projector version

python_app/hardware_full/laser_control/protocol.py

@@ -33,6 +33,7 @@ from .models import Measurements, VariationType
 from .exceptions import (
     CommunicationError,
     PortNotFoundError,
+    PortBusyError,
     CRCError,
     ProtocolError,
 )
@@ -184,20 +185,36 @@ class Protocol:
     # ---- Connection management
 
     def connect(self) -> None:
-        """Open the serial port. Auto-detects the device path when port is None."""
+        """Open the serial port. Auto-detects the device path when port is None.
+
+        The port is opened with ``exclusive=True`` so only one program can own the
+        laser controller at a time. This is required because the board is a single
+        half-duplex request/response bus: if two processes opened it, their
+        commands and responses would interleave and corrupt each other. A second
+        opener fails fast with :class:`PortBusyError` instead of silently sharing
+        the bus. Note the lock is advisory (``flock``): every program that opens
+        this port must also use ``exclusive=True`` for the guarantee to hold.
+        """
         port = self._port_name or self._detect_port()
         try:
             self._serial = serial.Serial(
                 port=port,
                 baudrate=BAUDRATE,
                 timeout=SERIAL_TIMEOUT_SEC,
+                exclusive=True,
             )
+        except serial.SerialException as exc:
+            # pyserial raises SerialException with errno EAGAIN/EWOULDBLOCK (11)
+            # when another process already holds the exclusive flock.
+            if getattr(exc, "errno", None) in (11,) or "exclusively lock" in str(exc).lower():
+                logger.error("Laser serial port '%s' is busy (held by another program)", port)
+                raise PortBusyError(port) from exc
+            logger.error("Cannot open laser serial port '%s': %s", port, exc)
+            raise CommunicationError(f"Cannot connect to port '{port}': {exc}") from exc
         except Exception as exc:
             logger.error("Cannot open laser serial port '%s': %s", port, exc)
-            raise CommunicationError(
-                f"Cannot connect to port '{port}': {exc}"
-            ) from exc
-        logger.debug("Laser serial port opened: %s @ %d baud", port, BAUDRATE)
+            raise CommunicationError(f"Cannot connect to port '{port}': {exc}") from exc
+        logger.debug("Laser serial port opened (exclusive): %s @ %d baud", port, BAUDRATE)
 
     def disconnect(self) -> None:
         """Close the serial port if open."""