rpc: Add MaybeArg() and Arg() default helper

src/rpc/mining.cpp

@@ -110,7 +110,7 @@ static RPCHelpMan getnetworkhashps()
 {
     ChainstateManager& chainman = EnsureAnyChainman(request.context);
     LOCK(cs_main);
-    return GetNetworkHashPS(!request.params[0].isNull() ? request.params[0].getInt<int>() : 120, !request.params[1].isNull() ? request.params[1].getInt<int>() : -1, chainman.ActiveChain());
+    return GetNetworkHashPS(self.Arg<int>(0), self.Arg<int>(1), chainman.ActiveChain());
 },
     };
 }
@@ -217,12 +217,12 @@ static RPCHelpMan generatetodescriptor()
             "\nGenerate 11 blocks to mydesc\n" + HelpExampleCli("generatetodescriptor", "11 \"mydesc\"")},
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
-    const int num_blocks{request.params[0].getInt<int>()};
-    const uint64_t max_tries{request.params[2].isNull() ? DEFAULT_MAX_TRIES : request.params[2].getInt<int>()};
+    const auto num_blocks{self.Arg<int>(0)};
+    const auto max_tries{self.Arg<uint64_t>(2)};
 
     CScript coinbase_script;
     std::string error;
-    if (!getScriptFromDescriptor(request.params[1].get_str(), coinbase_script, error)) {
+    if (!getScriptFromDescriptor(self.Arg<std::string>(1), coinbase_script, error)) {
         throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, error);
     }
 
@@ -468,9 +468,10 @@ static RPCHelpMan prioritisetransaction()
     LOCK(cs_main);
 
     uint256 hash(ParseHashV(request.params[0], "txid"));
+    const auto dummy{self.MaybeArg<double>(1)};
     CAmount nAmount = request.params[2].getInt<int64_t>();
 
-    if (!(request.params[1].isNull() || request.params[1].get_real() == 0)) {
+    if (dummy && *dummy != 0) {
         throw JSONRPCError(RPC_INVALID_PARAMETER, "Priority is no longer supported, dummy argument to prioritisetransaction must be 0.");
     }
 

src/rpc/util.cpp

@@ -609,7 +609,10 @@ UniValue RPCHelpMan::HandleRequest(const JSONRPCRequest& request) const
     if (!arg_mismatch.empty()) {
         throw JSONRPCError(RPC_TYPE_ERROR, strprintf("Wrong type passed:\n%s", arg_mismatch.write(4)));
     }
+    CHECK_NONFATAL(m_req == nullptr);
+    m_req = &request;
     UniValue ret = m_fun(*this, request);
+    m_req = nullptr;
     if (gArgs.GetBoolArg("-rpcdoccheck", DEFAULT_RPC_DOC_CHECK)) {
         UniValue mismatch{UniValue::VARR};
         for (const auto& res : m_results.m_results) {
@@ -635,6 +638,49 @@ UniValue RPCHelpMan::HandleRequest(const JSONRPCRequest& request) const
     return ret;
 }
 
+using CheckFn = void(const RPCArg&);
+static const UniValue* DetailMaybeArg(CheckFn* check, const std::vector<RPCArg>& params, const JSONRPCRequest* req, size_t i)
+{
+    CHECK_NONFATAL(i < params.size());
+    const UniValue& arg{CHECK_NONFATAL(req)->params[i]};
+    const RPCArg& param{params.at(i)};
+    if (check) check(param);
+
+    if (!arg.isNull()) return &arg;
+    if (!std::holds_alternative<RPCArg::Default>(param.m_fallback)) return nullptr;
+    return &std::get<RPCArg::Default>(param.m_fallback);
+}
+
+static void CheckRequiredOrDefault(const RPCArg& param)
+{
+    // Must use `Arg<Type>(i)` to get the argument or its default value.
+    const bool required{
+        std::holds_alternative<RPCArg::Optional>(param.m_fallback) && RPCArg::Optional::NO == std::get<RPCArg::Optional>(param.m_fallback),
+    };
+    CHECK_NONFATAL(required || std::holds_alternative<RPCArg::Default>(param.m_fallback));
+}
+
+#define TMPL_INST(check_param, ret_type, return_code)       \
+    template <>                                             \
+    ret_type RPCHelpMan::ArgValue<ret_type>(size_t i) const \
+    {                                                       \
+        const UniValue* maybe_arg{                          \
+            DetailMaybeArg(check_param, m_args, m_req, i),  \
+        };                                                  \
+        return return_code                                  \
+    }                                                       \
+    void force_semicolon(ret_type)
+
+// Optional arg (without default). Can also be called on required args, if needed.
+TMPL_INST(nullptr, std::optional<double>, maybe_arg ? std::optional{maybe_arg->get_real()} : std::nullopt;);
+TMPL_INST(nullptr, std::optional<bool>, maybe_arg ? std::optional{maybe_arg->get_bool()} : std::nullopt;);
+TMPL_INST(nullptr, const std::string*, maybe_arg ? &maybe_arg->get_str() : nullptr;);
+
+// Required arg or optional arg with default value.
+TMPL_INST(CheckRequiredOrDefault, int, CHECK_NONFATAL(maybe_arg)->getInt<int>(););
+TMPL_INST(CheckRequiredOrDefault, uint64_t, CHECK_NONFATAL(maybe_arg)->getInt<uint64_t>(););
+TMPL_INST(CheckRequiredOrDefault, const std::string&, CHECK_NONFATAL(maybe_arg)->get_str(););
+
 bool RPCHelpMan::IsValidNumArgs(size_t num_args) const
 {
     size_t num_required_args = 0;

src/rpc/util.h

@@ -6,6 +6,7 @@
 #define BITCOIN_RPC_UTIL_H
 
 #include <addresstype.h>
+#include <consensus/amount.h>
 #include <node/transaction.h>
 #include <outputtype.h>
 #include <protocol.h>
@@ -14,13 +15,29 @@
 #include <rpc/request.h>
 #include <script/script.h>
 #include <script/sign.h>
+#include <uint256.h>
 #include <univalue.h>
 #include <util/check.h>
 
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+#include <initializer_list>
+#include <map>
+#include <optional>
 #include <string>
+#include <type_traits>
+#include <utility>
 #include <variant>
 #include <vector>
 
+class JSONRPCRequest;
+enum ServiceFlags : uint64_t;
+enum class OutputType;
+enum class TransactionError;
+struct FlatSigningProvider;
+struct bilingual_str;
+
 static constexpr bool DEFAULT_RPC_DOC_CHECK{
 #ifdef RPC_DOC_CHECK
     true
@@ -383,6 +400,44 @@ public:
     RPCHelpMan(std::string name, std::string description, std::vector<RPCArg> args, RPCResults results, RPCExamples examples, RPCMethodImpl fun);
 
     UniValue HandleRequest(const JSONRPCRequest& request) const;
+    /**
+     * Helper to get a request argument.
+     * This function only works during m_fun(), i.e. it should only be used in
+     * RPC method implementations. The helper internally checks whether the
+     * user-passed argument isNull() and parses (from JSON) and returns the
+     * user-passed argument, or the default value derived from the RPCArg
+     * documention, or a falsy value if no default was given.
+     *
+     * Use Arg<Type>(i) to get the argument or its default value. Otherwise,
+     * use MaybeArg<Type>(i) to get the optional argument or a falsy value.
+     *
+     * The Type passed to this helper must match the corresponding
+     * RPCArg::Type.
+     */
+    template <typename R>
+    auto Arg(size_t i) const
+    {
+        // Return argument (required or with default value).
+        if constexpr (std::is_integral_v<R> || std::is_floating_point_v<R>) {
+            // Return numbers by value.
+            return ArgValue<R>(i);
+        } else {
+            // Return everything else by reference.
+            return ArgValue<const R&>(i);
+        }
+    }
+    template <typename R>
+    auto MaybeArg(size_t i) const
+    {
+        // Return optional argument (without default).
+        if constexpr (std::is_integral_v<R> || std::is_floating_point_v<R>) {
+            // Return numbers by value, wrapped in optional.
+            return ArgValue<std::optional<R>>(i);
+        } else {
+            // Return other types by pointer.
+            return ArgValue<const R*>(i);
+        }
+    }
     std::string ToString() const;
     /** Return the named args that need to be converted from string to another JSON type */
     UniValue GetArgMap() const;
@@ -399,6 +454,9 @@ private:
     const std::vector<RPCArg> m_args;
     const RPCResults m_results;
     const RPCExamples m_examples;
+    mutable const JSONRPCRequest* m_req{nullptr}; // A pointer to the request for the duration of m_fun()
+    template <typename R>
+    R ArgValue(size_t i) const;
 };
 
 /**

src/wallet/rpc/coins.cpp

@@ -194,8 +194,8 @@ RPCHelpMan getbalance()
 
     LOCK(pwallet->cs_wallet);
 
-    const UniValue& dummy_value = request.params[0];
-    if (!dummy_value.isNull() && dummy_value.get_str() != "*") {
+    const auto dummy_value{self.MaybeArg<std::string>(0)};
+    if (dummy_value && *dummy_value != "*") {
         throw JSONRPCError(RPC_METHOD_DEPRECATED, "dummy first argument must be excluded or set to \"*\".");
     }
 

src/wallet/rpc/wallet.cpp

@@ -478,7 +478,7 @@ static RPCHelpMan unloadwallet()
         // Release the "main" shared pointer and prevent further notifications.
         // Note that any attempt to load the same wallet would fail until the wallet
         // is destroyed (see CheckUniqueFileid).
-        std::optional<bool> load_on_start = request.params[1].isNull() ? std::nullopt : std::optional<bool>(request.params[1].get_bool());
+        std::optional<bool> load_on_start{self.MaybeArg<bool>(1)};
         if (!RemoveWallet(context, wallet, load_on_start, warnings)) {
             throw JSONRPCError(RPC_MISC_ERROR, "Requested wallet already unloaded");
         }