BLESession.setNotifications: Lookup CCCD to start or stop notifications

Per Bluetooth Core Specification Vol.3, Part G, Section 3.3.1.1, if
Notify property is set for a Characteristic, the Client Characteristic
Configuration Descriptor, or CCCD, shall exist. As noted in section
3.3.3.3 of the specification, notification of the Characteristic is
enabled or disabled by setting bit 0 of the CCCD.

This CCCD exists within the characteristic definition following the
Characteristic Value declaration, and section 3.3.3 of the specification
warns that the order of characteristic declaration shall not be assumed.
However, current pyscrlink code assumes that the CCCD comes just after
the Characteristic Value and CCCD handle is obtained just adding 1 to
the handle of the Characteristic Value. This works many of the BLE
devices but does not work all of them.

To get correct CCCD handle, look up descriptors of the Characteristic to
find out the CCCD.

Signed-off-by: Shin'ichiro Kawasaki <kawasaki@juno.dti.ne.jp>

pyscrlink/scratch_link.py

@@ -47,6 +47,8 @@
 HOSTNAME="device-manager.scratch.mit.edu"
 scan_seconds=10.0
 
+CCCD_UUID = 0x2902
+
 class Session():
     """Base class for BTSession and BLESession"""
     def __init__(self, websocket, loop):
@@ -705,11 +707,19 @@ def setNotifications(self, service_id, chara_id, value):
         service = self._get_service(service_id)
         c = self._get_characteristic(chara_id)
         handle = c.getHandle()
+        # get CCCD or Client Characterstic Configuration Descriptor
+        cccd = None
+        for d in c.getDescriptors():
+            if d.uuid == UUID(CCCD_UUID):
+                cccd = d
+        if not cccd:
+            logger.error("Characteristic {char_id} does not have CCCD")
+            return
         # prepare notification handler
         self.delegate.add_handle(service_id, chara_id, handle)
         # request notification to the BLE device
         with self.lock:
-            self.perip.writeCharacteristic(handle + 1, value, True)
+            self.perip.writeCharacteristic(cccd.handle, value, True)
 
     def startNotifications(self, service_id, chara_id):
         logger.debug(f"start notification for {chara_id}")