tests: Make sure that bumpfee feerate checks work when replacing outputs

When replacing the outputs of a transaction, we can end up with
fees that are drastically different from the original. This tests that
the feerate checks we perform will properly detect when the bumping tx
will have an insufficient feerate.

Author: achow101

test/functional/wallet_bumpfee.py

@@ -26,6 +26,7 @@
     assert_equal,
     assert_greater_than,
     assert_raises_rpc_error,
+    get_fee,
 )
 from test_framework.wallet import MiniWallet
 
@@ -100,6 +101,7 @@ def run_test(self):
         test_change_script_match(self, rbf_node, dest_address)
         test_settxfee(self, rbf_node, dest_address)
         test_maxtxfee_fails(self, rbf_node, dest_address)
+        test_feerate_checks_replaced_outputs(self, rbf_node)
         # These tests wipe out a number of utxos that are expected in other tests
         test_small_output_with_feerate_succeeds(self, rbf_node, dest_address)
         test_no_more_inputs_fails(self, rbf_node, dest_address)
@@ -668,5 +670,30 @@ def test_no_more_inputs_fails(self, rbf_node, dest_address):
     self.clear_mempool()
 
 
+def test_feerate_checks_replaced_outputs(self, rbf_node):
+    self.log.info("Test that feerate checks use replaced outputs")
+    outputs = []
+    for i in range(50):
+        outputs.append({rbf_node.getnewaddress(address_type="bech32"): 1})
+    tx_res = rbf_node.send(outputs=outputs, fee_rate=5)
+    tx_details = rbf_node.gettransaction(txid=tx_res["txid"], verbose=True)
+
+    # Calculate the minimum feerate required for the bump to work.
+    # Since the bumped tx will replace all of the outputs with a single output, we can estimate that its size will 31 * (len(outputs) - 1) bytes smaller
+    tx_size = tx_details["decoded"]["vsize"]
+    est_bumped_size = tx_size - (len(tx_details["decoded"]["vout"]) - 1) * 31
+    inc_fee_rate = max(rbf_node.getmempoolinfo()["incrementalrelayfee"], Decimal(0.00005000)) # Wallet has a fixed incremental relay fee of 5 sat/vb
+    # RPC gives us fee as negative
+    min_fee = (-tx_details["fee"] + get_fee(est_bumped_size, inc_fee_rate)) * Decimal(1e8)
+    min_fee_rate = (min_fee / est_bumped_size).quantize(Decimal("1.000"))
+
+    # Attempt to bumpfee and replace all outputs with a single one using a feerate slightly less than the minimum
+    new_outputs = [{rbf_node.getnewaddress(address_type="bech32"): 49}]
+    assert_raises_rpc_error(-8, "Insufficient total fee", rbf_node.bumpfee, tx_res["txid"], {"fee_rate": min_fee_rate - 1, "outputs": new_outputs})
+
+    # Bumpfee and replace all outputs with a single one using the minimum feerate
+    rbf_node.bumpfee(tx_res["txid"], {"fee_rate": min_fee_rate, "outputs": new_outputs})
+
+
 if __name__ == "__main__":
     BumpFeeTest().main()