mempool: Fix max descendants check

The chain limits check for max descendants would check the descendants of the transaction itself even though the description for -limitdescendantcount says 'any ancestor'. This commit corrects the descendant count check by finding the top parent transaction in the mempool and comparing against that.

src/txmempool.cpp

@@ -1055,11 +1055,22 @@ void CTxMemPool::TrimToSize(size_t sizelimit, std::vector<COutPoint>* pvNoSpends
     }
 }
 
+uint64_t CTxMemPool::CalculateDescendantMaximum(txiter entry) const {
+    // find top parent
+    txiter top = entry;
+    for (;;) {
+        const setEntries& parents = GetMemPoolParents(top);
+        if (parents.size() == 0) break;
+        top = *parents.begin();
+    }
+    return top->GetCountWithDescendants();
+}
+
 bool CTxMemPool::TransactionWithinChainLimit(const uint256& txid, size_t ancestor_limit, size_t descendant_limit) const {
     LOCK(cs);
     auto it = mapTx.find(txid);
     return it == mapTx.end() || (it->GetCountWithAncestors() < ancestor_limit &&
-       it->GetCountWithDescendants() < descendant_limit);
+                                 CalculateDescendantMaximum(it) < descendant_limit);
 }
 
 SaltedTxidHasher::SaltedTxidHasher() : k0(GetRand(std::numeric_limits<uint64_t>::max())), k1(GetRand(std::numeric_limits<uint64_t>::max())) {}

src/txmempool.h

@@ -498,6 +498,7 @@ public:
 
     const setEntries & GetMemPoolParents(txiter entry) const EXCLUSIVE_LOCKS_REQUIRED(cs);
     const setEntries & GetMemPoolChildren(txiter entry) const EXCLUSIVE_LOCKS_REQUIRED(cs);
+    uint64_t CalculateDescendantMaximum(txiter entry) const EXCLUSIVE_LOCKS_REQUIRED(cs);
 private:
     typedef std::map<txiter, setEntries, CompareIteratorByHash> cacheMap;