Use descriptor wallet for Bitcoin Core >= v0.21.0

[Sjors]

First part of #707.

This PR checks if Bitcoin Core is v0.21.0 or above. If so it creates a descriptor wallet.

We then check for each wallet if it's a descriptor wallet (since the node may have been upgraded after a wallet was created), using getwalletinfo.

Descriptor wallets use importdescriptors instead of importmulti. This means the getnewaddress RPC works as well as the Receive tab in the Bitcoin Core GUI.

Balance is checked with mine rather than watchonly for descriptor wallets.

For descriptor wallets we no longer refill the keypool. There's a bunch of other address management code that can probably be skipped, but I don't know how it works. I bypassed the self._addresses.add stuff in keypoolrefill, which seems to work.

[stepansnigirev]

Thanks a lot for the PR! Looks good.
I would like to test it a bit before merging, but I am really excited about this PR and how much we can simplify with Core 21!

self._addresses are currently used to get the derivation path and derive descriptors for receiving addresses without rpc calls to Core, but I think we can slightly refactor that and only store labels there. We can derive descriptors for receiving addresses from the index, without lookups (that makes more sense).

I'll have a look over the weekend what we need to change for that and also test your PR :)

[Sjors]

There's also the deriveaddressess RPC call. It might be best if you add some commits to this PR to deal with that stuff.

Another thing that would be useful is if the test suite can download Bitcoin Core binaries and test against different versions (since we really should test against 0.21 and an older version). This script makes that very easy: https://github.com/bitcoin/bitcoin/blob/master/test/get_previous_releases.py

[k9ert]

For the reference, i'm adding this link which seems to be the best source in order to understand what this PR is about:
bitcoin/bitcoin#15764

[Sjors]

I rebased on master, so there's no need for the merge commit...

[ben-kaufman]

Looks great, thanks! The only thing which is an issue currently is that without the keypool management from Specter the addresses caching in CSV stops working, and we currently rely on that for quite a bit. We'll have to figure out first how to keep the addresses caching up to date properly without relying on keypool refill events on Specter.

[ben-kaufman]

Hi @Sjors, I've worked on this PR now to fix the address caching issue, and also a coin control issue and support the new add_inputs flag in walletcreatefundedpsbt.

Would it be fine if I just push directly over here to this PR?

[Sjors]

@ben-kaufman that's fine by me

[ben-kaufman]

Great, just pushed it, tested with Core 0.21 and 0.20, and everything looks good to me now

[Sjors]

tACK c8dd881

Tested on Signet with Bitcoin Core v0.21 by creating a new 2-of-2 wallet, receiving coins on it and then sending them.

[k9ert]

In #1316 i'm currently migrating to v0.21.1 and the tests are hanging in this endless loop.
How does this loop is suppose to end, @Sjors ?

[Sjors]

@stepansnigirev a Python strangle?