Further improve comments around recentRejects

Author: sdaftuar

src/net_processing.cpp

@@ -190,6 +190,15 @@ namespace {
      * million to make it highly unlikely for users to have issues with this
      * filter.
      *
+     * We only need to add wtxids to this filter. For non-segwit
+     * transactions, the txid == wtxid, so this only prevents us from
+     * re-downloading non-segwit transactions when communicating with
+     * non-wtxidrelay peers -- which is important for avoiding malleation
+     * attacks that could otherwise interfere with transaction relay from
+     * non-wtxidrelay peers. For communicating with wtxidrelay peers, having
+     * the reject filter store wtxids is exactly what we want to avoid
+     * redownload of a rejected transaction.
+     *
      * Memory used: 1.3 MB
      */
     std::unique_ptr<CRollingBloomFilter> recentRejects GUARDED_BY(cs_main);
@@ -2033,6 +2042,7 @@ void static ProcessOrphanTx(CConnman& connman, CTxMemPool& mempool, std::set<uin
             // Probably non-standard or insufficient fee
             LogPrint(BCLog::MEMPOOL, "   removed orphan tx %s\n", orphanHash.ToString());
             if (orphan_state.GetResult() != TxValidationResult::TX_WITNESS_STRIPPED) {
+                // We can add the wtxid of this transaction to our reject filter.
                 // Do not add txids of witness transactions or witness-stripped
                 // transactions to the filter, as they can have been malleated;
                 // adding such txids to the reject filter would potentially
@@ -3004,11 +3014,16 @@ void ProcessMessage(
                 LogPrint(BCLog::MEMPOOL, "not keeping orphan with rejected parents %s\n",tx.GetHash().ToString());
                 // We will continue to reject this tx since it has rejected
                 // parents so avoid re-requesting it from other peers.
+                // Here we add both the txid and the wtxid, as we know that
+                // regardless of what witness is provided, we will not accept
+                // this, so we don't need to allow for redownload of this txid
+                // from any of our non-wtxidrelay peers.
                 recentRejects->insert(tx.GetHash());
                 recentRejects->insert(tx.GetWitnessHash());
             }
         } else {
             if (state.GetResult() != TxValidationResult::TX_WITNESS_STRIPPED) {
+                // We can add the wtxid of this transaction to our reject filter.
                 // Do not add txids of witness transactions or witness-stripped
                 // transactions to the filter, as they can have been malleated;
                 // adding such txids to the reject filter would potentially