Merge bitcoin/bitcoin#27021: Implement Mini version of BlockAssembler to calculate mining scores

6b605b91c1 [fuzz] Add MiniMiner target + diff fuzz against BlockAssembler (glozow)
3f3f2d59ea [unit test] GatherClusters and MiniMiner unit tests (glozow)
59afcc8354 Implement Mini version of BlockAssembler to calculate mining scores (glozow)
56484f0fdc [mempool] find connected mempool entries with GatherClusters(…) (glozow)

Pull request description:

  Implement Mini version of BlockAssembler to calculate mining scores

  Run the mining algorithm on a subset of the mempool, only disturbing the
  mempool to copy out fee information for relevant entries. Intended to be
  used by wallet to calculate amounts needed for fee-bumping unconfirmed
  transactions.

  From comments of sipa and glozow below:

  > > In what way does the code added here differ from the real block assembly code?
  >
  >    * Only operates on the relevant transactions rather than full mempool
  >    * Has the ability to remove transactions that will be replaced so they don't impact their ancestors
  >    * Does not hold mempool lock outside of the constructor, makes copies of the entries it needs instead (though I'm not sure if this has an effect in practice)
  >    * Doesn't do the sanity checks like keeping weight within max block weight and `IsFinalTx()`
  >    * After the block template is built, additionally calculates fees to bump remaining ancestor packages to target feerate

ACKs for top commit:
  achow101:
    ACK 6b605b91c1
  Xekyo:
    > ACK [6b605b9](6b605b91c1) modulo `miniminer_overlap` test.
  furszy:
    ACK 6b605b91 modulo `miniminer_overlap` test.
  theStack:
    Code-review ACK 6b605b91c1

Tree-SHA512: f86a8b4ae0506858a7b15d90f417ebceea5038b395c05c825e3796123ad3b6cb8a98ebb948521316802a4c6d60ebd7041093356b1e2c2922a06b3b96b3b8acb6

src/txmempool.cpp

@@ -26,6 +26,7 @@
 #include <validationinterface.h>
 
 #include <cmath>
+#include <numeric>
 #include <optional>
 #include <string_view>
 #include <utility>
@@ -918,6 +919,19 @@ CTxMemPool::setEntries CTxMemPool::GetIterSet(const std::set<uint256>& hashes) c
     return ret;
 }
 
+std::vector<CTxMemPool::txiter> CTxMemPool::GetIterVec(const std::vector<uint256>& txids) const
+{
+    AssertLockHeld(cs);
+    std::vector<txiter> ret;
+    ret.reserve(txids.size());
+    for (const auto& txid : txids) {
+        const auto it{GetIter(txid)};
+        if (!it) return {};
+        ret.push_back(*it);
+    }
+    return ret;
+}
+
 bool CTxMemPool::HasNoInputsOf(const CTransaction &tx) const
 {
     for (unsigned int i = 0; i < tx.vin.size(); i++)
@@ -1147,7 +1161,6 @@ void CTxMemPool::SetLoadTried(bool load_tried)
     m_load_tried = load_tried;
 }
 
-
 std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept
 {
     switch (r) {
@@ -1160,3 +1173,30 @@ std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept
     }
     assert(false);
 }
+
+std::vector<CTxMemPool::txiter> CTxMemPool::GatherClusters(const std::vector<uint256>& txids) const
+{
+    AssertLockHeld(cs);
+    std::vector<txiter> clustered_txs{GetIterVec(txids)};
+    // Use epoch: visiting an entry means we have added it to the clustered_txs vector. It does not
+    // necessarily mean the entry has been processed.
+    WITH_FRESH_EPOCH(m_epoch);
+    for (const auto& it : clustered_txs) {
+        visited(it);
+    }
+    // i = index of where the list of entries to process starts
+    for (size_t i{0}; i < clustered_txs.size(); ++i) {
+        // DoS protection: if there are 500 or more entries to process, just quit.
+        if (clustered_txs.size() > 500) return {};
+        const txiter& tx_iter = clustered_txs.at(i);
+        for (const auto& entries : {tx_iter->GetMemPoolParentsConst(), tx_iter->GetMemPoolChildrenConst()}) {
+            for (const CTxMemPoolEntry& entry : entries) {
+                const auto entry_it = mapTx.iterator_to(entry);
+                if (!visited(entry_it)) {
+                    clustered_txs.push_back(entry_it);
+                }
+            }
+        }
+    }
+    return clustered_txs;
+}