From 68cadb51fa627b6130f2188979991bee91a60e79 Mon Sep 17 00:00:00 2001
From: Gary Yendell <gary.yendell@diamond.ac.uk>
Date: Thu, 12 Mar 2026 10:21:55 +0000
Subject: [PATCH] Fix EPICA CA string truncation

This accounts for the null terminator added when storing to the record
---
 src/fastcs/transports/epics/ca/util.py    |  8 ++++++--
 tests/transports/epics/ca/test_softioc.py | 17 ++++++++++++++++-
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/fastcs/transports/epics/ca/util.py b/src/fastcs/transports/epics/ca/util.py
index 7a7b8347..b5892a47 100644
--- a/src/fastcs/transports/epics/ca/util.py
+++ b/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():
diff --git a/tests/transports/epics/ca/test_softioc.py b/tests/transports/epics/ca/test_softioc.py
index 0bf571d3..d695abab 100644
--- a/tests/transports/epics/ca/test_softioc.py
+++ b/tests/transports/epics/ca/test_softioc.py
@@ -72,7 +72,12 @@ 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(String(length=10)),
+            "longStringIn",
+            {"length": 11, "DESC": None, "initial_value": ""},
         ),
         (
             AttrR(Enum(ColourEnum)),
@@ -197,6 +202,16 @@ class LongEnum(enum.Enum):
                 "initial_value": 0,
             },
         ),
+        (
+            AttrW(String()),
+            "longStringOut",
+            {"length": 257, "DESC": None, "initial_value": ""},
+        ),
+        (
+            AttrW(String(length=10)),
+            "longStringOut",
+            {"length": 11, "DESC": None, "initial_value": ""},
+        ),
     ),
 )
 def test_make_output_record(