[mempool] sanity check that all unbroadcast txns are in mempool

- before reattempting broadcast for unbroadcast txns, check they are in mempool and remove if not
- this protects from memory leaks and network spam just in case unbroadcast set (incorrectly) has extra txns
- check that tx is in mempool before adding to unbroadcast set to try to prevent this from happening

src/net_processing.cpp

@@ -819,7 +819,12 @@ void PeerLogicValidation::ReattemptInitialBroadcast(CScheduler& scheduler) const
     std::set<uint256> unbroadcast_txids = m_mempool.GetUnbroadcastTxs();
 
     for (const uint256& txid : unbroadcast_txids) {
+        // Sanity check: all unbroadcast txns should exist in the mempool
+        if (m_mempool.exists(txid)) {
             RelayTransaction(txid, *connman);
+        } else {
+            m_mempool.RemoveUnbroadcastTx(txid, true);
+        }
     }
 
     // schedule next run for 10-15 minutes in the future

src/txmempool.h

@@ -704,8 +704,11 @@ public:
     /** Adds a transaction to the unbroadcast set */
     void AddUnbroadcastTx(const uint256& txid) {
         LOCK(cs);
+        /** Sanity Check: the transaction should also be in the mempool */
+        if (exists(txid)) {
             m_unbroadcast_txids.insert(txid);
         }
+    }
 
     /** Removes a transaction from the unbroadcast set */
     void RemoveUnbroadcastTx(const uint256& txid, const bool unchecked = false);