Merge bitcoin/bitcoin#27620: test: miner: add coverage for -blockmintxfee setting

bbbb89d238 test: miner: add coverage for `-blockmintxfee` setting (Sebastian Falbesoner)

Pull request description:

  This PR adds missing test coverage for the `-blockmintxfee` option, which can be used by miners to specify the lowest fee-rate for transactions to be included in blocks. The setting was introduced in PR #9380 (commit daec955fd6), with the rationale to decouple different minimum fees from `-minrelaytxfee`. According to the PR description it _"should be set by miners to reflect their marginal cost of transmitting extra bytes."_.

  On each iteration, the test creates and submits two txs using MiniWallet: one with the the minimum fee-rate as specified for `-blockmintxfee` and a second one with a fee-rate a little below that (-0.01 sats/vbyte). Then it checks that  only the first one is picked for the block template and accordingly also only exists in the block that is mined after. This is repeatedly done for a fixed (but obviously somewhat arbitrary) list of different `-blockmintxfee` settings on a single node, including the default and zero (i.e. no minimum fee a.k.a. "include even zero-fee txs") settings.

ACKs for top commit:
  ryanofsky:
    Code review ACK bbbb89d238, nice test
  brunoerg:
    reACK bbbb89d238
  glozow:
    ACK bbbb89d238, sorry for the late re-review!

Tree-SHA512: 7b72612971e6a1667b4b3913ec27109953fd17a1020a4bde6941a93666b2e10a23fb6fe7df23471a5671ffe31e42cd992d2efb8b31903915b3dfc1d6478df757

test/functional/mining_basic.py

@@ -18,9 +18,10 @@ from test_framework.blocktools import (
     TIME_GENESIS_BLOCK,
 )
 from test_framework.messages import (
+    BLOCK_HEADER_SIZE,
     CBlock,
     CBlockHeader,
-    BLOCK_HEADER_SIZE,
+    COIN,
     ser_uint256,
 )
 from test_framework.p2p import P2PDataStore
@@ -28,12 +29,14 @@ from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
     assert_raises_rpc_error,
+    get_fee,
 )
 from test_framework.wallet import MiniWallet
 
 
 VERSIONBITS_TOP_BITS = 0x20000000
 VERSIONBITS_DEPLOYMENT_TESTDUMMY_BIT = 28
+DEFAULT_BLOCK_MIN_TX_FEE = 1000  # default `-blockmintxfee` setting [sat/kvB]
 
 
 def assert_template(node, block, expect, rehash=True):
@@ -73,6 +76,45 @@ class MiningTest(BitcoinTestFramework):
         self.restart_node(0)
         self.connect_nodes(0, 1)
 
+    def test_blockmintxfee_parameter(self):
+        self.log.info("Test -blockmintxfee setting")
+        self.restart_node(0, extra_args=['-minrelaytxfee=0', '-persistmempool=0'])
+        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, 50, 100, 500, 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)...")
+            else:
+                blockmintxfee_parameter = f"-blockmintxfee={blockmintxfee_btc_kvb:.8f}"
+                self.log.info(f"-> Test {blockmintxfee_parameter} ({blockmintxfee_sat_kvb} sat/kvB)...")
+                self.restart_node(0, extra_args=[blockmintxfee_parameter, '-minrelaytxfee=0', '-persistmempool=0'])
+                self.wallet.rescan_utxos()  # to avoid spending outputs of txs that are not in mempool anymore after restart
+
+            # submit one tx with exactly the blockmintxfee rate, and one slightly below
+            tx_with_min_feerate = self.wallet.send_self_transfer(from_node=node, fee_rate=blockmintxfee_btc_kvb)
+            assert_equal(tx_with_min_feerate["fee"], get_fee(tx_with_min_feerate["tx"].get_vsize(), blockmintxfee_btc_kvb))
+            if blockmintxfee_btc_kvb > 0:
+                lowerfee_btc_kvb = blockmintxfee_btc_kvb - Decimal(10)/COIN  # 0.01 sat/vbyte lower
+                tx_below_min_feerate = self.wallet.send_self_transfer(from_node=node, fee_rate=lowerfee_btc_kvb)
+                assert_equal(tx_below_min_feerate["fee"], get_fee(tx_below_min_feerate["tx"].get_vsize(), lowerfee_btc_kvb))
+            else:  # go below zero fee by using modified fees
+                tx_below_min_feerate = self.wallet.send_self_transfer(from_node=node, fee_rate=blockmintxfee_btc_kvb)
+                node.prioritisetransaction(tx_below_min_feerate["txid"], 0, -1)
+
+            # check that tx below specified fee-rate is neither in template nor in the actual block
+            block_template = node.getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)
+            block_template_txids = [tx['txid'] for tx in block_template['transactions']]
+            self.generate(self.wallet, 1, sync_fun=self.no_op)
+            block = node.getblock(node.getbestblockhash(), verbosity=2)
+            block_txids = [tx['txid'] for tx in block['tx']]
+
+            assert tx_with_min_feerate['txid'] in block_template_txids
+            assert tx_with_min_feerate['txid'] in block_txids
+            assert tx_below_min_feerate['txid'] not in block_template_txids
+            assert tx_below_min_feerate['txid'] not in block_txids
+
     def run_test(self):
         node = self.nodes[0]
         self.wallet = MiniWallet(node)
@@ -279,6 +321,8 @@ class MiningTest(BitcoinTestFramework):
         node.submitheader(hexdata=CBlockHeader(bad_block_root).serialize().hex())
         assert_equal(node.submitblock(hexdata=block.serialize().hex()), 'duplicate')  # valid
 
+        self.test_blockmintxfee_parameter()
+
 
 if __name__ == '__main__':
     MiningTest().main()