rpc: refactor: use string_view in Arg/MaybeArg

Modernizes interface by not forcing users to deal with raw pointers,
without adding copying overhead. Generalizes the logic of whether
we return by value or by optional/pointer.

In cases where functions take a `const std::string&` and it would
be too much work to update them, a string copy is made (which was
already happening anyway).

src/wallet/rpc/coins.cpp

@@ -196,8 +196,7 @@ RPCHelpMan getbalance()
 
     LOCK(pwallet->cs_wallet);
 
-    const auto dummy_value{self.MaybeArg<std::string>("dummy")};
-    if (dummy_value && *dummy_value != "*") {
+    if (self.MaybeArg<std::string_view>("dummy").value_or("*") != "*") {
         throw JSONRPCError(RPC_METHOD_DEPRECATED, "dummy first argument must be excluded or set to \"*\".");
     }
 

src/wallet/rpc/util.cpp

@@ -11,6 +11,7 @@
 #include <wallet/context.h>
 #include <wallet/wallet.h>
 
+#include <optional>
 #include <string_view>
 #include <univalue.h>
 
@@ -29,7 +30,7 @@ bool GetAvoidReuseFlag(const CWallet& wallet, const UniValue& param) {
     return avoid_reuse;
 }
 
-std::string EnsureUniqueWalletName(const JSONRPCRequest& request, const std::string* wallet_name)
+std::string EnsureUniqueWalletName(const JSONRPCRequest& request, std::optional<std::string_view> wallet_name)
 {
     std::string endpoint_wallet;
     if (GetWalletNameFromJSONRPCRequest(request, endpoint_wallet)) {
@@ -47,7 +48,7 @@ std::string EnsureUniqueWalletName(const JSONRPCRequest& request, const std::str
             "Either the RPC endpoint wallet or the wallet name parameter must be provided");
     }
 
-    return *wallet_name;
+    return std::string{*wallet_name};
 }
 
 bool GetWalletNameFromJSONRPCRequest(const JSONRPCRequest& request, std::string& wallet_name)

src/wallet/rpc/util.h

@@ -11,7 +11,9 @@
 
 #include <any>
 #include <memory>
+#include <optional>
 #include <string>
+#include <string_view>
 #include <vector>
 
 class JSONRPCRequest;
@@ -42,7 +44,7 @@ bool GetWalletNameFromJSONRPCRequest(const JSONRPCRequest& request, std::string&
  * Ensures that a wallet name is specified across the endpoint and wallet_name.
  * Throws `RPC_INVALID_PARAMETER` if none or different wallet names are specified.
  */
-std::string EnsureUniqueWalletName(const JSONRPCRequest& request, const std::string* wallet_name);
+std::string EnsureUniqueWalletName(const JSONRPCRequest& request, std::optional<std::string_view> wallet_name);
 
 void EnsureWalletIsUnlocked(const CWallet&);
 WalletContext& EnsureWalletContext(const std::any& context);

src/wallet/rpc/wallet.cpp

@@ -19,6 +19,7 @@
 #include <wallet/walletutil.h>
 
 #include <optional>
+#include <string_view>
 
 
 namespace wallet {
@@ -456,7 +457,7 @@ static RPCHelpMan unloadwallet()
                 },
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
-    const std::string wallet_name{EnsureUniqueWalletName(request, self.MaybeArg<std::string>("wallet_name"))};
+    const std::string wallet_name{EnsureUniqueWalletName(request, self.MaybeArg<std::string_view>("wallet_name"))};
 
     WalletContext& context = EnsureWalletContext(request.context);
     std::shared_ptr<CWallet> wallet = GetWallet(context, wallet_name);
@@ -613,7 +614,7 @@ static RPCHelpMan migratewallet()
         },
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
         {
-            const std::string wallet_name{EnsureUniqueWalletName(request, self.MaybeArg<std::string>("wallet_name"))};
+            const std::string wallet_name{EnsureUniqueWalletName(request, self.MaybeArg<std::string_view>("wallet_name"))};
 
             SecureString wallet_pass;
             wallet_pass.reserve(100);

src/wallet/test/wallet_rpc_tests.cpp

@@ -17,7 +17,7 @@ static std::string TestWalletName(const std::string& endpoint, std::optional<std
 {
     JSONRPCRequest req;
     req.URI = endpoint;
-    return EnsureUniqueWalletName(req, parameter ? &*parameter : nullptr);
+    return EnsureUniqueWalletName(req, parameter);
 }
 
 BOOST_FIXTURE_TEST_SUITE(wallet_rpc_tests, BasicTestingSetup)