Fixed mempool pruning

- Fixed undefined behavior after a call to `remove_tx_from_transient_lists` (it used an invalid iterator)
- Fixed `txCompare` (it wasn't strictly weak ordered)

src/cryptonote_core/tx_pool.cpp

@@ -435,8 +435,14 @@ namespace cryptonote
   void tx_memory_pool::prune(size_t bytes)
   {
     CRITICAL_REGION_LOCAL(m_transactions_lock);
+
+    // Nothing to do if already empty
+    if (m_txs_by_fee_and_receive_time.empty())
+      return;
+
     if (bytes == 0)
       bytes = m_txpool_max_weight;
+
     CRITICAL_REGION_LOCAL1(m_blockchain);
     LockedTXN lock(m_blockchain.get_db());
     bool changed = false;
@@ -481,8 +487,13 @@ namespace cryptonote
         reduce_txpool_weight(meta.weight);
         remove_transaction_keyimages(tx, txid);
         MINFO("Pruned tx " << txid << " from txpool: weight: " << meta.weight << ", fee/byte: " << it->first.first);
+
+        auto it_prev = it;
+        --it_prev;
+
         remove_tx_from_transient_lists(it, txid, !meta.matches(relay_category::broadcasted));
-        it--;
+        it = it_prev;
+
         changed = true;
       }
       catch (const std::exception &e)
@@ -1827,7 +1838,7 @@ namespace cryptonote
       auto sorted_it = find_tx_in_sorted_container(txid);
       if (sorted_it == m_txs_by_fee_and_receive_time.end())
       {
-        MERROR("Re-adding tx " << txid << " to tx pool, but it was not found in the sorted txs container");
+        MDEBUG("Re-adding tx " << txid << " to tx pool, but it was not found in the sorted txs container");
       }
       else
       {

src/cryptonote_core/tx_pool.h

@@ -69,11 +69,12 @@ namespace cryptonote
     {
       // sort by greatest first, not least
       if (a.first.first > b.first.first) return true;
-      else if (a.first.first < b.first.first) return false;
-      else if (a.first.second < b.first.second) return true;
-      else if (a.first.second > b.first.second) return false;
-      else if (a.second != b.second) return true;
-      else return false;
+      if (a.first.first < b.first.first) return false;
+
+      if (a.first.second < b.first.second) return true;
+      if (a.first.second > b.first.second) return false;
+
+      return memcmp(a.second.data, b.second.data, sizeof(crypto::hash)) < 0;
     }
   };