core: quantize per kB fee to 8 decimals

src/cryptonote_config.h

@@ -128,6 +128,7 @@
 #define THREAD_STACK_SIZE                       5 * 1024 * 1024
 
 #define HF_VERSION_DYNAMIC_FEE                  4
+#define PER_KB_FEE_QUANTIZATION_DECIMALS        8
 
 // New constants are intended to go here
 namespace config

src/cryptonote_core/blockchain.cpp

@@ -2795,6 +2795,19 @@ void Blockchain::check_ring_signature(const crypto::hash &tx_prefix_hash, const
   result = crypto::check_ring_signature(tx_prefix_hash, key_image, p_output_keys, sig.data()) ? 1 : 0;
 }
 
+//------------------------------------------------------------------
+static uint64_t get_fee_quantization_mask()
+{
+  static uint64_t mask = 0;
+  if (mask == 0)
+  {
+    mask = 1;
+    for (size_t n = PER_KB_FEE_QUANTIZATION_DECIMALS; n < CRYPTONOTE_DISPLAY_DECIMAL_POINT; ++n)
+      mask *= 10;
+  }
+  return mask;
+}
+
 //------------------------------------------------------------------
 uint64_t Blockchain::get_dynamic_per_kb_fee(uint64_t block_reward, size_t median_block_size)
 {
@@ -2810,7 +2823,12 @@ uint64_t Blockchain::get_dynamic_per_kb_fee(uint64_t block_reward, size_t median
   div128_32(hi, lo, 1000000, &hi, &lo);
   assert(hi == 0);
 
-  return lo;
+  // quantize fee up to 8 decimals
+  uint64_t mask = get_fee_quantization_mask();
+  uint64_t qlo = (lo + mask - 1) / mask * mask;
+  MDEBUG("lo " << print_money(lo) << ", qlo " << print_money(qlo) << ", mask " << mask);
+
+  return qlo;
 }
 
 //------------------------------------------------------------------