Fix issue #78 by ensuring _read_value() returns true copy

It turns out that the value returned by record.read_val() can be a shared
reference to the value in the underlying EPICS record.  This means that
this value can change unexpectedly!

The fix is simply to apply the same logic as already done for string
records, and take a true copy instead.

Author: Araneidae

softioc/device.py

@@ -54,7 +54,10 @@ def _compare_values(self, value1, value2):
     # This method is called during Out record processing to return the
     # underlying value in EPICS format.
     def _read_value(self, record):
-        return record.read_val()
+        # Take a true copy of the value read to avoid accidental sharing
+        result = self._ctype_()
+        result.value = record.read_val().value
+        return result
 
     # This method is called during In record processing to update the
     # underlying value (the value must be in EPICS compatible format).  This is
@@ -270,12 +273,6 @@ def _value_to_epics(self, value):
     def _epics_to_value(self, epics):
         return _string_at(epics, 40)
 
-    def _read_value(self, record):
-        # For strings we need to take a copy of the value read
-        result = self._ctype_()
-        result.value = record.read_val().value
-        return result
-
 
 class stringin(EpicsString, ProcessDeviceSupportIn):
     _record_type_ = 'stringin'