fix(abcip): stride user-string arrays by the read buffer, not the built-in STRING size

Live read-only check against a production CompactLogix press: Program:UN01_Press._ModeNames (STR0016[32], 4 elements requested) returned an 80-byte buffer with libplctag.NET 1.5.2's ElementSize unset. The driver fell back to the static 88-byte STRING stride, read element 1 past the buffer, and the whole read failed (ErrorOutOfBounds -> BadCommunicationError). When the buffer divides evenly by the element count, that quotient is now the stride; the read returns [undefined, Production, Maintenance, Manual] as the controller holds.
2026-10-07 14:11:08 -04:00
parent 0ca56cb1f6
commit 68cf3ae8b7
2 changed files with 66 additions and 2 deletions
@@ -0,0 +1,45 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;
+
+/// <summary>
+///     Array element stride. Seen live on a production CompactLogix: reading
+///     <c>Program:UN01_Press._ModeNames</c> (<c>STR0016[32]</c>, 4 elements requested) returned an
+///     80-byte buffer with libplctag's <c>ElementSize</c> unset; the driver strode 88 bytes (built-in
+///     STRING) and the read failed <c>ErrorOutOfBounds</c>. The real values were
+///     "undefined" / "Production" / "Maintenance" / "Manual" at a 20-byte stride.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbCipArrayStrideTests
+{
+    /// <summary>A user string type's stride comes from the buffer, not the built-in STRING size.</summary>
+    [Fact]
+    public void User_string_array_strides_by_buffer_size_over_count()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 80, count: 4, AbCipDataType.String).ShouldBe(20);
+    }
+
+    /// <summary>The built-in STRING still strides 88.</summary>
+    [Fact]
+    public void Builtin_STRING_array_strides_88()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 3 * 88, count: 3, AbCipDataType.String).ShouldBe(88);
+    }
+
+    /// <summary>libplctag's own element size wins when it is populated.</summary>
+    [Fact]
+    public void Reported_element_size_wins()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(4, bufferSize: 80, count: 4, AbCipDataType.DInt).ShouldBe(4);
+    }
+
+    /// <summary>An empty or uneven buffer falls back to the static per-type size.</summary>
+    [Fact]
+    public void Empty_or_uneven_buffer_falls_back_to_the_type_size()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 0, count: 4, AbCipDataType.Real).ShouldBe(4);
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 81, count: 4, AbCipDataType.String).ShouldBe(88);
+    }
+}