[miner] lower default -blockmintxfee to 1sat/kvB

Back when we implemented coin age priority as a miner policy, miners
mempools might admit transactions paying very low fees, but then want to
set a higher fee for block inclusion. However, since coin age priority
was removed in v0.15, the block assembly policy is solely based on fees,
so we do not need to apply minimum feerate rules in multiple places. In
fact, the block assembly policy ignoring transactions that are added to
the mempool is likely undesirable as we waste resources accepting and
storing this transaction.

Instead, rely on mempool policy to enforce a minimum entry feerate to
the mempool (minrelaytxfee). Set the minimum block feerate to the
minimum non-zero amount (1sat/kvB) so it collects everything it finds in
mempool into the block.

Author: glozow

src/policy/policy.h

@@ -29,7 +29,7 @@ static constexpr unsigned int DEFAULT_BLOCK_RESERVED_WEIGHT{8000};
  * Setting a lower value is prevented at startup. */
 static constexpr unsigned int MINIMUM_BLOCK_RESERVED_WEIGHT{2000};
 /** Default for -blockmintxfee, which sets the minimum feerate for a transaction in blocks created by mining code **/
-static constexpr unsigned int DEFAULT_BLOCK_MIN_TX_FEE{1000};
+static constexpr unsigned int DEFAULT_BLOCK_MIN_TX_FEE{1};
 /** The maximum weight for transactions we're willing to relay/mine */
 static constexpr int32_t MAX_STANDARD_TX_WEIGHT{400000};
 /** The minimum non-witness size for transactions we're willing to relay/mine: one larger than 64  */

src/test/miner_tests.cpp

@@ -12,6 +12,7 @@
 #include <node/miner.h>
 #include <policy/policy.h>
 #include <test/util/random.h>
+#include <test/util/transaction_utils.h>
 #include <test/util/txmempool.h>
 #include <txmempool.h>
 #include <uint256.h>
@@ -216,6 +217,9 @@ void MinerTestingSetup::TestPackageSelection(const CScript& scriptPubKey, const
     tx.vout.resize(2);
     tx.vout[0].nValue = 5000000000LL - 100000000;
     tx.vout[1].nValue = 100000000; // 1BTC output
+    // Increase size to avoid rounding errors: when the feerate is extremely small (i.e. 1sat/kvB), evaluating the fee
+    // at a smaller transaction size gives us a rounded value of 0.
+    BulkTransaction(tx, 4000);
     Txid hashFreeTx2 = tx.GetHash();
     AddToMempool(tx_mempool, entry.Fee(0).SpendsCoinbase(true).FromTx(tx));
 

test/functional/mining_basic.py

@@ -55,7 +55,7 @@
 MAX_TIMEWARP = 600
 VERSIONBITS_TOP_BITS = 0x20000000
 VERSIONBITS_DEPLOYMENT_TESTDUMMY_BIT = 28
-DEFAULT_BLOCK_MIN_TX_FEE = 1000  # default `-blockmintxfee` setting [sat/kvB]
+DEFAULT_BLOCK_MIN_TX_FEE = 1 # default `-blockmintxfee` setting [sat/kvB]
 
 class MiningTest(BitcoinTestFramework):
     def set_test_params(self):
@@ -144,7 +144,7 @@ def test_blockmintxfee_parameter(self):
         node = self.nodes[0]
 
         # test default (no parameter), zero and a bunch of arbitrary blockmintxfee rates [sat/kvB]
-        for blockmintxfee_sat_kvb in (DEFAULT_BLOCK_MIN_TX_FEE, 0, 1, 5, 10, 50, 100, 500, 2500, 5000, 21000, 333333, 2500000):
+        for blockmintxfee_sat_kvb in (DEFAULT_BLOCK_MIN_TX_FEE, 0, 5, 10, 50, 100, 500, 1000, 2500, 5000, 21000, 333333, 2500000):
             blockmintxfee_btc_kvb = blockmintxfee_sat_kvb / Decimal(COIN)
             if blockmintxfee_sat_kvb == DEFAULT_BLOCK_MIN_TX_FEE:
                 self.log.info(f"-> Default -blockmintxfee setting ({blockmintxfee_sat_kvb} sat/kvB)...")