CSHARP-1117: Fix issue in ByteBufferSlice AccessBackingBytes when slice spans multiple chunks.

Author: rstam

src/MongoDB.Bson.Tests/IO/ByteBufferSliceTests.cs

@@ -41,6 +41,21 @@ public void AccessBackingBytes_should_adjust_count()
             result.Count.Should().Be(2); // not 3
         }
 
+        [Test]
+        public void AccessBackingBytes_should_adjust_count_when_multiple_chunks_are_present()
+        {
+            var arrays = new[] { new byte[] { 1, 2 }, new byte[] { 3, 4 } };
+            var chunks = arrays.Select(a => new ByteArrayChunk(a));
+            var buffer = new MultiChunkBuffer(chunks, isReadOnly: true);
+            var subject = new ByteBufferSlice(buffer, 1, 2);
+
+            var result = subject.AccessBackingBytes(0);
+
+            result.Array.Should().BeSameAs(arrays[0]);
+            result.Offset.Should().Be(1);
+            result.Count.Should().Be(1); // not 2 or 3
+        }
+
         [Test]
         public void AccessBackingBytes_should_adjust_position()
         {

src/MongoDB.Bson/IO/ByteBufferSlice.cs

@@ -114,7 +114,8 @@ public ArraySegment<byte> AccessBackingBytes(int position)
             ThrowIfDisposed();
 
             var segment = _buffer.AccessBackingBytes(position + _offset);
-            return new ArraySegment<byte>(segment.Array, segment.Offset, _length - position);
+            var count = Math.Min(segment.Count, _length - position);
+            return new ArraySegment<byte>(segment.Array, segment.Offset, count);
         }
 
         /// <inheritdoc/>