Odd return value for GetByteLength on Boolean StackItem about neo-vm

[igormcoelho]

This is an interesting question @ixje, let me explain. The Boolean stackitem needs a definition for true/false, and personally I don't know which is the oldest version that was adopted, but like shargon said, we will need to preserve this on Neo 2.X, but we could improve for Neo 3.
When you code a smart contract that returns any int zero (like int i = 0;), compiler will usually issue a PUSH0. However, PUSH0 is not actually pushing the value zero to the stack, but an empty array. This is fine, because on BigInteger format, all left-zeros can be removed without issue until you get a byte with top bit set to one (the zero before this you cannot remove, otherwise you change sign). So, an empty array is in fact a value false (as usual, integer 0). However, I'm not sure its byte size would indeed be zero after conversion Int->ByteArray. But I'm sure that an empty bytearray is also zero (obviously every language would have somewhere at least size control that would make it not perfectly zero, but officially the byte array is zero length).
So: [ ] is false. [ 1 ] is true. [ 2 ] should be true as well, and [ -1 ] too. One thing I don't know, is that if [ 0 ] is false too. Is it? I was hoping in fact that [ 0 0 0 0 0 ] is also false. Is it? So, we could indeed improve this check, as long as it does not break any past compatibility.

[ixje]

@shargon what are your thoughts on this? The above issue got triggered by a recent test you added so I'm assuming you'll have an idea about it :)

neo-vm/tests/neo-vm.Tests/Tests/OpCodes/Splice/SIZE.json

Lines 173 to 174 in a3f9444

	"type": "Integer",
	"value": 0

[ixje]

To elaborate; this is my expected behaviour but in a normal language like C#

            bool boolValue = false;

            Console.WriteLine("Bool value: " + boolValue.ToString());

            byte[] bytes = BitConverter.GetBytes(boolValue);

            Console.WriteLine("Byte array value: {0}", BitConverter.ToString(bytes));
            Console.WriteLine("bytes len {0}", bytes.Length);

returns

Bool value: False
Byte array value: 00
bytes len 1

The neo-vm returns an empty array instead

neo-vm/src/neo-vm/Types/Boolean.cs

Line 9 in a3f9444

private static readonly byte[] FALSE = new byte[0];

[shargon]

I just know that is the expected behaviour inside the virtual machine ... it could be strange, but new byte[]{ 0x00} is false too inside the virtual machine.

[ixje]

I understand that this is how the VM has been working up to now. I raise the question if that is actually what we want. I think

neo-vm/src/neo-vm/Types/Boolean.cs

Line 9 in a3f9444

private static readonly byte[] FALSE = new byte[0];

should be

private static readonly byte[] FALSE = { 0x00 }; 

because then this

            StackItem true_item = new Neo.VM.Types.Boolean(true);
            StackItem false_item = new Neo.VM.Types.Boolean(false);
            Console.WriteLine(true_item.GetByteLength());
            Console.WriteLine(false_item.GetByteLength());

would not output

1
0

but

1
1

which is inline with how C#'s build in BitConverter works and how many other languages convert.

Let's ask for more opinions: @igormcoelho @vncoelho