fix(telemetrix): a dropped serial link reads as disconnected

src/glider/hal/boards/telemetrix_board.py

@@ -173,7 +173,23 @@ def telemetrix(self):
 
     @property
     def is_running(self) -> bool:
-        return self._thread is not None and self._thread.is_alive()
+        return self._thread is not None and self._thread.is_alive() and self.link_error is None
+
+    @property
+    def link_error(self) -> str | None:
+        """Why the serial link died under a live thread, or None if it hasn't.
+
+        Once the port drops, telemetrix-aio's serial write swallows the
+        SerialException and returns None, so the thread outlives its board and
+        every later write "succeeds" in silence. What does die with the port is
+        its report reader task. A deliberate shutdown() ends the reader too, so
+        that is not counted.
+        """
+        tmx = self._telemetrix
+        task = getattr(tmx, "the_task", None)
+        if task is None or not task.done() or getattr(tmx, "shutdown_flag", False):
+            return None
+        return "serial write failed" if task.cancelled() else repr(task.exception())
 
 
 # Standard Arduino Uno pin capabilities
@@ -317,10 +333,11 @@ def is_connected(self) -> bool:
             return False
 
         if not self._telemetrix_thread.is_running:
-            # Thread died but state wasn't updated - fix it now
+            # Thread or its serial link died but state wasn't updated - fix it now
             # Only log once by checking if we're transitioning state
             if self._state == BoardConnectionState.CONNECTED:
-                logger.warning("Telemetrix thread died unexpectedly - marking as disconnected")
+                reason = self._telemetrix_thread.link_error or "worker thread died"
+                logger.warning(f"Telemetrix link lost ({reason}) - marking as disconnected")
                 self._set_state(BoardConnectionState.DISCONNECTED)
                 # The cached pin state is stale the moment the link drops
                 # (the Arduino resets on the next serial open); clear it so

tests/unit/hal/test_telemetrix_board.py

@@ -7,7 +7,7 @@
 import pytest
 
 from glider.hal.base_board import BoardConnectionState, PinMode, PinType
-from glider.hal.boards.telemetrix_board import TelemetrixBoard
+from glider.hal.boards.telemetrix_board import TelemetrixBoard, TelemetrixThread
 
 
 @pytest.fixture
@@ -160,6 +160,8 @@ async def test_dead_thread_triggers_auto_reconnect():
     board = TelemetrixBoard(port="COM3", auto_reconnect=True)
 
     class DeadThread:
+        link_error = None
+
         @property
         def is_running(self):
             return False
@@ -195,6 +197,31 @@ def telemetrix(self):
                 pass
 
 
+@pytest.mark.asyncio
+async def test_dropped_serial_link_reads_as_disconnected():
+    """A live worker thread is not a live board.
+
+    Once the USB link drops, telemetrix-aio's serial write swallows the
+    SerialException and returns None, so every later write "succeeds" and the
+    rig just stops. The one thing that does die is its report reader task.
+    """
+    reader = asyncio.get_running_loop().create_future()
+    thread = TelemetrixThread()
+    thread._thread = types.SimpleNamespace(is_alive=lambda: True)
+    thread._telemetrix = types.SimpleNamespace(the_task=reader, shutdown_flag=False)
+    board = TelemetrixBoard(port="COM3")
+    board._telemetrix_thread = thread
+    board._state = BoardConnectionState.CONNECTED
+
+    assert board.is_connected is True
+
+    reader.set_exception(OSError(6, "Device not configured"))
+
+    assert board.is_connected is False
+    with pytest.raises(RuntimeError, match="not connected"):
+        await board.write_digital(13, True)
+
+
 @pytest.mark.asyncio
 async def test_disconnect_clears_pin_state_caches():
     board = TelemetrixBoard(port="COM3")