added availability check to BSCache.get_var()

bscache.py

@@ -1,5 +1,5 @@
 from time import sleep
-from bsread import source
+from bsread import source, dispatcher
 from bsvar import BSVar
 from prodthread import ProdThread
 
@@ -38,6 +38,8 @@ class BSCache:
 
     def get_var(self, name, modulo=None, offset=None):
         if name is not "pid" and not name in self.channels:
+            if not is_available(name):
+                raise ValueError(f"channel {name} is not available")
             cfg = make_channel_config(name, modulo, offset)
             self.update_source(name, cfg)
         return BSVar(name, self)
@@ -57,6 +59,16 @@ class BSCache:
 
 
 
+def is_available(name):
+    available = get_available_channels()
+    return name in available
+
+def get_available_channels():
+    channels = dispatcher.get_current_channels()
+    return set(ch["name"] for ch in channels)
+
+
+
 def make_channel_config(name, modulo, offset):
     res = {}
     if modulo is not None:
@@ -70,8 +82,6 @@ def make_channel_config(name, modulo, offset):
 
 
 
-
-
 def repack(channels, message):
     data     = message.data.data
     pulse_id = message.data.pulse_id