Merge bitcoin/bitcoin#33106: policy: lower the default blockmintxfee, incrementalrelayfee, minrelaytxfee

ba84a25dee [doc] update mempool-replacements.md for incremental relay feerate change (glozow)
18720bc5d5 [doc] release note for min feerate changes (glozow)
6da5de58ca [policy] lower default minrelaytxfee and incrementalrelayfee to 100sat/kvB (glozow)
2e515d2897 [prep/test] make wallet_fundrawtransaction's minrelaytxfee assumption explicit (glozow)
457cfb61b5 [prep/util] help MockMempoolMinFee handle more precise feerates (glozow)
3eab8b7240 [prep/test] replace magic number 1000 with respective feerate vars (glozow)
5f2df0ef78 [miner] lower default -blockmintxfee to 1sat/kvB (glozow)
d6213d6aa1 [doc] assert that default min relay feerate and incremental are the same (glozow)
1fbee5d7b6 [test] explicitly check default -minrelaytxfee and -incrementalrelayfee (glozow)
72dc18467d [test] RBF rule 4 for various incrementalrelayfee settings (glozow)
85f498893f [test] check bypass of minrelay for various minrelaytxfee settings (glozow)
e5f896bb1f [test] check miner doesn't select 0fee transactions (glozow)

Pull request description:

  ML post for discussion about the general concept, how this impacts the wider ecosystem, philosophy about minimum feerates, etc: https://delvingbitcoin.org/t/changing-the-minimum-relay-feerate/1886

  This PR is inspired by #13922 and #32959 to lower the minimum relay feerate in response to bitcoin's exchange rate changes in the last ~10 years. It lowers the default `-minrelaytxfee` and `-incrementalrelayfee`, and knocks `-blockmintxfee` down to the minimum nonzero setting. Also adds some tests for the settings and pulls in #32750.

  The minimum relay feerate is a DoS protection rule, representing a price on the network bandwidth used to relay transactions that have no PoW. While relay nodes don't all collect fees, the assumption is that if nodes on the network use their resources to relay this transaction, it will reach a miner and the attacker's money will be spent once it is mined. The incremental relay feerate is similar: it's used to price the relay of replacement transactions (the additional fees need to cover the new transactions at this feerate) and evicted transactions (following a trim, the new mempool minimum feerate is the package feerate of what was removed + incremental).

  Also note that many nodes on the network have elected to relay/mine lower feerate transactions. Miners (some say up to 85%) are choosing to mine these low feerate transactions instead of leaving block space unfilled, but these blocks have extremely poor compact block reconstruction rates with nodes that rejected or didn't hear about those transactions earlier.
  - https://github.com/bitcoin/bitcoin/pull/33106#issuecomment-3155627414
  - https://x.com/caesrcd/status/1947022514267230302
  - https://mempool.space/block/00000000000000000001305770e0aa279dcd8ba8be18c3d5cf736a26f77e06fd
  - https://mempool.space/block/00000000000000000001b491649ec030aa8e003e1f4f9d3b24bb99ba16f91e97
  - https://x.com/mononautical/status/1949452586391855121

  While it wouldn't make sense to loosen DoS restrictions recklessly in response to these events, I think the current price is higher than necessary, and this motivates us changing the default soon. Since the minimum relay feerate defines an amount as too small based on what it costs the attacker, it makes sense to consider BTC's conversion rate to what resources you can buy in the "real world."

  Going off of [this comment](https://github.com/bitcoin/bitcoin/pull/32959#issuecomment-3095260286) and [this comment](https://github.com/bitcoin/bitcoin/pull/33106#issuecomment-3142444090)
  - Let's say an attacker wants to use/exhaust the network's bandwidth, and has the choice between renting resources from a commercial provider and getting the network to "spam" itself it by sending unconfirmed transactions. We'd like the latter to be more expensive than the former.
  - The bandwidth for relaying a transaction across the network is roughly its serialized size (plus relay overhead) x number of nodes. A 1000vB transaction is 1000-4000B serialized. With 100k nodes, that's 0.1-0.4GB
  - If the going rate for ec2 bandwidth is 10c/GB, that's like 1-4c per kvB of transaction data
  - Then a 1000vB transaction should pay at least 4c
  - $0.04 USD is 40 satoshis at 100k USD/BTC
  - Baking in some margin for changes in USD/BTC conversion rate, number of nodes (and thus bandwidth), and commercial service costs, I think 50-100 satoshis is on the conservative end but in the right ballpark
  - At least 97% of the recent sub-1sat/vB transactions would be accepted with a new threshold of 0.1sat/vB: https://github.com/bitcoin/bitcoin/pull/33106#issuecomment-3156213089

  List of feerates that are changed and why:
  - min relay feerate: significant conversion rate changes, see above
  - incremental relay feerate: should follow min relay feerate, see above
  - block minimum feerate: shouldn’t be above min relay feerate, otherwise the node accepts transactions it will never mine. I've knocked it down to the bare minimum of 1sat/kvB. Now that we no longer have coin age priority (removed in v0.15), I think we can leave it to the `CheckFeeRate` policy rule to enforce a minimum entry price, and the block assembly code should just fill up the block with whatever it finds in mempool.

  List of feerates that are not changed and why:
  - dust feerate: this feerate cannot be changed as flexibly as the minrelay feerate. A much longer record of low feerate transactions being mined is needed to motivate a decrease there.
  - maxfeerate (RPC, wallet): I think the conversion rate is relevant as well, but out of scope for this PR
  - minimum feerate returned by fee estimator: should be done later. In the past, we've excluded new policy defaults from fee estimation until we feel confident they represent miner policy (e.g. #9519). Also, the fee estimator itself doesn't have support for sub-1sat/vB yet.
  - all wallet feerates (mintxfee, fallbackfee, discardfee, consolidatefeerate, WALLET_INCREMENTAL_RELAY_FEE, etc.): should be done later. Our standard procedure is to do wallet changes at least 1 release after policy changes.

ACKs for top commit:
  achow101:
    ACK ba84a25dee
  gmaxwell:
    ACK ba84a25dee
  jsarenik:
    Tested ACK ba84a25dee
  darosior:
    ACK ba84a25dee
  ajtowns:
    ACK ba84a25dee
  davidgumberg:
    crACK  ba84a25dee
  w0xlt:
    ACK ba84a25dee
  caesrcd:
    reACK ba84a25dee
  ismaelsadeeq:
    re-ACK ba84a25dee

Tree-SHA512: b4c35e8b506b1184db466551a7e2e48bb1e535972a8dbcaa145ce3a8bfdcc70a8807dc129460f129a9d31024174d34077154a387c32f1a3e6831f6fa5e9c399e

doc/policy/mempool-replacements.md

@@ -32,8 +32,8 @@ other consensus and policy rules, each of the following conditions are met:
 4. The additional fees (difference between absolute fee paid by the replacement transaction and the
    sum paid by the original transactions) pays for the replacement transaction's bandwidth at or
    above the rate set by the node's incremental relay feerate. For example, if the incremental relay
-   feerate is 1 satoshi/vB and the replacement transaction is 500 virtual bytes total, then the
-   replacement pays a fee at least 500 satoshis higher than the sum of the original transactions.
+   feerate is 0.1 satoshi/vB and the replacement transaction is 500 virtual bytes total, then the
+   replacement pays a fee at least 50 satoshis higher than the sum of the original transactions.
 
    *Rationale*: Try to prevent DoS attacks where an attacker causes the network to repeatedly relay
    transactions each paying a tiny additional amount in fees, e.g. just 1 satoshi.
@@ -77,3 +77,5 @@ This set of rules is similar but distinct from BIP125.
 * Full replace-by-fee is the default policy as of **v28.0** ([PR #30493](https://github.com/bitcoin/bitcoin/pull/30493)).
 
 * Signaling for replace-by-fee is no longer required as of [PR 30592](https://github.com/bitcoin/bitcoin/pull/30592).
+
+* The incremental relay feerate default is 0.1sat/vB ([PR #33106](https://github.com/bitcoin/bitcoin/pull/33106)).

doc/release-notes-33106.md

@@ -0,0 +1,17 @@
+Mining and Transaction Relay Policy
+=========================
+
+The minimum block feerate (`-blockmintxfee`) has been changed to 1 satoshi per kvB. It can still be changed using the
+configuration option.
+
+The default minimum relay feerate (`-minrelaytxfee`) and incremental relay feerate (`-incrementalrelayfee`) have been
+changed to 100 satoshis per kvB. They can still be changed using their respective configuration options, but it is
+recommended to change both together if you decide to do so.
+
+Other minimum feerates (e.g. the dust feerate, the minimum returned by the fee estimator, and all feerates used by the
+wallet) remain unchanged. The mempool minimum feerate still changes in response to high volume but more gradually, as a
+result of the change to the incremental relay feerate.
+
+Note that unless these lower defaults are widely adopted across the network, transactions created with lower fee rates
+are not guaranteed to propagate or confirm. The wallet feerates remain unchanged; `-mintxfee` must be changed before
+attempting to create transactions with lower feerates using the wallet.