rpc: access some args by name

Use the new key-based Arg helper in a few locations to show how
it is used.

src/rpc/blockchain.cpp

@@ -2149,7 +2149,8 @@ static RPCHelpMan scantxoutset()
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
     UniValue result(UniValue::VOBJ);
-    if (request.params[0].get_str() == "status") {
+    const auto action{self.Arg<std::string>("action")};
+    if (action == "status") {
         CoinsViewScanReserver reserver;
         if (reserver.reserve()) {
             // no scan in progress
@@ -2157,7 +2158,7 @@ static RPCHelpMan scantxoutset()
         }
         result.pushKV("progress", g_scan_progress.load());
         return result;
-    } else if (request.params[0].get_str() == "abort") {
+    } else if (action == "abort") {
         CoinsViewScanReserver reserver;
         if (reserver.reserve()) {
             // reserve was possible which means no scan was running
@@ -2166,7 +2167,7 @@ static RPCHelpMan scantxoutset()
         // set the abort flag
         g_should_abort_scan = true;
         return true;
-    } else if (request.params[0].get_str() == "start") {
+    } else if (action == "start") {
         CoinsViewScanReserver reserver;
         if (!reserver.reserve()) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Scan already in progress, use action \"abort\" or \"status\"");
@@ -2235,7 +2236,7 @@ static RPCHelpMan scantxoutset()
         result.pushKV("unspents", unspents);
         result.pushKV("total_amount", ValueFromAmount(total_in));
     } else {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid action '%s'", request.params[0].get_str()));
+        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid action '%s'", action));
     }
     return result;
 },

src/rpc/mining.cpp

@@ -122,7 +122,7 @@ static RPCHelpMan getnetworkhashps()
 {
     ChainstateManager& chainman = EnsureAnyChainman(request.context);
     LOCK(cs_main);
-    return GetNetworkHashPS(self.Arg<int>(0), self.Arg<int>(1), chainman.ActiveChain());
+    return GetNetworkHashPS(self.Arg<int>("nblocks"), self.Arg<int>("height"), chainman.ActiveChain());
 },
     };
 }
@@ -229,12 +229,12 @@ static RPCHelpMan generatetodescriptor()
             "\nGenerate 11 blocks to mydesc\n" + HelpExampleCli("generatetodescriptor", "11 \"mydesc\"")},
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
-    const auto num_blocks{self.Arg<int>(0)};
-    const auto max_tries{self.Arg<uint64_t>(2)};
+    const auto num_blocks{self.Arg<int>("num_blocks")};
+    const auto max_tries{self.Arg<uint64_t>("maxtries")};
 
     CScript coinbase_script;
     std::string error;
-    if (!getScriptFromDescriptor(self.Arg<std::string>(1), coinbase_script, error)) {
+    if (!getScriptFromDescriptor(self.Arg<std::string>("descriptor"), coinbase_script, error)) {
         throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, error);
     }
 

src/rpc/net.cpp

@@ -322,7 +322,7 @@ static RPCHelpMan addnode()
                 },
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
-    const std::string command{request.params[1].get_str()};
+    const auto command{self.Arg<std::string>("command")};
     if (command != "onetry" && command != "add" && command != "remove") {
         throw std::runtime_error(
             self.ToString());
@@ -331,9 +331,9 @@ static RPCHelpMan addnode()
     NodeContext& node = EnsureAnyNodeContext(request.context);
     CConnman& connman = EnsureConnman(node);
 
-    const std::string node_arg{request.params[0].get_str()};
+    const auto node_arg{self.Arg<std::string>("node")};
     bool node_v2transport = connman.GetLocalServices() & NODE_P2P_V2;
-    bool use_v2transport = self.MaybeArg<bool>(2).value_or(node_v2transport);
+    bool use_v2transport = self.MaybeArg<bool>("v2transport").value_or(node_v2transport);
 
     if (use_v2transport && !node_v2transport) {
         throw JSONRPCError(RPC_INVALID_PARAMETER, "Error: v2transport requested but not enabled (see -v2transport)");

src/rpc/signmessage.cpp

@@ -38,9 +38,9 @@ static RPCHelpMan verifymessage()
         },
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
         {
-            std::string strAddress = request.params[0].get_str();
-            std::string strSign = request.params[1].get_str();
-            std::string strMessage = request.params[2].get_str();
+            std::string strAddress = self.Arg<std::string>("address");
+            std::string strSign = self.Arg<std::string>("signature");
+            std::string strMessage = self.Arg<std::string>("message");
 
             switch (MessageVerify(strAddress, strSign, strMessage)) {
             case MessageVerificationResult::ERR_INVALID_ADDRESS:

src/wallet/rpc/coins.cpp

@@ -194,15 +194,12 @@ RPCHelpMan getbalance()
 
     LOCK(pwallet->cs_wallet);
 
-    const auto dummy_value{self.MaybeArg<std::string>(0)};
+    const auto dummy_value{self.MaybeArg<std::string>("dummy")};
     if (dummy_value && *dummy_value != "*") {
         throw JSONRPCError(RPC_METHOD_DEPRECATED, "dummy first argument must be excluded or set to \"*\".");
     }
 
-    int min_depth = 0;
-    if (!request.params[1].isNull()) {
-        min_depth = request.params[1].getInt<int>();
-    }
+    const auto min_depth{self.Arg<int>("minconf")};
 
     bool include_watchonly = ParseIncludeWatchonly(request.params[2], *pwallet);