Fix exception thrown when streaming certain XML values via XmlReader

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlTypes/SqlTypeWorkarounds.cs

@@ -7,6 +7,7 @@
 using System.Diagnostics;
 using System.IO;
 using System.Runtime.CompilerServices;
+using System.Text;
 using System.Xml;
 using Microsoft.Data.SqlClient;
 using Microsoft.Data.SqlClient.Internal;
@@ -32,6 +33,13 @@ internal static class SqlTypeWorkarounds
         private static readonly XmlReaderSettings s_defaultXmlReaderSettingsCloseInput = new() { ConformanceLevel = ConformanceLevel.Fragment, CloseInput = true };
         private static readonly XmlReaderSettings s_defaultXmlReaderSettingsAsyncCloseInput = new() { Async = true, ConformanceLevel = ConformanceLevel.Fragment, CloseInput = true };
 
+        private static readonly XmlParserContext s_defaultXmlParserContext = new(
+            nt: null, nsMgr: null, xmlLang: string.Empty, xmlSpace: XmlSpace.None,
+            // Do not use SqlUnicodeEncoding here. It assumes no BOM, but some streams backing
+            // XmlReader (specifically, SqlStream) can have artificial BOMs. Consider removing
+            // artificial BOM behavior from SqlStream as a future optimization.
+            Encoding.Unicode);
+
         internal const SqlCompareOptions SqlStringValidSqlCompareOptionMask =
             SqlCompareOptions.BinarySort |
             SqlCompareOptions.BinarySort2 |
@@ -50,7 +58,7 @@ internal static XmlReader SqlXmlCreateSqlXmlReader(Stream stream, bool closeInpu
                     : s_defaultXmlReaderSettingsCloseInput
                 : s_defaultXmlReaderSettings;
 
-            return XmlReader.Create(stream, settingsToUse);
+            return XmlReader.Create(stream, settingsToUse, s_defaultXmlParserContext);
         }
 
         #endregion

src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/DataReaderTest/DataReaderStreamsTest.cs

@@ -89,7 +89,7 @@ public static async Task GetFieldValueAsync_OfXmlReader(CommandBehavior behavior
                         using (XmlReader xmlReader = await reader.GetFieldValueAsync<XmlReader>(1))
                         {
                             isAsync = xmlReader.Settings.Async;
-                            outputXml = GetXmlDocumentContents(xmlReader);
+                            outputXml = GetXmlReaderContents(xmlReader);
                         }
                     }
                 }
@@ -207,7 +207,7 @@ public static async Task GetFieldValue_OfXmlReader(CommandBehavior behavior, boo
                         using (XmlReader xmlReader = reader.GetFieldValue<XmlReader>(1))
                         {
                             isAsync = xmlReader.Settings.Async;
-                            outputXml = GetXmlDocumentContents(xmlReader);
+                            outputXml = GetXmlReaderContents(xmlReader);
                         }
                     }
                 }
@@ -364,7 +364,7 @@ public static async Task GetXmlReader(CommandBehavior behavior, bool isExecuteAs
                         using (XmlReader xmlReader = reader.GetXmlReader(1))
                         {
                             isAsync = xmlReader.Settings.Async;
-                            outputXml = GetXmlDocumentContents(xmlReader);
+                            outputXml = GetXmlReaderContents(xmlReader);
                         }
                     }
                 }
@@ -481,7 +481,7 @@ public static void NullStreamProperties(CommandBehavior behavior, AccessorType a
                     // get a clean reader over the same field and check that the value is empty
                     using (XmlReader xmlReader = GetValue<XmlReader>(reader, 0, accessorType))
                     {
-                        Assert.Equal(GetXmlDocumentContents(xmlReader), string.Empty);
+                        Assert.Equal(GetXmlReaderContents(xmlReader), string.Empty);
                     }
 
                     using (TextReader textReader = GetValue<TextReader>(reader, 1, accessorType))
@@ -1169,6 +1169,11 @@ private static string CreateXmlData(int packetSize, int forcedPacketCount)
             using (XmlWriter xmlWriter = XmlWriter.Create(stringWriter, settings))
             {
                 int index = 1;
+                // Purposefully start with root-level text to confuse the encoding-detection logic
+                // in XmlTextReaderImpl so it would assume UTF-8. This ensures tests will fail if a
+                // SqlDataReader creates an XmlReader over a UTF-16LE stream without explicitly
+                // specifying the encoding.
+                xmlWriter.WriteString("foo");
                 xmlWriter.WriteStartElement("root");
                 while (buffer.Length / 2 < (packetSize * forcedPacketCount))
                 {
@@ -1232,15 +1237,19 @@ private static string CreateCharDataQuery(string originalText)
             return queryBuilder.ToString();
         }
 
-        private static string GetXmlDocumentContents(XmlReader xmlReader)
+        /// <summary>
+        /// Serializes the contents of the specified <see cref="XmlReader"/> to a
+        /// <see langword="string"/>.
+        /// </summary>
+        /// <param name="xmlReader">The reader whose contents are to be serialized.</param>
+        /// <returns>
+        /// A string representation of the contents of <paramref name="xmlReader"/>.
+        /// </returns>
+        private static string GetXmlReaderContents(XmlReader xmlReader)
         {
-            string outputXml;
-            XmlDocument document = new XmlDocument();
-            document.Load(xmlReader);
-
             XmlWriterSettings settings = new XmlWriterSettings
             {
-                ConformanceLevel = ConformanceLevel.Document,
+                ConformanceLevel = ConformanceLevel.Fragment,
                 Encoding = Encoding.Unicode,
                 Indent = true,
                 OmitXmlDeclaration = true
@@ -1250,10 +1259,9 @@ private static string GetXmlDocumentContents(XmlReader xmlReader)
             using (StringWriter stringWriter = new StringWriter(buffer))
             using (XmlWriter xmlWriter = XmlWriter.Create(stringWriter, settings))
             {
-                document.WriteContentTo(xmlWriter);
+                xmlWriter.WriteNode(xmlReader, defattr: false);
             }
-            outputXml = buffer.ToString();
-            return outputXml;
+            return buffer.ToString();
         }
 
         private static byte[] GetStreamContents(Stream stream)