Add `min_relay_fee` checks for fee-estimation calls

[ariard]

See #985 (comment)

[dunxen]

Working on this

[dunxen]

Kind of related, but is this constant not supposed to be 250 for the default min relay fee in Bitcoin Core policy?

rust-lightning/lightning/src/chain/chaininterface.rs

Line 54 in 2a8a396

pub const MIN_RELAY_FEE_SAT_PER_1000_WEIGHT: u64 = 4000;

(Divide 1000 by 4 instead of multiplying?)

$$ {1000 sats \over 1000 vB} \times { 1 vB \over 4 WU } \times 1000 = 250 sats $$

I might be misunderstanding that variable name.

I'll create a const for the 253 sat/kvB min rate for the assertions needed for this issue. I think CLN calls it FEERATE_FLOOR to distinguish it from Bitcoin Core policy one they name BITCOIND_MINRELAYTXFEE_PER_KW which is 250.

[ariard]

If you would like the rational Bitcoin Core rounds up during the conversion from weight to vbytes : https://github.com/bitcoin/bitcoin/blob/master/src/policy/policy.cpp#L296

Therefore, if your transaction is let's say 1003 WU offering a feerate of 250 sats/kW because you think 1003 / WITNESS_SCALE_FACTOR = 250, you might think you're okay. You're not. Your transaction should be scaled to 251 vbytes and the required min fee by Core should be 251 satoshis (see CFeeRate::GetFee).