contract download - remove exception on empty validatedrootindex

[ixje]

The contract download feature is intended to support local development. There is a security check that seems unnecessary when used for this purpose, specifically here

neo-express/src/neoxp/Node/NodeUtility.cs

Lines 244 to 245 in c6424c0

	stateHeight = validatedRootIndex.HasValue ? validatedRootIndex.Value
	: throw new Exception($"Null \"{nameof(validatedRootIndex)}\" in state height response");

this basically forbids using any node that is not voted into the list of state validators by the consensus committee. It should be possible to use nodes that you trust e.g. I want to use our COZ nodes because the NEO nodes are very slow. This could be done by using localrootindex instead of validatedrootindex.

I see 3 ways of solving this

1. always use localrootindex instead of validatedrootindex
2. have a flag like --allow-localroot to use localrootindex if validatedrootindex is null
3. the inverse of 2. where we allow localrootindex by default unless passing some flag like --require-rootvalidated-state

[devhawk]

I wouldn't characterize this as a security check. If the user doesn't explicitly specify a block height to download, the command gets "most recent available". How the command chooses "most recent available" doesn't seem that critical.

How about a slight variation on option 1: Use localrootindex if validatedrootindex is not available. Something like this maybe?

if (stateHeight == 0)
{
    uint? localRootIndex, validatedRootIndex;
    try
    {
        (localRootIndex, validatedRootIndex) = await stateAPI.GetStateHeightAsync().ConfigureAwait(false);
    }
    catch (RpcException e) when (e.Message.Contains("Method not found"))
    {
        throw new Exception(
            "Could not get state information. Make sure the remote RPC server has state service support");
    }

    stateHeight = validatedRootIndex.HasValue 
        ? validatedRootIndex.Value
        : localRootIndex.HasValue
            ? localRootIndex.Value
            : throw new Exception($"GetStateHeight did not return local or validated root index");
}

[devhawk]

FYI, can use null coalesce operator instead:

stateHeight = validatedRootIndex ?? localRootIndex 
    ?? throw new Exception($"GetStateHeight did not return local or validated root index");