Inconsistent checking of RBF rules

[LLFourn]

BIP125 rules 3 and 4 are not being checked properly according to my understanding:

https://github.com/bitcoindevkit/bdk/blob/c2b2da7601fb570952cfd858f3821b3a4fd87ec9/src/wallet/mod.rs#L730-L746

It should be checking rules 3 and 4 regardless of the fee policy chosen in txbuilder.

[afilini]

I think we can kinda ignore rule 3 at the moment, since we don't support merging multiple unconfirmed transactions, so this:

The replacement transaction pays an absolute fee of at least the sum paid by the original transactions.

For us basically means that we should just pay at least the previous fee.

I agree on rule 4 though, the FeeAmount branch should check for *amount < details.fees + _something_.

And now that I think about it, it's kind of a chicken-and-egg problem: rule 4 says that we should pay at least the size of the new transaction, but at this point we don't know it yet. So even the part above where required_feerate is computed is wrong.. Not sure what's the best way to handle this, I've gotta think about it a bit more..

[evanlinjin]

Coin selection algorithms should take min_absolute_fee and target_feerate into consideration on the get-go (and only succeed when both constraints are satisfied).

bdk_core will solve this issue, WIP here: LLFourn/bdk_core_staging#21

[nondiremanuel]

We discussed it in the call and decided to close this issue. Please comment if you think it should be re-opened

[notmandatory]

Per discussion in dev call today this doesn't seem to be fixed yet.

[notmandatory]

Per @evanlinjin we can fix this issue by using the new bdk_coin_select crate.