Fix and improve relay from whitelisted peers

This makes sure that retransmits by a whitelisted peer also actually
result in a retransmit.

Further, this changes the logic to never relay in case we would assign
a DoS score, as we expect to get DoS banned ourselves as a result.

src/main.cpp

@@ -4421,11 +4421,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
         mapAlreadyAskedFor.erase(inv);
 
-        // Check for recently rejected (and do other quick existence checks)
-        if (AlreadyHave(inv))
-            return true;
-
-        if (AcceptToMemoryPool(mempool, state, tx, true, &fMissingInputs))
+        if (!AlreadyHave(inv) && AcceptToMemoryPool(mempool, state, tx, true, &fMissingInputs))
         {
             mempool.check(pcoinsTip);
             RelayTransaction(tx);
@@ -4505,13 +4501,20 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
             if (pfrom->fWhitelisted && GetBoolArg("-whitelistalwaysrelay", DEFAULT_WHITELISTALWAYSRELAY)) {
                 // Always relay transactions received from whitelisted peers, even
-                // if they were rejected from the mempool, allowing the node to
-                // function as a gateway for nodes hidden behind it.
+                // if they were already in the mempool or rejected from it due
+                // to policy, allowing the node to function as a gateway for
+                // nodes hidden behind it.
                 //
-                // FIXME: This includes invalid transactions, which means a
-                // whitelisted peer could get us banned! We may want to change
-                // that.
-                RelayTransaction(tx);
+                // Never relay transactions that we would assign a non-zero DoS
+                // score for, as we expect peers to do the same with us in that
+                // case.
+                int nDoS = 0;
+                if (!state.IsInvalid(nDoS) || nDoS == 0) {
+                    LogPrintf("Force relaying tx %s from whitelisted peer=%d\n", tx.GetHash().ToString(), pfrom->id);
+                    RelayTransaction(tx);
+                } else {
+                    LogPrintf("Not relaying invalid transaction %s from whitelisted peer=%d (%s)\n", tx.GetHash().ToString(), pfrom->id, FormatStateMessage(state));
+                }
             }
         }
         int nDoS = 0;