allow fractional feerates (#4324)

lib/simple_config.py

@@ -3,6 +3,7 @@ import threading
 import time
 import os
 import stat
+from decimal import Decimal
 
 from copy import deepcopy
 
@@ -473,12 +474,9 @@ class SimpleConfig(PrintError):
 
     @classmethod
     def estimate_fee_for_feerate(cls, fee_per_kb, size):
-        # note: We only allow integer sat/byte values atm.
-        # The GUI for simplicity reasons only displays integer sat/byte,
-        # and for the sake of consistency, we thus only use integer sat/byte in
-        # the backend too.
-        fee_per_byte = int(fee_per_kb / 1000)
-        return int(fee_per_byte * size)
+        fee_per_kb = Decimal(fee_per_kb)
+        fee_per_byte = fee_per_kb / 1000
+        return round(fee_per_byte * size)
 
     def update_fee_estimates(self, key, value):
         self.fee_estimates[key] = value

lib/util.py

@@ -24,6 +24,7 @@ import binascii
 import os, sys, re, json
 from collections import defaultdict
 from datetime import datetime
+import decimal
 from decimal import Decimal
 import traceback
 import urllib
@@ -472,8 +473,21 @@ def format_satoshis(x, num_zeros=0, decimal_point=8, precision=None, is_diff=Fal
         result = " " * (15 - len(result)) + result
     return result
 
+
+FEERATE_PRECISION = 1  # num fractional decimal places for sat/byte fee rates
+_feerate_quanta = Decimal(10) ** (-FEERATE_PRECISION)
+
+
 def format_fee_satoshis(fee, num_zeros=0):
-    return format_satoshis(fee, num_zeros, 0, precision=1)
+    return format_satoshis(fee, num_zeros, 0, precision=FEERATE_PRECISION)
+
+
+def quantize_feerate(fee):
+    """Strip sat/byte fee rate of excess precision."""
+    if fee is None:
+        return None
+    return Decimal(fee).quantize(_feerate_quanta, rounding=decimal.ROUND_HALF_DOWN)
+
 
 def timestamp_to_datetime(timestamp):
     if timestamp is None: