Disable mainnet/testnet operation

To avoid the risk of bugs in new consensus code causing potential
chain splits or loss of funds, we disable use of this branch on mainnet
or testnet.  Use with either regtest or signet remains supported.

src/bitcoind.cpp

@@ -161,6 +161,10 @@ static bool AppInit(NodeContext& node, int argc, char* argv[])
         } catch (const std::exception& e) {
             return InitError(Untranslated(strprintf("%s\n", e.what())));
         }
+        // Disallow mainnet/testnet operation
+        if (Params().NetworkIDString() == CBaseChainParams::MAIN || Params().NetworkIDString() == CBaseChainParams::TESTNET) {
+            return InitError(Untranslated(strprintf("Selected network '%s' is unsupported for this client, select -regtest or -signet instead.\n", Params().NetworkIDString())));
+        }
 
         // Error out when loose non-argument tokens are encountered on command line
         for (int i = 1; i < argc; i++) {

src/qt/bitcoin.cpp

@@ -586,6 +586,11 @@ int GuiMain(int argc, char* argv[])
         QMessageBox::critical(nullptr, PACKAGE_NAME, QObject::tr("Error: %1").arg(e.what()));
         return EXIT_FAILURE;
     }
+    // Disallow mainnet/testnet operation
+    if (Params().NetworkIDString() == CBaseChainParams::MAIN || Params().NetworkIDString() == CBaseChainParams::TESTNET) {
+        return InitError(Untranslated(strprintf("Selected network '%s' is unsupported for this client, select -regtest or -signet instead.\n", Params().NetworkIDString())));
+    }
+
 #ifdef ENABLE_WALLET
     // Parse URIs on command line -- this can affect Params()
     PaymentServer::ipcParseCommandLine(argc, argv);