Fix EPICA CA string truncation

This accounts for the null terminator added when storing to the record

Author: GDYendell

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

@@ -83,7 +83,9 @@ def _make_in_record(pv: str, attribute: AttrR) -> RecordWrapper:
         case String():
             record = builder.longStringIn(
                 pv,
-                length=attribute.datatype.length or DEFAULT_STRING_WAVEFORM_LENGTH,
+                length=(attribute.datatype.length + 1)
+                if attribute.datatype.length
+                else DEFAULT_STRING_WAVEFORM_LENGTH + 1,
                 **common_fields,
             )
         case Enum():
@@ -157,7 +159,9 @@ def _make_out_record(pv: str, attribute: AttrW, on_update: Callable) -> RecordWr
         case String():
             record = builder.longStringOut(
                 pv,
-                length=attribute.datatype.length or DEFAULT_STRING_WAVEFORM_LENGTH,
+                length=(attribute.datatype.length + 1)
+                if attribute.datatype.length
+                else DEFAULT_STRING_WAVEFORM_LENGTH + 1,
                 **common_fields,
             )
         case Enum():

tests/transports/epics/ca/test_softioc.py

@@ -72,7 +72,7 @@ async def test_create_and_link_read_pv(mocker: MockerFixture):
         (
             AttrR(String()),
             "longStringIn",
-            {"length": 256, "DESC": None, "initial_value": ""},
+            {"length": 257, "DESC": None, "initial_value": ""},
         ),
         (
             AttrR(Enum(ColourEnum)),