Merge #14582: wallet: always do avoid partial spends if fees are within a specified range

7f13dfb587 test: test the implicit avoid partial spends functionality (Karl-Johan Alm)
b82067bf69 wallet: try -avoidpartialspends mode and use its result if fees are below threshold (Karl-Johan Alm)

Pull request description:

  The `-avoidpartialspends` feature is normally disabled, as it may affect the optimal fee for payments. This PR introduces a new parameter `-maxapsfee` (max avoid partial spends fee) which acts on the following values:
  * -1: disable partial spend avoidance completely (do not even try it)
  * 0: only do partial spend avoidance if fees are the same or better as the regular coin selection
  * 1..∞: use APS variant if the absolute fee difference is less than or equal to the max APS fee

  For values other than -1, the code will now try partial spend avoidance once, and if that gives a value within the accepted range, it will use that.

  Example: -maxapsfee=0.00001000 means the wallet will do regular coin select, APS coin select, and then pick AKS iff the absolute fee difference is <= 1000 satoshi.

  Edit: updated this to reflect the fact we are now using a max fee.

ACKs for top commit:
  fjahr:
    tested ACK 7f13dfb587
  achow101:
    ACK 7f13dfb587
  jonatack:
    ACK 7f13dfb58, code review, debug build, verified the test fails with `AssertionError: not(2 == 1)` for the number of vouts when `-maxapsfee=0.0001` is changed to 0, and verified the new logging with an added assertion.
  meshcollider:
    Code review ACK 7f13dfb587

Tree-SHA512: 475929df57f6191bb4e36bfbcad5a280a64bb0ecd8767b76cb2e44e2301235d0eb294a3f2fac5bbf15d35d7ecfba47acb2285feadb883c9ce31c08377e3afb3c

test/functional/wallet_groups.py

@@ -15,8 +15,8 @@ from test_framework.util import (
 class WalletGroupTest(BitcoinTestFramework):
     def set_test_params(self):
         self.setup_clean_chain = True
-        self.num_nodes = 3
-        self.extra_args = [[], [], ['-avoidpartialspends']]
+        self.num_nodes = 4
+        self.extra_args = [[], [], ['-avoidpartialspends'], ["-maxapsfee=0.0001"]]
         self.rpc_timeout = 480
 
     def skip_test_if_missing_module(self):
@@ -64,6 +64,52 @@ class WalletGroupTest(BitcoinTestFramework):
         assert_approx(v[0], 0.2)
         assert_approx(v[1], 1.3, 0.0001)
 
+        # Test 'avoid partial if warranted, even if disabled'
+        self.sync_all()
+        self.nodes[0].generate(1)
+        # Nodes 1-2 now have confirmed UTXOs (letters denote destinations):
+        # Node #1:      Node #2:
+        # - A  1.0      - D0 1.0
+        # - B0 1.0      - D1 0.5
+        # - B1 0.5      - E0 1.0
+        # - C0 1.0      - E1 0.5
+        # - C1 0.5      - F  ~1.3
+        # - D ~0.3
+        assert_approx(self.nodes[1].getbalance(), 4.3, 0.0001)
+        assert_approx(self.nodes[2].getbalance(), 4.3, 0.0001)
+        # Sending 1.4 btc should pick one 1.0 + one more. For node #1,
+        # this could be (A / B0 / C0) + (B1 / C1 / D). We ensure that it is
+        # B0 + B1 or C0 + C1, because this avoids partial spends while not being
+        # detrimental to transaction cost
+        txid3 = self.nodes[1].sendtoaddress(self.nodes[0].getnewaddress(), 1.4)
+        tx3 = self.nodes[1].getrawtransaction(txid3, True)
+        # tx3 should have 2 inputs and 2 outputs
+        assert_equal(2, len(tx3["vin"]))
+        assert_equal(2, len(tx3["vout"]))
+        # the accumulated value should be 1.5, so the outputs should be
+        # ~0.1 and 1.4 and should come from the same destination
+        values = [vout["value"] for vout in tx3["vout"]]
+        values.sort()
+        assert_approx(values[0], 0.1, 0.0001)
+        assert_approx(values[1], 1.4)
+
+        input_txids = [vin["txid"] for vin in tx3["vin"]]
+        input_addrs = [self.nodes[1].gettransaction(txid)['details'][0]['address'] for txid in input_txids]
+        assert_equal(input_addrs[0], input_addrs[1])
+        # Node 2 enforces avoidpartialspends so needs no checking here
+
+        # Test wallet option maxapsfee with Node 3
+        addr_aps = self.nodes[3].getnewaddress()
+        self.nodes[0].sendtoaddress(addr_aps, 1.0)
+        self.nodes[0].sendtoaddress(addr_aps, 1.0)
+        self.nodes[0].generate(1)
+        txid4 = self.nodes[3].sendtoaddress(self.nodes[0].getnewaddress(), 0.1)
+        tx4 = self.nodes[3].getrawtransaction(txid4, True)
+        # tx4 should have 2 inputs and 2 outputs although one output would
+        # have been enough and the transaction caused higher fees
+        assert_equal(2, len(tx4["vin"]))
+        assert_equal(2, len(tx4["vout"]))
+
         # Empty out node2's wallet
         self.nodes[2].sendtoaddress(address=self.nodes[0].getnewaddress(), amount=self.nodes[2].getbalance(), subtractfeefromamount=True)
         self.sync_all()