Add option 'mempooltxinputlimit' so the mempool can reject a transaction

based on the number of transparent inputs.

src/init.cpp

@@ -332,6 +332,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-dbcache=<n>", strprintf(_("Set database cache size in megabytes (%d to %d, default: %d)"), nMinDbCache, nMaxDbCache, nDefaultDbCache));
     strUsage += HelpMessageOpt("-loadblock=<file>", _("Imports blocks from external blk000??.dat file") + " " + _("on startup"));
     strUsage += HelpMessageOpt("-maxorphantx=<n>", strprintf(_("Keep at most <n> unconnectable transactions in memory (default: %u)"), DEFAULT_MAX_ORPHAN_TRANSACTIONS));
+    strUsage += HelpMessageOpt("-mempooltxinputlimit=<n>", _("Set the maximum number of transparent inputs in a transaction that the mempool will accept (default: 0 = no limit applied)"));
     strUsage += HelpMessageOpt("-par=<n>", strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"),
         -(int)boost::thread::hardware_concurrency(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
 #ifndef WIN32
@@ -1009,6 +1010,17 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     }
 #endif
 
+    // Default value of 0 for mempooltxinputlimit means no limit is applied
+    if (mapArgs.count("-mempooltxinputlimit")) {
+        int64_t limit = GetArg("-mempooltxinputlimit", 0);
+        if (limit < 0) {
+            return InitError(_("Mempool limit on transparent inputs to a transaction cannot be negative"));
+        } else if (limit > 0) {
+            LogPrintf("Mempool configured to reject transactions with greater than %lld transparent inputs\n", limit);
+        }
+    }
+
+
     // ********************************************************* Step 4: application initialization: dir lock, daemonize, pidfile, debug log
 
     // Initialize libsodium

src/main.cpp

@@ -1057,6 +1057,17 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     if (pfMissingInputs)
         *pfMissingInputs = false;
 
+    // Node operator can choose to reject tx by number of transparent inputs
+    static_assert(std::numeric_limits<size_t>::max() >= std::numeric_limits<int64_t>::max(), "size_t too small");
+    size_t limit = (size_t) GetArg("-mempooltxinputlimit", 0);
+    if (limit > 0) {
+        size_t n = tx.vin.size();
+        if (n > limit) {
+            LogPrint("mempool", "Dropping txid %s : too many transparent inputs %zu > limit %zu\n", tx.GetHash().ToString(), n, limit );
+            return false;
+        }
+    }
+
     auto verifier = libzcash::ProofVerifier::Strict();
     if (!CheckTransaction(tx, state, verifier))
         return error("AcceptToMemoryPool: CheckTransaction failed");