Merge bitcoin/bitcoin#30412: MiniMiner: use FeeFrac in AncestorFeerateComparator

09370529fb9f6d06f6d16bdb1fb336f7a265d8ba fuzz: mini_miner_selection fixups. (glozow)
de273d53004f48e4c8c965f7ce0bd375fd8d0d69 MiniMiner: use FeeFrac in AncestorFeerateComparator (glozow)

Pull request description:

  Closes #30284. Closes #30367, see https://github.com/bitcoin/bitcoin/issues/30367#issuecomment-2217459257

  Previously, we were only comparing feerates up to 1/1000 precision, since CFeeRate comparison just looks at their respective nSatoshisPerK. This could lead to MiniMiner selecting packages in the wrong order (i.e. by txid) if their feerates were less than 0.001sat/vB different. Fix this by creating + comparing `FeeFrac`s instead.

  Also, `FeeFrac::Mul` doesn't have the overflow problem.

  Also added a few minor fuzzer fixups that caught my eye while I was debugging this.

ACKs for top commit:
  ismaelsadeeq:
    Tested ACK 09370529fb9f6d06f6d16bdb1fb336f7a265d8ba
  murchandamus:
    ACK 09370529fb9f6d06f6d16bdb1fb336f7a265d8ba with nits
  dergoegge:
    tACK 09370529fb9f6d06f6d16bdb1fb336f7a265d8ba

Tree-SHA512: e5b6d6c3f7289f30cd8280d0a47cd852d0180b83d1b27ff9514f50c97103b0f069484e48cba2ca3a57419beadc1996c1b9dd8d0a0f34bc4f4223d8adaf414ce5

2 files changed, 10 insertions, 18 deletions

diff --git a/src/node/mini_miner.cpp b/src/node/mini_miner.cpp
index 58422c4439..d7d15554b3 100644
--- a/src/node/mini_miner.cpp
+++ b/src/node/mini_miner.cpp
@@ -174,7 +174,7 @@ MiniMiner::MiniMiner(const std::vector<MiniMinerMempoolEntry>& manual_entries,
     SanityCheck();
 }
 
-// Compare by min(ancestor feerate, individual feerate), then iterator
+// Compare by min(ancestor feerate, individual feerate), then txid
 //
 // Under the ancestor-based mining approach, high-feerate children can pay for parents, but high-feerate
 // parents do not incentive inclusion of their children. Therefore the mining algorithm only considers
@@ -183,21 +183,13 @@ struct AncestorFeerateComparator
 {
     template<typename I>
     bool operator()(const I& a, const I& b) const {
-        auto min_feerate = [](const MiniMinerMempoolEntry& e) -> CFeeRate {
-            const CAmount ancestor_fee{e.GetModFeesWithAncestors()};
-            const int64_t ancestor_size{e.GetSizeWithAncestors()};
-            const CAmount tx_fee{e.GetModifiedFee()};
-            const int64_t tx_size{e.GetTxSize()};
-            // Comparing ancestor feerate with individual feerate:
-            //     ancestor_fee / ancestor_size <= tx_fee / tx_size
-            // Avoid division and possible loss of precision by
-            // multiplying both sides by the sizes:
-            return ancestor_fee * tx_size < tx_fee * ancestor_size ?
-                       CFeeRate(ancestor_fee, ancestor_size) :
-                       CFeeRate(tx_fee, tx_size);
+        auto min_feerate = [](const MiniMinerMempoolEntry& e) -> FeeFrac {
+            FeeFrac self_feerate(e.GetModifiedFee(), e.GetTxSize());
+            FeeFrac ancestor_feerate(e.GetModFeesWithAncestors(), e.GetSizeWithAncestors());
+            return std::min(ancestor_feerate, self_feerate);
         };
-        CFeeRate a_feerate{min_feerate(a->second)};
-        CFeeRate b_feerate{min_feerate(b->second)};
+        FeeFrac a_feerate{min_feerate(a->second)};
+        FeeFrac b_feerate{min_feerate(b->second)};
         if (a_feerate != b_feerate) {
             return a_feerate > b_feerate;
         }
diff --git a/src/test/fuzz/mini_miner.cpp b/src/test/fuzz/mini_miner.cpp
index 3a1663364f..51de4d0166 100644
--- a/src/test/fuzz/mini_miner.cpp
+++ b/src/test/fuzz/mini_miner.cpp
@@ -188,9 +188,9 @@ FUZZ_TARGET(mini_miner_selection, .init = initialize_miner)
     auto mock_template_txids = mini_miner.GetMockTemplateTxids();
     // MiniMiner doesn't add a coinbase tx.
     assert(mock_template_txids.count(blocktemplate->block.vtx[0]->GetHash()) == 0);
-    mock_template_txids.emplace(blocktemplate->block.vtx[0]->GetHash());
-    assert(mock_template_txids.size() <= blocktemplate->block.vtx.size());
-    assert(mock_template_txids.size() >= blocktemplate->block.vtx.size());
+    auto [iter, new_entry] = mock_template_txids.emplace(blocktemplate->block.vtx[0]->GetHash());
+    assert(new_entry);
+
     assert(mock_template_txids.size() == blocktemplate->block.vtx.size());
     for (const auto& tx : blocktemplate->block.vtx) {
         assert(mock_template_txids.count(tx->GetHash()));