Implement AsnReader.Clone (#86913)

* Implement AsnReader.Clone

* Test that the clone is created from the correct slice

Author: vcsjones

src/libraries/System.Formats.Asn1/ref/System.Formats.Asn1.cs

@@ -100,6 +100,7 @@ public partial class AsnReader
         public AsnReader(System.ReadOnlyMemory<byte> data, System.Formats.Asn1.AsnEncodingRules ruleSet, System.Formats.Asn1.AsnReaderOptions options = default(System.Formats.Asn1.AsnReaderOptions)) { }
         public bool HasData { get { throw null; } }
         public System.Formats.Asn1.AsnEncodingRules RuleSet { get { throw null; } }
+        public System.Formats.Asn1.AsnReader Clone() { throw null; }
         public System.ReadOnlyMemory<byte> PeekContentBytes() { throw null; }
         public System.ReadOnlyMemory<byte> PeekEncodedValue() { throw null; }
         public System.Formats.Asn1.Asn1Tag PeekTag() { throw null; }

src/libraries/System.Formats.Asn1/src/System/Formats/Asn1/AsnDecoder.cs

@@ -807,6 +807,15 @@ public ReadOnlyMemory<byte> ReadEncodedValue()
             return encodedValue;
         }
 
+        /// <summary>
+        /// Clones the current reader.
+        /// </summary>
+        /// <returns>A clone of the current reader.</returns>
+        /// <remarks>
+        /// This does not create a clone of the ASN.1 data, only the reader's state is cloned.
+        /// </remarks>
+        public AsnReader Clone() => new AsnReader(_data, RuleSet, _options);
+
         private AsnReader CloneAtSlice(int start, int length)
         {
             return new AsnReader(_data.Slice(start, length), RuleSet, _options);

src/libraries/System.Formats.Asn1/tests/Reader/ReaderStateTests.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Test.Cryptography;
 using Xunit;
 
 namespace System.Formats.Asn1.Tests.Reader
@@ -30,5 +31,71 @@ public static void HasDataAndThrowIfNotEmpty_StartsEmpty()
             // Assert.NoThrow
             reader.ThrowIfNotEmpty();
         }
+
+        [Fact]
+        public static void Clone_CopiesCurrentState()
+        {
+            // Sequence {
+            //   SetOf {
+            //     UtcTime 500405000012Z
+            //     Null
+            //   }
+            // }
+            // Verify the options are preserved in the clone by observing them:
+            // this is an incorrectly sorted SET OF with a date of 50/04/05 that should be 2050, not 1950.
+            ReadOnlyMemory<byte> asn = "30133111170D3530303430353030303031325A0500".HexToByteArray();
+
+            AsnReaderOptions options = new AsnReaderOptions
+            {
+                UtcTimeTwoDigitYearMax = 2050,
+                SkipSetSortOrderVerification = true,
+            };
+
+            AsnReader sequence = new AsnReader(asn, AsnEncodingRules.DER, options);
+            AsnReader reader = sequence.ReadSequence();
+            sequence.ThrowIfNotEmpty();
+
+            AsnReader clone = reader.Clone();
+            Assert.Equal(reader.RuleSet, clone.RuleSet);
+
+            AssertReader(reader);
+            Assert.False(reader.HasData, "reader.HasData");
+            Assert.True(clone.HasData, "clone.HasData");
+
+            AssertReader(clone);
+            Assert.False(clone.HasData, "clone.HasData");
+
+            static void AssertReader(AsnReader reader)
+            {
+                AsnReader setOf = reader.ReadSetOf();
+                reader.ThrowIfNotEmpty();
+
+                DateTimeOffset dateTime = setOf.ReadUtcTime();
+                Assert.Equal(2050, dateTime.Year);
+                setOf.ReadNull();
+                setOf.ThrowIfNotEmpty();
+            }
+        }
+
+        [Fact]
+        public static void Clone_Empty()
+        {
+            AsnReader reader = new AsnReader(ReadOnlyMemory<byte>.Empty, AsnEncodingRules.DER);
+            AsnReader clone = reader.Clone();
+            Assert.False(reader.HasData, "reader.HasData");
+            Assert.False(clone.HasData, "clone.HasData");
+        }
+
+        [Fact]
+        public static void Clone_SameUnderlyingData()
+        {
+            ReadOnlyMemory<byte> data = "04050102030405".HexToByteArray();
+            AsnReader reader = new AsnReader(data, AsnEncodingRules.DER);
+            AsnReader clone = reader.Clone();
+
+            Assert.True(reader.TryReadPrimitiveOctetString(out ReadOnlyMemory<byte> readerData));
+            Assert.True(clone.TryReadPrimitiveOctetString(out ReadOnlyMemory<byte> cloneData));
+            Assert.True(readerData.Span == cloneData.Span, "readerData == cloneData");
+        }
     }
 }