[mempool] speed up check() by using coins cache and iterating in topo order

No behavior changes.

Before, we're always adding transactions to the "check later" queue if
they have any parents in the mempool. But there's no reason to do this
if all of its inputs are already available from mempoolDuplicate.
Instead, check for inputs, and only mark fDependsWait=true if the
parents haven't been processed yet.

Reduce the amount of "check later" transactions by looking at
ancestors before descendants. Do this by iterating through them in
ascending order by ancestor count. This works because a child will
always have more in-mempool ancestors than its parent.

We should never have any entries in the "check later" queue
after this commit.

Author: glozow

src/txmempool.cpp

@@ -693,13 +693,14 @@ void CTxMemPool::check(CChainState& active_chainstate) const
     uint64_t checkTotal = 0;
     CAmount check_total_fee{0};
     uint64_t innerUsage = 0;
+    uint64_t prev_ancestor_count{0};
 
     CCoinsViewCache& active_coins_tip = active_chainstate.CoinsTip();
     CCoinsViewCache mempoolDuplicate(const_cast<CCoinsViewCache*>(&active_coins_tip));
     const int64_t spendheight = active_chainstate.m_chain.Height() + 1;
 
     std::list<const CTxMemPoolEntry*> waitingOnDependants;
-    for (indexed_transaction_set::const_iterator it = mapTx.begin(); it != mapTx.end(); it++) {
+    for (const auto& it : GetSortedDepthAndScore()) {
         unsigned int i = 0;
         checkTotal += it->GetTxSize();
         check_total_fee += it->GetFee();
@@ -714,7 +715,7 @@ void CTxMemPool::check(CChainState& active_chainstate) const
             if (it2 != mapTx.end()) {
                 const CTransaction& tx2 = it2->GetTx();
                 assert(tx2.vout.size() > txin.prevout.n && !tx2.vout[txin.prevout.n].IsNull());
-                fDependsWait = true;
+                if (!mempoolDuplicate.HaveCoin(txin.prevout)) fDependsWait = true;
                 setParentCheck.insert(*it2);
             } else {
                 assert(active_coins_tip.HaveCoin(txin.prevout));
@@ -751,6 +752,9 @@ void CTxMemPool::check(CChainState& active_chainstate) const
         assert(it->GetSizeWithAncestors() == nSizeCheck);
         assert(it->GetSigOpCostWithAncestors() == nSigOpCheck);
         assert(it->GetModFeesWithAncestors() == nFeesCheck);
+        // Sanity check: we are walking in ascending ancestor count order.
+        assert(prev_ancestor_count <= it->GetCountWithAncestors());
+        prev_ancestor_count = it->GetCountWithAncestors();
 
         // Check children against mapNextTx
         CTxMemPoolEntry::Children setChildrenCheck;