This introduces Taproot wallet support. I fixed all the merge conflicts
and ensured that the tests pass, but this is still using the old sighash
(before Russell/Sanket/I redid it) so is not actually production ready.
Will be fixed when we bring Elements #1002 in.
Does the bare minimum to introduce Taproot wallet support with CT; just
adds a CPubKey blinding_pubkey to the taproot destination variant and
updates some visitors.
In future when we define blech32 we will need to make sure we are using
that encoding and using the pubkey.
This removes a lot of the craziness that was added during the 0.21
rebase 240f03f586 (Bitcoin PR #17290)
as well as a lot of other upstream craziness. It's a very positive
change but will probably be a bear to review.
There are two places where I (purposely) nontrivially changed the
logic beyond what the upstream PR did:
1. When setting the change output positions, I used a uniform
distribution for all change outputs and also obeyed the user's
`change_position` choice. The old code was non-uniform and
had a bug where it would offset the change position.
For non-policy assets, I do not create zero-sized change
since we would just remove them later. For the policy asset,
I do sometimes create zero-sized change since the upstream
logic expects it to exist.
2. Rather than "reblinding whenever something changes" we blind
once for size/fee estimation purposes and then again at the
end after all potential adjustments have been made. The
resulting code should be closer to upstream.
We should really redo this code though because our use of the
`BlindDetails` structure is very confusing and stateful.
3. We consider whether or not we're using CT/CA when estimating
the size of change outpust. This is because we now use branch
and bound far more often (yay!) which selects coins such that
there will be no change output. This is implemented however
by adding a change output then dumping it to fees ... meaning
that if we dramatically overestimate the size of a change out
then we'll end up dramatically dumping too many coins to fees.
There are another couple small things, which I apologize for .. this
PR took me 2 hours to get compiling and 11 hours to get working(!!)
and some things got away from me.
One fun thing is that I changed CWallet::SelectCoinsMinConf to gate
an addition on coin_selection_params.m_subtract_fee_outputs ... this
is actually an upstream bug that I'll file whenever I get around to
producing a test that triggers it. (A bit hard as I have to gin up
a scenario where branch-and-bound fails to hit this codepath.)
Also, in test/functional/wallet_bumpfee.py I had to change the feerate
required to force selection to add another input from 500 to 800. I
don't know why and I don't care to figure it out.
