Fix XDocument.LoadAsync to not use sync reads (#123800)

Co-authored-by: Stephen Toub <[email protected]>

Author: lilinus

src/libraries/System.Private.Xml.Linq/src/System/Xml/Linq/XDeclaration.cs

@@ -3,6 +3,7 @@
 
 using System.IO;
 using System.Text;
+using System.Threading.Tasks;
 using StringBuilder = System.Text.StringBuilder;
 
 namespace System.Xml.Linq
@@ -65,6 +66,15 @@ internal XDeclaration(XmlReader r)
             r.Read();
         }
 
+        internal static async Task<XDeclaration> CreateAsync(XmlReader r)
+        {
+            string? version = r.GetAttribute("version");
+            string? encoding = r.GetAttribute("encoding");
+            string? standalone = r.GetAttribute("standalone");
+            await r.ReadAsync().ConfigureAwait(false);
+            return new XDeclaration(version, encoding, standalone);
+        }
+
         /// <summary>
         /// Gets or sets the encoding for this document.
         /// </summary>

src/libraries/System.Private.Xml.Linq/src/System/Xml/Linq/XDocument.cs

@@ -425,6 +425,12 @@ public static XDocument Load(XmlReader reader, LoadOptions options)
             if (reader.ReadState == ReadState.Initial) reader.Read();
 
             XDocument d = InitLoad(reader, options);
+
+            if (reader.NodeType == XmlNodeType.XmlDeclaration)
+            {
+                d.Declaration = new XDeclaration(reader);
+            }
+
             d.ReadContentFrom(reader, options);
 
             if (!reader.EOF) throw new InvalidOperationException(SR.InvalidOperation_ExpectedEndOfFile);
@@ -467,6 +473,12 @@ private static async Task<XDocument> LoadAsyncInternal(XmlReader reader, LoadOpt
             }
 
             XDocument d = InitLoad(reader, options);
+
+            if (reader.NodeType == XmlNodeType.XmlDeclaration)
+            {
+                d.Declaration = await XDeclaration.CreateAsync(reader).ConfigureAwait(false);
+            }
+
             await d.ReadContentFromAsync(reader, options, cancellationToken).ConfigureAwait(false);
 
             if (!reader.EOF) throw new InvalidOperationException(SR.InvalidOperation_ExpectedEndOfFile);
@@ -496,10 +508,6 @@ private static XDocument InitLoad(XmlReader reader, LoadOptions options)
                     d.SetLineInfo(li.LineNumber, li.LinePosition);
                 }
             }
-            if (reader.NodeType == XmlNodeType.XmlDeclaration)
-            {
-                d.Declaration = new XDeclaration(reader);
-            }
             return d;
         }
 

src/libraries/System.Private.Xml.Linq/tests/misc/LoadSaveAsyncTests.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using System.Xml;
@@ -232,6 +233,39 @@ public async Task SaveAsync_CallsAsyncOnly_SaveSync_CallsSyncOnly(bool isAsync,
             }
         }
 
+        public static IEnumerable<object[]> IsAsync_LoadOptions_XmlString_Data
+        {
+            get
+            {
+                foreach (bool isAsync in new[] { true, false })
+                {
+                    foreach (LoadOptions loadOptions in Enum.GetValues(typeof(LoadOptions)))
+                    {
+                        yield return new object[] { isAsync, loadOptions, "<root>Test</root>" };
+                        yield return new object[] { isAsync, loadOptions, "<?xml version=\"1.0\" encoding=\"utf-8\"?><root>Test</root>" };
+                    }
+                }
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(IsAsync_LoadOptions_XmlString_Data))]
+        public async Task XDocument_LoadAsync_CallsAsyncOnly_LoadSync_CallsSyncOnly(bool isAsync, LoadOptions loadOptions, string xmlString)
+        {
+            using MemoryStream memoryStream = new MemoryStream(Encoding.UTF8.GetBytes(xmlString));
+            using (CheckSyncAsyncStream stream = new CheckSyncAsyncStream(isAsync, memoryStream))
+            {
+                if (isAsync)
+                {
+                    await XDocument.LoadAsync(stream, loadOptions, CancellationToken.None);
+                }
+                else
+                {
+                    XDocument.Load(stream, loadOptions);
+                }
+            }
+        }
+
         [Theory]
         [MemberData(nameof(RoundtripOptions_MemberData))]
         public static async Task RoundtripSyncAsyncCheckAndMatches_XmlReader(bool document, LoadOptions loadOptions, SaveOptions saveOptions)