Merge bitcoin/bitcoin#28903: refactor: Make CTxMemPoolEntry only explicitly copyable

705e3f1de0 refactor: Make CTxMemPoolEntry only explicitly copyable (TheCharlatan)

Pull request description:

  This has the goal of prohibiting users from accidentally creating runtime failures, e.g. by interacting with iterator_to with a copied entry. This was brought up here:  https://github.com/bitcoin/bitcoin/pull/28886#issuecomment-1814794954.

  CTxMemPoolEntry is already implicitly not move-constructable. So be explicit about this and use a std::list to collect the values in the policy_estimator fuzz test instead of a std::vector.

ACKs for top commit:
  maflcko:
    ACK 705e3f1de0 🌯
  achow101:
    ACK 705e3f1de0
  ajtowns:
    ACK 705e3f1de0
  ismaelsadeeq:
    ACK 705e3f1de0

Tree-SHA512: 62056905c679c919d00f9ae065ed66ac986e7e7062015aea542843d8deecda57104d7a68d002f7b20afa3164f8e9215d2d2d002c167224129540e3b1bd0712cc

src/kernel/mempool_entry.h

@@ -71,6 +71,11 @@ public:
     typedef std::set<CTxMemPoolEntryRef, CompareIteratorByHash> Children;
 
 private:
+    CTxMemPoolEntry(const CTxMemPoolEntry&) = default;
+    struct ExplicitCopyTag {
+        explicit ExplicitCopyTag() = default;
+    };
+
     const CTransactionRef tx;
     mutable Parents m_parents;
     mutable Children m_children;
@@ -122,6 +127,13 @@ public:
           nModFeesWithAncestors{nFee},
           nSigOpCostWithAncestors{sigOpCost} {}
 
+    CTxMemPoolEntry(ExplicitCopyTag, const CTxMemPoolEntry& entry) : CTxMemPoolEntry(entry) {}
+    CTxMemPoolEntry& operator=(const CTxMemPoolEntry&) = delete;
+    CTxMemPoolEntry(CTxMemPoolEntry&&) = delete;
+    CTxMemPoolEntry& operator=(CTxMemPoolEntry&&) = delete;
+
+    static constexpr ExplicitCopyTag ExplicitCopy{};
+
     const CTransaction& GetTx() const { return *this->tx; }
     CTransactionRef GetSharedTx() const { return this->tx; }
     const CAmount& GetFee() const { return nFee; }