harden UBX parsing/dispatch and report peer-side disconnects in GUI

A struct size-mismatch or any other parse error in a listener could
propagate uncaught out of the socket-reading thread and silently kill
it. Catch broadly in the event loop and per-listener in call_listener,
and make the NAV-SAT/NAV-SIG/RXM-RAWX group parsers tolerate a
payload length that isn't an exact multiple of the group size instead
of crashing on a short struct.unpack.

Also wire NetworkBackend's socket-loss detection through to
ReceiverManager (via a queued signal, since the callback fires from
the backend thread) so the GUI reflects a peer-initiated disconnect
instead of leaving the row stuck showing "connected". Verified against
a real ZED-X20P: it drops the TCP session on its own after ~20-28s
regardless of traffic; the GUI now marks the receiver disconnected and
frees its ID for reconnecting.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AzmCaNjDb3TAqPpTwunKTY

backend/network_backend.py

@@ -10,11 +10,12 @@ from backend.a_backend import ABackend
 
 
 class NetworkBackend(ABackend):
-	def __init__(self, servers: list[dict]):
+	def __init__(self, servers: list[dict], on_disconnect=None):
 		self.sel = None
 		self.sock_list = [{'name': s['name'], 'addr': (socket.gethostbyname(s['host']), s['port']), 'xcvr': None, 'queue': Queue()} for s in servers]
 		self.thread = Thread(target=self.event_loop)
 		self.loop_enable = True
+		self.on_disconnect = on_disconnect
 
 	def send(self, xcvr: ATransceiver, data: bytes):
 		sock = self.find_by_name(xcvr.name)
@@ -89,6 +90,10 @@ class NetworkBackend(ABackend):
 						print(f"Connection {sock['name']} to {sock['addr']} lost: {exc}")
 						self.sel.unregister(key.fileobj)
 						key.fileobj.close()
+						if self.on_disconnect:
+							self.on_disconnect(sock['name'])
+					except Exception as exc:
+						print(f"Connection {sock['name']}: error processing data, ignoring: {exc}")
 
 		except KeyboardInterrupt:
 			print("Caught keyboard interrupt, exiting")

gui/data_model.py

@@ -93,12 +93,14 @@ class ReceiverManager(QObject):
     connection_changed = pyqtSignal(str, bool)           # rid, connected
     receiver_added     = pyqtSignal(str)
     receiver_removed   = pyqtSignal(str)
+    _connection_lost   = pyqtSignal(str)                 # internal: backend thread -> GUI thread
 
     def __init__(self):
         super().__init__()
         self._states:  dict = {}
         self._configs: dict = {}
         self._lru:     list = self._load_lru()
+        self._connection_lost.connect(self._handle_connection_lost)
 
     # ── LRU ──────────────────────────────────────────────────────────────────
 
@@ -164,7 +166,8 @@ class ReceiverManager(QObject):
 
         if config.conn_type == 'tcp':
             backend = NetworkBackend(
-                [{'name': rid, 'host': config.host, 'port': config.port}]
+                [{'name': rid, 'host': config.host, 'port': config.port}],
+                on_disconnect=lambda name, r=rid: self._connection_lost.emit(r),
             )
         elif config.conn_type == 'serial':
             backend = SerialBackend(config.serial_port, config.baud_rate)
@@ -211,6 +214,13 @@ class ReceiverManager(QObject):
         self._push_lru(config)
         self.connection_changed.emit(rid, True)
 
+    def _handle_connection_lost(self, rid: str):
+        state = self._states.get(rid)
+        if not state or not state.connected:
+            return
+        print(f"Receiver '{rid}': connection lost")
+        self.remove_receiver(rid)
+
     def disconnect_receiver(self, rid: str):
         state = self._states.get(rid)
         if not state or not state.connected:

msg/talker.py

@@ -11,5 +11,8 @@ class MsgTalker(ABC):
 
 	def call_listener(self, msg: MsgContainer):
 		for sink in self.sinks:
-			if sink.is_msg(msg):
-				sink.on_recv(msg)
+			try:
+				if sink.is_msg(msg):
+					sink.on_recv(msg)
+			except Exception as exc:
+				print(f"Listener {sink.__class__.__name__} failed on {msg}: {exc}")

ubx/messages/nav.py

@@ -34,11 +34,13 @@ class Sig(UbxMsg):
 		remain = len(data) - offset
 		group_size = 16
 		n = offset
-		while remain:
+		while remain >= group_size:
 			signal = cls.Group1.from_bytes(data[n:n + group_size])
 			n += group_size
 			remain -= group_size
 			obj.signals.append(signal)
+		if remain:
+			print(f"Sig.from_bytes: {remain} trailing bytes unparsed (payload size mismatch)")
 		return obj.__dict__
 
 class Sat(UbxMsg):
@@ -70,9 +72,11 @@ class Sat(UbxMsg):
 		remain = len(data) - offset
 		group_size = 12
 		n = offset
-		while remain:
+		while remain >= group_size:
 			satellite = cls.Group1.from_bytes(data[n:n + group_size])
 			n += group_size
 			remain -= group_size
 			obj.satellites.append(satellite)
+		if remain:
+			print(f"Sat.from_bytes: {remain} trailing bytes unparsed (payload size mismatch)")
 		return obj.__dict__

ubx/messages/rxm.py

@@ -40,9 +40,11 @@ class RawX(UbxMsg):
 		remain = len(data) - offset
 		group_size = 32
 		n = offset
-		while remain:
+		while remain >= group_size:
 			group = cls.Group1.from_bytes(data[n:n + 32])
 			n += group_size
 			remain -= group_size
 			obj.meas.append(group)
+		if remain:
+			print(f"RawX.from_bytes: {remain} trailing bytes unparsed (payload size mismatch)")
 		return obj.__dict__