Merge bitcoin#24239: test: fix ceildiv division by using integers

d1fab9d test: Call ceildiv helper with integer (Martin Zumsande)

Pull request description:

  On master,

  `assert_fee_amount(Decimal("0.00000993"), 217, Decimal("0.00004531"))` passes
  `assert_fee_amount(Decimal("0.00000993"), Decimal("217"), Decimal("0.00004531"))` fails.

  the reason is that the // operator in  `ceildiv(a,b) = -(-a//b)`  has a different behavior for Decimals, see [doc](https://docs.python.org/3/library/decimal.html#decimal-objects).

  `wallet_send.py` calls this function with Decimals, and I think this is the reason for the failure reported in the OP of bitcoin#24151 (`wallet_send.py --legacy-wallet` line 332, the numbers used in the example above are from there). However, the other failures reported there cannot be explained by this, so this is just a partial fix.

ACKs for top commit:
  ryanofsky:
    Code review ACK d1fab9d. Tracking down this problem was a good find, and code seems safer and easier to understand now

Tree-SHA512: 5bf0568cd1a0824f6b1a15a03580b6e9391b4f51112a97c1d00469d255bf6dda45c49a36fa567a5ba9b9973efe1d9cdd480db91965c9f4c2aa963629a8a32cba

test/functional/test_framework/util.py

@@ -36,6 +36,7 @@ def assert_approx(v, vexp, vspan=0.00001):
 
 def assert_fee_amount(fee, tx_size, feerate_BTC_kvB):
     """Assert the fee is in range."""
+    assert isinstance(tx_size, int)
     target_fee = get_fee(tx_size, feerate_BTC_kvB)
     if fee < target_fee:
         raise AssertionError("Fee of %s BTC too low! (Should be %s BTC)" % (str(fee), str(target_fee)))
@@ -219,15 +220,21 @@ def str_to_b64str(string):
 
 
 def ceildiv(a, b):
-    """Divide 2 ints and round up to next int rather than round down"""
+    """
+    Divide 2 ints and round up to next int rather than round down
+    Implementation requires python integers, which have a // operator that does floor division.
+    Other types like decimal.Decimal whose // operator truncates towards 0 will not work.
+    """
+    assert isinstance(a, int)
+    assert isinstance(b, int)
     return -(-a // b)
 
 
 def get_fee(tx_size, feerate_btc_kvb):
     """Calculate the fee in BTC given a feerate is BTC/kvB. Reflects CFeeRate::GetFee"""
     feerate_sat_kvb = int(feerate_btc_kvb * Decimal(1e8)) # Fee in sat/kvb as an int to avoid float precision errors
     target_fee_sat = ceildiv(feerate_sat_kvb * tx_size, 1000) # Round calculated fee up to nearest sat
-    return satoshi_round(target_fee_sat / Decimal(1e8)) # Truncate BTC result to nearest sat
+    return target_fee_sat / Decimal(1e8) # Return result in  BTC
 
 
 def satoshi_round(amount):

test/functional/wallet_send.py

@@ -16,6 +16,7 @@
     assert_fee_amount,
     assert_greater_than,
     assert_raises_rpc_error,
+    count_bytes,
 )
 from test_framework.wallet_util import bytes_to_wif
 
@@ -320,20 +321,20 @@ def run_test(self):
 
         res = self.test_send(from_wallet=w0, to_wallet=w1, amount=1, fee_rate=7, add_to_wallet=False)
         fee = self.nodes[1].decodepsbt(res["psbt"])["fee"]
-        assert_fee_amount(fee, Decimal(len(res["hex"]) / 2), Decimal("0.00007"))
+        assert_fee_amount(fee, count_bytes(res["hex"]), Decimal("0.00007"))
 
         # "unset" and None are treated the same for estimate_mode
         res = self.test_send(from_wallet=w0, to_wallet=w1, amount=1, fee_rate=2, estimate_mode="unset", add_to_wallet=False)
         fee = self.nodes[1].decodepsbt(res["psbt"])["fee"]
-        assert_fee_amount(fee, Decimal(len(res["hex"]) / 2), Decimal("0.00002"))
+        assert_fee_amount(fee, count_bytes(res["hex"]), Decimal("0.00002"))
 
         res = self.test_send(from_wallet=w0, to_wallet=w1, amount=1, arg_fee_rate=4.531, add_to_wallet=False)
         fee = self.nodes[1].decodepsbt(res["psbt"])["fee"]
-        assert_fee_amount(fee, Decimal(len(res["hex"]) / 2), Decimal("0.00004531"))
+        assert_fee_amount(fee, count_bytes(res["hex"]), Decimal("0.00004531"))
 
         res = self.test_send(from_wallet=w0, to_wallet=w1, amount=1, arg_fee_rate=3, add_to_wallet=False)
         fee = self.nodes[1].decodepsbt(res["psbt"])["fee"]
-        assert_fee_amount(fee, Decimal(len(res["hex"]) / 2), Decimal("0.00003"))
+        assert_fee_amount(fee, count_bytes(res["hex"]), Decimal("0.00003"))
 
         # Test that passing fee_rate as both an argument and an option raises.
         self.test_send(from_wallet=w0, to_wallet=w1, amount=1, arg_fee_rate=1, fee_rate=1, add_to_wallet=False,