Merge #18017: txmempool: split epoch logic into class

fd6580e405699ccb051fd2a34525e48d3253673d [refactor] txmempool: split epoch logic into class (Anthony Towns)

Pull request description:

  Splits the epoch logic introduced in #17925 into a separate class.

  Uses clang's thread safety annotations and encapsulates the data more strongly to reduce chances of bugs from API misuse.

ACKs for top commit:
  jonatack:
    ACK fd6580e405699ccb051fd2a34525e48d3253673d using clang thread safety annotations looks like a very good idea, and the encapsulation this change adds should improve robustness (and possible unit test-ability) of the code. Verified that changing some of the locking duly provoked build-time warnings with Clang 9 on Debian and that small changes in the new `Epoch` class were covered by failing functional test assertions in `mempool_updatefromblock.py`, `mempool_resurrect.py`, and `mempool_reorg.py`
  hebasto:
    re-ACK fd6580e405699ccb051fd2a34525e48d3253673d, since my [previous](https://github.com/bitcoin/bitcoin/pull/18017#pullrequestreview-569619362) review:

Tree-SHA512: 7004623faa02b56639aa05ab7a078320a6d8d54ec62d8022876221e33f350f47df51ddff056c0de5be798f8eb39b5c03c2d3f035698555d70abc218e950f2f8c

1 files changed, 2 insertions, 21 deletions

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 899835019a..48424a75d0 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -23,7 +23,7 @@ CTxMemPoolEntry::CTxMemPoolEntry(const CTransactionRef& _tx, const CAmount& _nFe
                                  int64_t _nTime, unsigned int _entryHeight,
                                  bool _spendsCoinbase, int64_t _sigOpsCost, LockPoints lp)
     : tx(_tx), nFee(_nFee), nTxWeight(GetTransactionWeight(*tx)), nUsageSize(RecursiveDynamicUsage(tx)), nTime(_nTime), entryHeight(_entryHeight),
-    spendsCoinbase(_spendsCoinbase), sigOpCost(_sigOpsCost), lockPoints(lp), m_epoch(0)
+    spendsCoinbase(_spendsCoinbase), sigOpCost(_sigOpsCost), lockPoints(lp)
 {
     nCountWithDescendants = 1;
     nSizeWithDescendants = GetTxSize();
@@ -132,7 +132,7 @@ void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashes
         // include them, and update their CTxMemPoolEntry::m_parents to include this tx.
         // we cache the in-mempool children to avoid duplicate updates
         {
-            const auto epoch = GetFreshEpoch();
+            WITH_FRESH_EPOCH(m_epoch);
             for (; iter != mapNextTx.end() && iter->first->hash == hash; ++iter) {
                 const uint256 &childHash = iter->second->GetHash();
                 txiter childIter = mapTx.find(childHash);
@@ -1119,22 +1119,3 @@ void CTxMemPool::SetIsLoaded(bool loaded)
     LOCK(cs);
     m_is_loaded = loaded;
 }
-
-
-CTxMemPool::EpochGuard CTxMemPool::GetFreshEpoch() const
-{
-    return EpochGuard(*this);
-}
-CTxMemPool::EpochGuard::EpochGuard(const CTxMemPool& in) : pool(in)
-{
-    assert(!pool.m_has_epoch_guard);
-    ++pool.m_epoch;
-    pool.m_has_epoch_guard = true;
-}
-
-CTxMemPool::EpochGuard::~EpochGuard()
-{
-    // prevents stale results being used
-    ++pool.m_epoch;
-    pool.m_has_epoch_guard = false;
-}