Bugfix: RPC/blockchain: Actually round feerates down for getmempoolinfo fee histograms

Author: luke-jr

src/rpc/mempool.cpp

@@ -697,12 +697,14 @@ UniValue MempoolInfoToJSON(const CTxMemPool& pool, const std::optional<MempoolHi
             const CAmount afees{e.GetModFeesWithAncestors()}, dfees{e.GetModFeesWithDescendants()};
             const uint32_t asize{uint32_t(e.GetSizeWithAncestors())}, dsize{uint32_t(e.GetSizeWithDescendants())};
 
-            const CAmount fpb{CFeeRate{fee, size}.GetFee(1)};     // Fee rate per byte
-            const CAmount afpb{CFeeRate{afees, asize}.GetFee(1)}; // Fee rate per byte including ancestors
-            const CAmount dfpb{CFeeRate{dfees, dsize}.GetFee(1)}; // Fee rate per byte including descendants
+            // Do not use CFeeRate here, since it rounds up, and this should be rounding down
+            const CAmount fpb{fee / size};     // Fee rate per byte
+            const CAmount afpb{afees / asize}; // Fee rate per byte including ancestors
+            const CAmount dfpb{dfees / dsize}; // Fee rate per byte including descendants
 
             // Fee rate per byte including ancestors & descendants
-            const CAmount tfpb{CFeeRate{afees + dfees - fee, asize + dsize - size}.GetFee(1)};
+            // (fee/size are included in both, so subtracted to avoid double-counting)
+            const CAmount tfpb{(afees + dfees - fee) / (asize + dsize - size)};
 
             const CAmount fee_rate{std::max(std::min(dfpb, tfpb), std::min(fpb, afpb))};
 

test/functional/mempool_fee_histogram.py

@@ -110,7 +110,7 @@ def run_test(self):
         self.log.info(f"Test fee rate histogram when mempool contains 2 transactions (tx1: {tx1_info['feerate']} sat/vB, tx2: {tx2_info['feerate']} sat/vB)")
         info = node.getmempoolinfo([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15])
 
-        # Verify that both tx1 and tx2 are reported in 9 sat/vB fee rate group
+        # Verify that both tx1 and tx2 are reported in 8 sat/vB fee rate group
         (non_empty_groups, empty_groups, total_fees) = self.histogram_stats(info['fee_histogram'])
         tx1p2_feerate = get_actual_fee_rate(tx1_info['fee'] + tx2_info['fee'], tx1_info['vsize'] + tx2_info['vsize'])
         assert_equal(1, non_empty_groups)