Fix EpicsCATransport to pass length with initial_value for string records (#262)

softioc sets the length of the initial value as the max length, so it
can never be set if it is empty initially. Override with the driver set
length or 256 by default.

Author: GDYendell

src/fastcs/datatypes.py

@@ -128,6 +128,9 @@ def initial_value(self) -> bool:
 class String(DataType[str]):
     """`DataType` mapping to builtin ``str``."""
 
+    length: int | None = None
+    """Maximum length of string to display in transports"""
+
     @property
     def dtype(self) -> type[str]:
         return str

src/fastcs/transport/epics/ca/util.py

@@ -32,7 +32,7 @@
 
 
 EPICS_ALLOWED_DATATYPES = (Bool, DataType, Enum, Float, Int, String, Waveform)
-EPICS_WAVEFORM_LENGTH = 256
+DEFAULT_STRING_WAVEFORM_LENGTH = 256
 
 DATATYPE_FIELD_TO_RECORD_FIELD = {
     "prec": "PREC",
@@ -79,6 +79,8 @@ def record_metadata_from_datatype(
         arguments.pop("DRVH", None)
 
     match datatype:
+        case String():
+            arguments["length"] = datatype.length or DEFAULT_STRING_WAVEFORM_LENGTH
         case Waveform():
             if len(datatype.shape) != 1:
                 raise TypeError(
@@ -138,8 +140,8 @@ def cast_to_epics_type(datatype: DataType[T], value: T) -> Any:
                 return datatype.index_of(datatype.validate(value))
             else:  # enum backed by string record
                 return datatype.validate(value).name
-        case String():
-            return value[: EPICS_WAVEFORM_LENGTH - 1]
+        case String() as string:
+            return value[: string.length]
         case datatype if issubclass(type(datatype), EPICS_ALLOWED_DATATYPES):
             return value
         case _: