Fix #485: SqlFixedString round-trip truncates values that fill capacity

src/Lightweight/DataBinder/BasicStringBinder.hpp

@@ -286,12 +286,18 @@ struct SqlDataBinder<AnsiStringType>
             cb.PlanPostProcessOutputColumn(
                 [stmt, column, indicator, result]() { PostProcessOutputColumn(stmt, column, result, indicator); });
 
-        return SQLBindCol(stmt,
-                          column,
-                          SQL_C_CHAR,
-                          (SQLPOINTER) StringTraits::Data(result),
-                          (SQLLEN) StringTraits::Size(result),
-                          indicator);
+        // Per ODBC spec, BufferLength for SQL_C_CHAR must include space for the
+        // null terminator. SqlFixedString<N> backs its data with _data[N+1] for
+        // exactly this reason; pass Capacity+1 to let the driver write a full
+        // N-char value plus null. Dynamic strings keep the current behaviour.
+        SQLLEN const bufferLength = [&]() -> SQLLEN {
+            if constexpr (requires { AnsiStringType::Capacity; })
+                return static_cast<SQLLEN>(AnsiStringType::Capacity) + 1;
+            else
+                return static_cast<SQLLEN>(StringTraits::Size(result));
+        }();
+
+        return SQLBindCol(stmt, column, SQL_C_CHAR, (SQLPOINTER) StringTraits::Data(result), bufferLength, indicator);
     }
 
     static void PostProcessOutputColumn(SQLHSTMT stmt, SQLUSMALLINT column, AnsiStringType* result, SQLLEN* indicator)
@@ -338,8 +344,10 @@ struct SqlDataBinder<AnsiStringType>
         if constexpr (requires { AnsiStringType::Capacity; })
         {
             StringTraits::Resize(result, AnsiStringType::Capacity);
+            // BufferLength for SQL_C_CHAR must include space for the null terminator;
+            // _data is sized N+1 to accommodate it.
             SQLRETURN const rv =
-                SQLGetData(stmt, column, SQL_C_CHAR, StringTraits::Data(result), AnsiStringType::Capacity, indicator);
+                SQLGetData(stmt, column, SQL_C_CHAR, StringTraits::Data(result), AnsiStringType::Capacity + 1, indicator);
             if (rv == SQL_SUCCESS || rv == SQL_NO_DATA)
             {
                 if (*indicator == SQL_NULL_DATA)

src/Lightweight/DataBinder/SqlFixedString.hpp

@@ -471,9 +471,9 @@ struct SqlBasicStringOperations<SqlFixedString<N, T, Mode>>
     LIGHTWEIGHT_FORCE_INLINE static void TrimRight(ValueType* boundOutputString, SQLLEN indicator) noexcept
     {
 #if defined(_WIN32)
-        size_t n = (std::min) (static_cast<size_t>(indicator) / sizeof(CharType), N - 1);
+        size_t n = (std::min) (static_cast<size_t>(indicator) / sizeof(CharType), N);
 #else
-        size_t n = std::min(static_cast<size_t>(indicator), N - 1);
+        size_t n = std::min(static_cast<size_t>(indicator), N);
 #endif
         // Strip trailing whitespace and null characters
         while (n > 0 && (std::isspace((*boundOutputString)[n - 1]) || (*boundOutputString)[n - 1] == '\0'))

src/tests/DataBinderTests.cpp

@@ -1450,6 +1450,34 @@ TEST_CASE_METHOD(SqlTestFixture, "Trimmed fixed strings strip trailing padding t
     CHECK(result->stringWide == SqlTrimmedWideFixedString<32> { L"Hellö" });
 }
 
+// Regression for #485: BasicStringBinder::OutputColumn passes BufferLength=N
+// (not N+1) to SQLBindCol, so ODBC truncates any value of length N to N-1
+// data chars + null. The bug only surfaces when the stored value fills the
+// full capacity with no trailing whitespace -- e.g. a 3-letter currency code
+// stored in a CHAR(3) column.
+struct FullCapacityFixedRow
+{
+    Field<uint64_t, PrimaryKey::ServerSideAutoIncrement> id {};
+    Field<SqlTrimmedFixedString<3>> code {};
+};
+
+TEST_CASE_METHOD(SqlTestFixture,
+                 "Trimmed fixed string preserves a value that fills its capacity",
+                 "[DataMapper][SqlFixedString][regression]")
+{
+    auto dm = DataMapper {};
+    dm.CreateTable<FullCapacityFixedRow>();
+
+    FullCapacityFixedRow row {};
+    row.code = "EUR";
+    dm.Create(row);
+
+    auto const result = dm.QuerySingle<FullCapacityFixedRow>(row.id);
+    REQUIRE(result.has_value());
+    CHECK(result->code.Value().size() == 3);
+    CHECK(result->code.Value().str() == "EUR");
+}
+
 // Coverage for the SQL_C_WCHAR truncation arithmetic in
 // detail::GetRawColumnArrayData (BasicStringBinder.hpp). The function has two
 // re-fetch branches: (a) the driver reports total length up front and we re-