Ensure data handling exceptions are propogated to callers

For both camonitor and caget we have to do data conversion on callbacks
called from ctypes, so this fix catches any conversion errors and
propogates them back to the original caller.

Author: Araneidae

cothread/catools.py

@@ -325,8 +325,15 @@ def __on_event(args):
         self = args.usr
 
         if args.status == cadef.ECA_NORMAL:
-            # Good data: extract value from the dbr.
-            value = self.dbr_to_value(args.raw_dbr, args.type, args.count)
+            # Good data: extract value from the dbr.  Note that this can fail,
+            # for example if there is malformed UTF-8 in the result.
+            try:
+                value = self.dbr_to_value(args.raw_dbr, args.type, args.count)
+            except:
+                # If there is an error, bypass update merging and force this
+                # exception tuple into the processing chain.
+                self.__Callback(self.__signal, sys.exc_info())
+                return
         elif self.notify_disconnect:
             # Something is wrong: let the subscriber know, if they've requested
             # disconnect nofication.
@@ -361,7 +368,12 @@ def __signal(self, value):
                     self.__update_count = 0
 
             try:
-                self.callback(value)
+                if isinstance(value, tuple):
+                    # This should only happen if the asynchronous callback
+                    # caught an exception for us to re-raise here.
+                    py23.raise_with_traceback(value)
+                else:
+                    self.callback(value)
             except:
                 # We try and be robust about exceptions in handlers, but to
                 # prevent a perpetual storm of exceptions, we close the
@@ -583,8 +595,12 @@ def _caget_event_handler(args):
     ctypes.pythonapi.Py_DecRef(args.usr)
 
     if args.status == cadef.ECA_NORMAL:
-        cothread.Callback(done.Signal, dbr_to_value(
-            args.raw_dbr, args.type, args.count))
+        try:
+            value = dbr_to_value(args.raw_dbr, args.type, args.count)
+        except:
+            cothread.Callback(done.SignalException, sys.exc_info()[1])
+        else:
+            cothread.Callback(done.Signal, value)
     else:
         cothread.Callback(done.SignalException, ca_nothing(pv, args.status))