Fix Length for ReadOnlySequence created out of sliced Memory owned by MemoryManager (#57479)

Author: adamsitnik

src/libraries/System.Memory/src/System/Buffers/ReadOnlySequence.cs

@@ -150,7 +150,7 @@ public ReadOnlySequence(ReadOnlyMemory<T> memory)
                 _startObject = manager;
                 _endObject = manager;
                 _startInteger = ReadOnlySequence.MemoryManagerToSequenceStart(index);
-                _endInteger = length;
+                _endInteger = index + length;
             }
             else if (MemoryMarshal.TryGetArray(memory, out ArraySegment<T> segment))
             {

src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceFactory.byte.cs

@@ -4,13 +4,17 @@
 using System.Buffers;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using System.Threading;
 
 namespace System.Memory.Tests
 {
     public abstract class ReadOnlySequenceFactory<T>
     {
         public static ReadOnlySequenceFactory<T> ArrayFactory { get; } = new ArrayTestSequenceFactory();
         public static ReadOnlySequenceFactory<T> MemoryFactory { get; } = new MemoryTestSequenceFactory();
+        public static ReadOnlySequenceFactory<T> MemoryManagerFactory { get; } = new MemoryManagerTestSequenceFactory();
         public static ReadOnlySequenceFactory<T> SingleSegmentFactory { get; } = new SingleSegmentTestSequenceFactory();
         public static ReadOnlySequenceFactory<T> SegmentPerItemFactory { get; } = new BytePerSegmentTestSequenceFactory();
         public static ReadOnlySequenceFactory<T> SplitInThree { get; } = new SegmentsTestSequenceFactory(3);
@@ -37,7 +41,11 @@ internal class MemoryTestSequenceFactory : ReadOnlySequenceFactory<T>
         {
             public override ReadOnlySequence<T> CreateOfSize(int size)
             {
-                return CreateWithContent(new T[size]);
+#if DEBUG
+                return new ReadOnlySequence<T>(new ReadOnlyMemory<T>(new T[size + 1]).Slice(1));
+#else
+                return new ReadOnlySequence<T>(new ReadOnlyMemory<T>(new T[size]));
+#endif
             }
 
             public override ReadOnlySequence<T> CreateWithContent(T[] data)
@@ -112,6 +120,49 @@ public override ReadOnlySequence<T> CreateWithContent(T[] data)
             }
         }
 
+        internal class MemoryManagerTestSequenceFactory : ReadOnlySequenceFactory<T>
+        {
+            public override ReadOnlySequence<T> CreateOfSize(int size)
+            {
+#if DEBUG
+                return new ReadOnlySequence<T>(new CustomMemoryManager(size + 1).Memory.Slice(1));
+#else
+                return new ReadOnlySequence<T>(new CustomMemoryManager(size).Memory);
+#endif
+            }
+
+            public override ReadOnlySequence<T> CreateWithContent(T[] data)
+            {
+                return new ReadOnlySequence<T>(new CustomMemoryManager(data).Memory);
+            }
+
+            private unsafe class CustomMemoryManager : MemoryManager<T>
+            {
+                private readonly T[] _buffer;
+
+                public CustomMemoryManager(int size) => _buffer = new T[size];
+
+                public CustomMemoryManager(T[] content) => _buffer = content;
+
+                public unsafe override Span<T> GetSpan() => _buffer;
+
+                public override unsafe MemoryHandle Pin(int elementIndex = 0)
+                {
+                    if ((uint)elementIndex > (uint)_buffer.Length)
+                    {
+                        throw new ArgumentOutOfRangeException(nameof(elementIndex));
+                    }
+
+                    var handle = GCHandle.Alloc(_buffer, GCHandleType.Pinned);
+                    return new MemoryHandle(Unsafe.Add<T>((void*)handle.AddrOfPinnedObject(), elementIndex), handle, this);
+                }
+
+                public override void Unpin() { }
+
+                protected override void Dispose(bool disposing) { }
+            }
+        }
+
         public static ReadOnlySequence<T> CreateSegments(params T[][] inputs) => CreateSegments(inputs.Select(input => (ReadOnlyMemory<T>)input.AsMemory()));
 
         public static ReadOnlySequence<T> CreateSegments(IEnumerable<ReadOnlyMemory<T>> inputs)

src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceTests.byte.cs

@@ -20,6 +20,11 @@ public class Memory : ReadOnlySequenceTestsByte
             public Memory() : base(ReadOnlySequenceFactory<byte>.MemoryFactory) { }
         }
 
+        public class MemoryManager : ReadOnlySequenceTestsByte
+        {
+            public MemoryManager() : base(ReadOnlySequenceFactory<byte>.MemoryManagerFactory) { }
+        }
+
         public class SingleSegment : ReadOnlySequenceTestsByte
         {
             public SingleSegment() : base(ReadOnlySequenceFactory<byte>.SingleSegmentFactory) { }