Introduce option to disable relay/mining of bare multisig scripts in TX outputs

First and foremost, this defaults to OFF.

This option lets a node consider such transactions non-standard,
meaning they will not be relayed or mined by default, but other miners
are free to mine these as usual.

src/init.cpp

@@ -248,6 +248,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += "  -maxsendbuffer=<n>     " + _("Maximum per-connection send buffer, <n>*1000 bytes (default: 1000)") + "\n";
     strUsage += "  -onion=<ip:port>       " + _("Use separate SOCKS5 proxy to reach peers via Tor hidden services (default: -proxy)") + "\n";
     strUsage += "  -onlynet=<net>         " + _("Only connect to nodes in network <net> (IPv4, IPv6 or Tor)") + "\n";
+    strUsage += "  -permitbaremultisig    " + _("Relay non-P2SH multisig (default: 1)") + "\n";
     strUsage += "  -port=<port>           " + _("Listen for connections on <port> (default: 8333 or testnet: 18333)") + "\n";
     strUsage += "  -proxy=<ip:port>       " + _("Connect through SOCKS5 proxy") + "\n";
     strUsage += "  -seednode=<ip>         " + _("Connect to a node to retrieve peer addresses, and disconnect") + "\n";
@@ -676,7 +677,10 @@ bool AppInit2(boost::thread_group& threadGroup)
     bSpendZeroConfChange = GetArg("-spendzeroconfchange", true);
 
     std::string strWalletFile = GetArg("-wallet", "wallet.dat");
-#endif
+#endif // ENABLE_WALLET
+
+    fIsBareMultisigStd = GetArg("-permitbaremultisig", true);
+
     // ********************************************************* Step 4: application initialization: dir lock, daemonize, pidfile, debug log
     // Sanity check
     if (!InitSanityCheck())

src/main.cpp

@@ -48,6 +48,7 @@ bool fImporting = false;
 bool fReindex = false;
 bool fBenchmark = false;
 bool fTxIndex = false;
+bool fIsBareMultisigStd = true;
 unsigned int nCoinCacheSize = 5000;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for relaying and mining) */
@@ -604,9 +605,13 @@ bool IsStandardTx(const CTransaction& tx, string& reason)
             reason = "scriptpubkey";
             return false;
         }
+
         if (whichType == TX_NULL_DATA)
             nDataOut++;
-        else if (txout.IsDust(::minRelayTxFee)) {
+        else if ((whichType == TX_MULTISIG) && (!fIsBareMultisigStd)) {
+            reason = "bare-multisig";
+            return false;
+        } else if (txout.IsDust(::minRelayTxFee)) {
             reason = "dust";
             return false;
         }

src/main.h

@@ -94,6 +94,7 @@ extern bool fReindex;
 extern bool fBenchmark;
 extern int nScriptCheckThreads;
 extern bool fTxIndex;
+extern bool fIsBareMultisigStd;
 extern unsigned int nCoinCacheSize;
 extern CFeeRate minRelayTxFee;