Fee(rate) mismatch between min relay and fee filter #16499

I was messing around with min relay policies and found that under certain circumstances the mempool will accept things, then not relay to any peers.

Example made using wallet: Transaction of size 6195, fee of 619 satoshis, minrelay of 100 satoshis per kvB. Mempool logic allows it:

        // No transactions are allowed below minRelayTxFee except from disconnected blocks
        if (!bypass_limits && nModifiedFees < ::minRelayTxFee.GetFee(nSize)) {
            return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "min relay fee not met", false, strprintf("%d < %d", nModifiedFees, ::minRelayTxFee.GetFee(nSize)));
        }

and lets it into mempool

but in net_processing.cpp:

                    if (filterrate && txinfo.feeRate.GetFeePerK() < filterrate) {
                        continue;
                    }

this gets hit because the computed feerate is insufficient(printing I added):

0.00000099 BTC/kB

I'm not super at arithmetic so I'm opening this up in case someone has an easy fix.

let me also get a reproducible test failure with this...

Reproduced here https://github.com/instagibbs/bitcoin/commit/4538d6bfd1bddcfb741f04dbcd61a4eb41162e8f if you run rpc_fundrawtransaction.py

Ok it's pretty straight-forward:

Whenever you have a feerate in satoshis per kvB that's not divisible by 1000 exactly, GetFee may undershoot its intended results when it's checked for fee filter. See:

https://github.com/instagibbs/bitcoin/commit/c480e4ddbf611dd0d9de9aa39317de15381625b8

Applying this fix means that previously-accepted things into mempool are no longer accepted, but these transactions won't propagate anyways, so I think it's fairly conservative.