[bitcoin-dev] Segwit v2

[Luke Dashjr <luke@dashjr.org>]

Since BIP 141's version bit assignment will timeout soon, and needing renewal, 
I was thinking it might make sense to make some minor tweaks to the spec for 
the next deployment. These aren't critical, so it's perfectly fine if BIP 141 
activates as-is (potentially with BIP 148), but IMO would be an improvement if 
a new deployment (non-BIP148 UASF and/or new versionbit) is needed.

1. Change the dummy marker to 0xFF instead of 0. Using 0 creates ambiguity 
with incomplete zero-input transactions, which has been a source of confusion 
for raw transaction APIs. 0xFF would normally indicate a >32-bit input count, 
which is impossible right now (it'd require a >=158 GB transaction) and 
unlikely to ever be useful.

2. Relax the consensus rules on when witness data is allowed for an input. 
Currently, it is only allowed when the scriptSig is null, and the scriptPubKey 
being spent matches a very specific pattern. It is ignored by "upgrade-safe" 
policy when the scriptPubKey doesn't match an even-more-specific pattern. 
Instead, I suggest we allow it (in the consensus layer only) in combination 
with scriptSig and with any scriptPubKey, and consider these cases to be 
"upgrade-safe" policy ignoring.

The purpose of the second change is to be more flexible to any future 
softforks. I consider it minor because we don't know of any possibilities 
where it would actually be useful.

Thoughts?

Luke