Don't close serial port before switching to state "closed serial port"

Depending on what was happening in the monitoring thread this could lead to an attempt to write to the already closed port, logging an error and killing the interface in the process. Also fixed a timing issue in the state reporting towards the frontend, state updates and readings for clients are now wrapped in a mutex

Closes #492

src/octoprint/printer.py

@@ -688,6 +688,7 @@ class StateMonitor(object):
 		self._offsets = {}
 
 		self._changeEvent = threading.Event()
+		self._stateMutex = threading.Lock()
 
 		self._lastUpdate = time.time()
 		self._worker = threading.Thread(target=self._work)
@@ -717,6 +718,7 @@ class StateMonitor(object):
 		self._changeEvent.set()
 
 	def setState(self, state):
+		with self._stateMutex:
 			self._state = state
 			self._changeEvent.set()
 
@@ -736,6 +738,7 @@ class StateMonitor(object):
 		while True:
 			self._changeEvent.wait()
 
+			with self._stateMutex:
 				now = time.time()
 				delta = now - self._lastUpdate
 				additionalWaitTime = self._ratelimit - delta

src/octoprint/util/comm.py

@@ -340,11 +340,11 @@ class MachineCom(object):
 	def close(self, isError = False):
 		printing = self.isPrinting() or self.isPaused()
 		if self._serial is not None:
-			self._serial.close()
 			if isError:
 				self._changeState(self.STATE_CLOSED_WITH_ERROR)
 			else:
 				self._changeState(self.STATE_CLOSED)
+			self._serial.close()
 		self._serial = None
 
 		if settings().get(["feature", "sdSupport"]):