bdp_guard: one lock for all I2C transfers of the bd_pressure

The guard's timer read the sensor while G28 ran; bdpressure.py writes the
probe mode at every homing move, and Klipper allows only one I2C request
at a time per device: KeyError ('i2c_response', 1), Klipper shut down
during HOME. The guard now wraps the sensor's i2c in a reactor mutex at
connect; bdpressure and bdpressure_check (now looks up sensor.i2c at
every use) go through it too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extras/bdp_guard.py

@@ -46,6 +46,38 @@ EXT_MAGIC = 0xBD
 MODES = ('log', 'stop')
 
 
+# Klipper allows only one I2C request at a time per device: a second one
+# while the first waits for its answer raises KeyError ('i2c_response', oid)
+# and shuts Klipper down. The guard reads from a timer while G-code runs,
+# and bdpressure.py writes the probe mode at every homing move (seen: G28
+# during an armed guard), so all transfers of the sensor take this lock.
+# The guard puts it on the sensor object at connect; bdpressure and
+# bdpressure_check use sensor.i2c and so get it too.
+class LockedI2C:
+    def __init__(self, i2c, mutex):
+        self._i2c = i2c
+        self._mutex = mutex
+
+    def i2c_read(self, *args, **kwargs):
+        with self._mutex:
+            return self._i2c.i2c_read(*args, **kwargs)
+
+    def i2c_write(self, *args, **kwargs):
+        with self._mutex:
+            return self._i2c.i2c_write(*args, **kwargs)
+
+    def i2c_write_wait_ack(self, *args, **kwargs):
+        with self._mutex:
+            return self._i2c.i2c_write_wait_ack(*args, **kwargs)
+
+    def i2c_transfer(self, *args, **kwargs):
+        with self._mutex:
+            return self._i2c.i2c_transfer(*args, **kwargs)
+
+    def __getattr__(self, name):
+        return getattr(self._i2c, name)
+
+
 class BDPGuard:
     def __init__(self, config):
         self.printer = config.get_printer()
@@ -76,7 +108,13 @@ class BDPGuard:
 
     def _handle_connect(self):
         self.sensor = self.printer.lookup_object(self.sensor_name, None)
-        self.i2c = getattr(self.sensor, 'i2c', None)
+        i2c = getattr(self.sensor, 'i2c', None)
+        if i2c is not None and not isinstance(i2c, LockedI2C):
+            # Every user of the sensor (bdpressure, bdpressure_check, this
+            # guard) now goes through one lock, see LockedI2C
+            i2c = LockedI2C(i2c, self.reactor.mutex())
+            self.sensor.i2c = i2c
+        self.i2c = i2c
 
     def _handle_shutdown(self):
         self._stop_timer()