Overlap Buffer.MemoryCopy causes unknown error

[neuecc]

@AArnott

Overlapped Buffer.MemoryCopy in WriteString_PostEncoding (or other places?) causes unknown error.
https://github.com/neuecc/MessagePack-CSharp/blob/15c5739/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/MessagePackWriter.cs#L1251

For example, in Unity, this string serialization is broken.

// 'あ' is [227, 129, 130]
var bin = MessagePackSerializer.Serialize(new string('あ', 11));

// expected: 217, 33, 227, 129, 130, 227, 129, 130, ....
// actual:  217, 33, 227, 227, 227, 227, 227, 227, 227, 227, ...
UnityEngine.Debug.Log(string.Join(", ", bin));

This code works fine in .NET Core but maybe it is undefined in overlapped memcpy.

The parameter of MemoryCopy is

// pBuffer = address
// estimatedOffset = 1
// byteCount = 33
Buffer.MemoryCopy(pBuffer + estimatedOffset, pBuffer + 2, byteCount, byteCount);

[neuecc]

In .NET Core, called memmove so it is safe.
https://source.dot.net/#System.Private.CoreLib/shared/System/Buffer.cs,146

But Unity's .NET (2018.3-mbe(MonoBleedingEdge)) maybe calls this code.
https://github.com/Unity-Technologies/mono/blob/unity-2018.3-mbe/mcs/class/corlib/ReferenceSources/Buffer.cs#L87

[AArnott]

Ya, Buffer.MemoryCopy is documented to work with overlapping buffers. We may want to avoid the bug in mono as well, but do you know where we can file a bug against mono?

[neuecc]

In GitHub?
https://www.mono-project.com/community/bugs/make-a-good-bug-report/

[AArnott]

I filed mono/mono#18516