Deduplicate missing parents of orphan transactions

In the logic for requesting missing parents of orphan transactions, parent
transactions with multiple outputs being spent by the given orphan were being
processed multiple times. Fix this by deduplicating the set of missing parent
txids first.

Co-authored-by: Anthony Towns <[email protected]>

Author: sdaftuar

src/net_processing.cpp

@@ -2990,8 +2990,19 @@ void ProcessMessage(
         else if (state.GetResult() == TxValidationResult::TX_MISSING_INPUTS)
         {
             bool fRejectedParents = false; // It may be the case that the orphans parents have all been rejected
+
+            // Deduplicate parent txids, so that we don't have to loop over
+            // the same parent txid more than once down below.
+            std::vector<uint256> unique_parents;
+            unique_parents.reserve(tx.vin.size());
             for (const CTxIn& txin : tx.vin) {
-                if (recentRejects->contains(txin.prevout.hash)) {
+                // We start with all parents, and then remove duplicates below.
+                unique_parents.push_back(txin.prevout.hash);
+            }
+            std::sort(unique_parents.begin(), unique_parents.end());
+            unique_parents.erase(std::unique(unique_parents.begin(), unique_parents.end()), unique_parents.end());
+            for (const uint256& parent_txid : unique_parents) {
+                if (recentRejects->contains(parent_txid)) {
                     fRejectedParents = true;
                     break;
                 }
@@ -3000,14 +3011,14 @@ void ProcessMessage(
                 uint32_t nFetchFlags = GetFetchFlags(pfrom);
                 const auto current_time = GetTime<std::chrono::microseconds>();
 
-                for (const CTxIn& txin : tx.vin) {
+                for (const uint256& parent_txid : unique_parents) {
                     // Here, we only have the txid (and not wtxid) of the
                     // inputs, so we only request in txid mode, even for
                     // wtxidrelay peers.
                     // Eventually we should replace this with an improved
                     // protocol for getting all unconfirmed parents.
-                    CInv _inv(MSG_TX | nFetchFlags, txin.prevout.hash);
-                    pfrom.AddKnownTx(txin.prevout.hash);
+                    CInv _inv(MSG_TX | nFetchFlags, parent_txid);
+                    pfrom.AddKnownTx(parent_txid);
                     if (!AlreadyHave(_inv, mempool)) RequestTx(State(pfrom.GetId()), ToGenTxid(_inv), current_time);
                 }
                 AddOrphanTx(ptx, pfrom.GetId());