Make TX_WITNESS_STRIPPED its own rejection reason

Previously, TX_WITNESS_MUTATED could be returned during transaction validation
for either transactions that had a witness that was non-standard, or for
transactions that had no witness but were invalid due to segwit validation
rules.

However, for txid/wtxid-relay considerations, net_processing distinguishes the
witness stripped case separately, because it affects whether a wtxid should be
able to be added to the reject filter. It is safe to add the wtxid of a
witness-mutated transaction to the filter (as that wtxid shouldn't collide with
the txid, and hence it wouldn't interfere with transaction relay from
txid-relay peers), but it is not safe to add the wtxid (== txid) of a
witness-stripped transaction to the filter, because that would interfere with
relay of another transaction with the same txid (but different wtxid) when
relaying from txid-relay peers.

Also updates the comment explaining this logic, and explaining that we can get
rid of this complexity once there's a sufficient deployment of wtxid-relaying
peers on the network.

src/consensus/validation.h

@@ -30,11 +30,15 @@ enum class TxValidationResult {
     TX_MISSING_INPUTS,        //!< transaction was missing some of its inputs
     TX_PREMATURE_SPEND,       //!< transaction spends a coinbase too early, or violates locktime/sequence locks
     /**
-     * Transaction might be missing a witness, have a witness prior to SegWit
+     * Transaction might have a witness prior to SegWit
      * activation, or witness may have been malleated (which includes
      * non-standard witnesses).
      */
     TX_WITNESS_MUTATED,
+    /**
+     * Transaction is missing a witness.
+     */
+    TX_WITNESS_STRIPPED,
     /**
      * Tx already in mempool or conflicts with a tx in the chain
      * (if it conflicts with another tx in mempool, we use MEMPOOL_POLICY as it failed to reach the RBF threshold)

src/net_processing.cpp

@@ -1160,6 +1160,7 @@ static bool MaybePunishNodeForTx(NodeId nodeid, const TxValidationState& state,
     case TxValidationResult::TX_MISSING_INPUTS:
     case TxValidationResult::TX_PREMATURE_SPEND:
     case TxValidationResult::TX_WITNESS_MUTATED:
+    case TxValidationResult::TX_WITNESS_STRIPPED:
     case TxValidationResult::TX_CONFLICT:
     case TxValidationResult::TX_MEMPOOL_POLICY:
         break;
@@ -2031,10 +2032,19 @@ void static ProcessOrphanTx(CConnman& connman, CTxMemPool& mempool, std::set<uin
             // Has inputs but not accepted to mempool
             // Probably non-standard or insufficient fee
             LogPrint(BCLog::MEMPOOL, "   removed orphan tx %s\n", orphanHash.ToString());
-            if (orphanTx.HasWitness() || orphan_state.GetResult() != TxValidationResult::TX_WITNESS_MUTATED) {
+            if (orphan_state.GetResult() != TxValidationResult::TX_WITNESS_STRIPPED) {
-                // Do not use rejection cache for witness transactions or
+                // Do not add txids of witness transactions or witness-stripped
-                // witness-stripped transactions, as they can have been malleated.
+                // transactions to the filter, as they can have been malleated;
-                // See https://github.com/bitcoin/bitcoin/issues/8279 for details.
+                // adding such txids to the reject filter would potentially
+                // interfere with relay of valid transactions from peers that
+                // do not support wtxid-based relay. See
+                // https://github.com/bitcoin/bitcoin/issues/8279 for details.
+                // We can remove this restriction (and always add wtxids to
+                // the filter even for witness stripped transactions) once
+                // wtxid-based relay is broadly deployed.
+                // See also comments in https://github.com/bitcoin/bitcoin/pull/18044#discussion_r443419034
+                // for concerns around weakening security of unupgraded nodes
+                // if we start doing this too early.
                 assert(recentRejects);
                 recentRejects->insert(orphanTx.GetWitnessHash());
             }
@@ -2992,10 +3002,19 @@ void ProcessMessage(
                 recentRejects->insert(tx.GetWitnessHash());
             }
         } else {
-            if (tx.HasWitness() || state.GetResult() != TxValidationResult::TX_WITNESS_MUTATED) {
+            if (state.GetResult() != TxValidationResult::TX_WITNESS_STRIPPED) {
-                // Do not use rejection cache for witness transactions or
+                // Do not add txids of witness transactions or witness-stripped
-                // witness-stripped transactions, as they can have been malleated.
+                // transactions to the filter, as they can have been malleated;
-                // See https://github.com/bitcoin/bitcoin/issues/8279 for details.
+                // adding such txids to the reject filter would potentially
+                // interfere with relay of valid transactions from peers that
+                // do not support wtxid-based relay. See
+                // https://github.com/bitcoin/bitcoin/issues/8279 for details.
+                // We can remove this restriction (and always add wtxids to
+                // the filter even for witness stripped transactions) once
+                // wtxid-based relay is broadly deployed.
+                // See also comments in https://github.com/bitcoin/bitcoin/pull/18044#discussion_r443419034
+                // for concerns around weakening security of unupgraded nodes
+                // if we start doing this too early.
                 assert(recentRejects);
                 recentRejects->insert(tx.GetWitnessHash());
                 if (RecursiveDynamicUsage(*ptx) < 100000) {

src/validation.cpp

@@ -939,7 +939,7 @@ bool MemPoolAccept::PolicyScriptChecks(ATMPArgs& args, Workspace& ws, Precompute
         if (!tx.HasWitness() && CheckInputScripts(tx, state_dummy, m_view, scriptVerifyFlags & ~(SCRIPT_VERIFY_WITNESS | SCRIPT_VERIFY_CLEANSTACK), true, false, txdata) &&
                 !CheckInputScripts(tx, state_dummy, m_view, scriptVerifyFlags & ~SCRIPT_VERIFY_CLEANSTACK, true, false, txdata)) {
             // Only the witness is missing, so the transaction itself may be fine.
-            state.Invalid(TxValidationResult::TX_WITNESS_MUTATED,
+            state.Invalid(TxValidationResult::TX_WITNESS_STRIPPED,
                     state.GetRejectReason(), state.GetDebugMessage());
         }
         return false; // state filled in by CheckInputScripts