
+		try:
+			parsed = self._parse_data(data)
+		except (KeyError, TypeError):
+			return False
+
+		self.name, self.description, self.schedule, self.pot_mass, self.pot_material = parsed
+		with open(self.path, 'w') as f:
+			json.dump(data, f, indent='\t')
+
+		self.index = -1
+		self.hold_remaining = 0.0
+		self.state = SudState.IDLE
+		self.step = None
+		self.user_message = None
+		return True
+
 	def derive_plant_params(self, grain_mass, water_mass):
 		"""Lumped (mass, specific heat) lifted from pot_mass/pot_material and
 		the current step's grain_mass/water_mass, for use as Pot's "M"/"C"