Ensure we don't create zero length waveforms

Also check and abort writes that are too long for the waveform

Author: Araneidae

softioc/builder.py

@@ -179,6 +179,8 @@ def _waveform(value, fields):
         length = fields.pop('length')
     datatype = initial_value.dtype
 
+    assert length > 0, 'Array cannot be of zero length'
+
     fields['initial_value'] = initial_value
     fields['_wf_nelm'] = length
     fields['_wf_dtype'] = datatype

softioc/device.py

@@ -374,8 +374,9 @@ def _value_to_epics(self, value):
         # Because arrays are mutable values it's ever so easy to accidentially
         # call set() with a value which subsequently changes.  To avoid this
         # common class of bug, at the cost of duplicated code and data, here we
-        # ensure a copy is taken of the value, after pruning to length.
-        return +value[:self._nelm]
+        # ensure a copy is taken of the value.
+        assert len(value) <= self._nelm, 'Value too long for waveform'
+        return +value
 
     def _epics_to_value(self, value):
         return value