Add Feature Extension for User Agent

src/Microsoft.Data.SqlClient/tests/UnitTests/TdsParserInternalsTest.cs

@@ -0,0 +1,115 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+using System;
+using System.Linq;
+using System.Reflection;
+using System.Text;
+using Xunit;
+
+#nullable enable
+
+namespace Microsoft.Data.SqlClient.UnitTests
+{
+    public class TdsParserInternalsTest
+    {
+        // Selects and returns the first non-public instance constructor for TdsParser 
+        private static TdsParser CreateParserInstance()
+        {
+            Type parserType = typeof(TdsParser);
+
+            ConstructorInfo ctor = parserType
+                .GetConstructors(BindingFlags.Instance | BindingFlags.NonPublic)
+                .First();
+
+            // build default args for each parameter
+            object?[] ctorArgs = ctor.GetParameters()
+                .Select(p => p.ParameterType.IsValueType
+                    ? Activator.CreateInstance(p.ParameterType)
+                    : null)
+                .ToArray();
+
+            return (TdsParser)ctor.Invoke(ctorArgs);
+        }
+
+        // Helper function to extract private _physicalStateObj fields raw buffer and no. of bytes written so far 
+        private static (byte[] buffer, int count) ExtractOutputBuffer(TdsParser parser)
+        {
+            FieldInfo stateField = typeof(TdsParser)
+                .GetField("_physicalStateObj", BindingFlags.Instance | BindingFlags.NonPublic)
+                ?? throw new InvalidOperationException("_physicalStateObj not found");
+
+            object stateObj = stateField.GetValue(parser)
+                ?? throw new InvalidOperationException("physical state object is null");
+
+            Type stateType = stateObj.GetType();
+
+            FieldInfo buffField = stateType
+                .GetField("_outBuff", BindingFlags.Instance | BindingFlags.NonPublic)
+                ?? throw new InvalidOperationException("_outBuff not found");
+
+            byte[] buffer = (byte[])buffField.GetValue(stateObj)!;
+
+            FieldInfo usedField = stateType
+                .GetField("_outBytesUsed", BindingFlags.Instance | BindingFlags.NonPublic)
+                ?? throw new InvalidOperationException("_outBytesUsed not found");
+
+            int count = (int)usedField.GetValue(stateObj)!;
+
+            return (buffer, count);
+        }
+
+        [Fact]
+        public void WriteUserAgentFeatureRequest_WriteFalse_LengthOnlyReturn()
+        {
+            byte[] payload = Encoding.UTF8.GetBytes("{\"kel\":\"sier\"}");
+            var parser = CreateParserInstance();
+
+            int lengthOnly = parser.WriteUserAgentFeatureRequest(payload, write: false);
+
+            // assert: total = 1 (feat-ID) + 4 (len field) + [1 (version) + payload.Length]
+            int expectedDataLen = 1 + payload.Length;
+            int expectedTotalLen = 1 + 4 + expectedDataLen;
+            Assert.Equal(expectedTotalLen, lengthOnly);
+        }
+
+        [Fact]
+        public void WriteUserAgentFeatureRequest_WriteTrue_AppendsOnlyExtensionBytes()
+        {
+            byte[] payload = Encoding.UTF8.GetBytes("{\"kel\":\"sier\"}");
+            var parser = CreateParserInstance();
+
+            var (bufferBefore, countBefore) = ExtractOutputBuffer(parser);
+
+            int returnedLength = parser.WriteUserAgentFeatureRequest(payload, write: true);
+
+            var (bufferAfter, countAfter) = ExtractOutputBuffer(parser);
+
+            int appended = countAfter - countBefore;
+            Assert.Equal(returnedLength, appended);
+
+            int start = countBefore;
+
+            Assert.Equal(
+                TdsEnums.FEATUREEXT_USERAGENT,
+                bufferAfter[start]);
+
+            int dataLenFromStream = BitConverter.ToInt32(bufferAfter, start + 1);
+            int expectedDataLen = 1 + payload.Length;
+            Assert.Equal(expectedDataLen, dataLenFromStream);
+
+            Assert.Equal(
+                TdsEnums.SUPPORTED_USER_AGENT_VERSION,
+                bufferAfter[start + 5]);
+
+            byte[] writtenPayload = bufferAfter
+                .Skip(start + 6)
+                .Take(payload.Length)
+                .ToArray();
+            Assert.Equal(payload, writtenPayload);
+
+            Assert.Equal(returnedLength, appended);
+        }
+
+    }
+}