[mempool] find connected mempool entries with GatherClusters(…)

We limit GatherClusters’s result to a maximum of 500 transactions as
clusters can be made arbitrarily large by third parties.

Co-authored-by: Murch <murch@murch.one>

src/txmempool.cpp

@@ -24,6 +24,7 @@
 #include <validationinterface.h>
 
 #include <cmath>
+#include <numeric>
 #include <optional>
 #include <string_view>
 #include <utility>
@@ -898,6 +899,19 @@ 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++)
@@ -1127,7 +1141,6 @@ void CTxMemPool::SetLoadTried(bool load_tried)
     m_load_tried = load_tried;
 }
 
-
 std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept
 {
     switch (r) {
@@ -1140,3 +1153,30 @@ 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;
+}

src/txmempool.h

@@ -522,9 +522,16 @@ public:
     /** Returns an iterator to the given hash, if found */
     std::optional<txiter> GetIter(const uint256& txid) const EXCLUSIVE_LOCKS_REQUIRED(cs);
 
-    /** Translate a set of hashes into a set of pool iterators to avoid repeated lookups */
+    /** Translate a set of hashes into a set of pool iterators to avoid repeated lookups.
+     * Does not require that all of the hashes correspond to actual transactions in the mempool,
+     * only returns the ones that exist. */
     setEntries GetIterSet(const std::set<uint256>& hashes) const EXCLUSIVE_LOCKS_REQUIRED(cs);
 
+    /** Translate a list of hashes into a list of mempool iterators to avoid repeated lookups.
+     * The nth element in txids becomes the nth element in the returned vector. If any of the txids
+     * don't actually exist in the mempool, returns an empty vector. */
+    std::vector<txiter> GetIterVec(const std::vector<uint256>& txids) const EXCLUSIVE_LOCKS_REQUIRED(cs);
+
     /** Remove a set of transactions from the mempool.
      *  If a transaction is in this set, then all in-mempool descendants must
      *  also be in the set, unless this transaction is being removed for being
@@ -585,6 +592,12 @@ public:
         const Limits& limits,
         bool fSearchForParents = true) const EXCLUSIVE_LOCKS_REQUIRED(cs);
 
+    /** Collect the entire cluster of connected transactions for each transaction in txids.
+     * All txids must correspond to transaction entries in the mempool, otherwise this returns an
+     * empty vector. This call will also exit early and return an empty vector if it collects 500 or
+     * more transactions as a DoS protection. */
+    std::vector<txiter> GatherClusters(const std::vector<uint256>& txids) const EXCLUSIVE_LOCKS_REQUIRED(cs);
+
     /** Calculate all in-mempool ancestors of a set of transactions not already in the mempool and
      * check ancestor and descendant limits. Heuristics are used to estimate the ancestor and
      * descendant count of all entries if the package were to be added to the mempool.  The limits