Remove pointer cast in CRPCTable::dumpArgMap

CRPCTable::dumpArgMap currently works by casting RPC command unique_id
integer field to a function pointer, and then calling the function. The
unique_id field wasn't supposed to be used this way (it's meant to be
used to detect RPC aliases), and this code segfaults in the rpc_help.py
test in multiprocess PR https://github.com/bitcoin/bitcoin/pull/10102
because wallet RPC functions aren't directly accessible from the node
process.

Fix this by adding a new GET_ARGS request mode to retrieve argument
information similar to the way the GET_HELP mode retrieves help
information.

src/rpc/request.h

@@ -34,7 +34,7 @@ public:
     UniValue id;
     std::string strMethod;
     UniValue params;
-    enum Mode { EXECUTE, GET_HELP } mode = EXECUTE;
+    enum Mode { EXECUTE, GET_HELP, GET_ARGS } mode = EXECUTE;
     std::string URI;
     std::string authUser;
     std::string peerAddr;

src/rpc/server.cpp

@@ -149,7 +149,7 @@ static RPCHelpMan help()
     }
     if (strCommand == "dump_all_command_conversions") {
         // Used for testing only, undocumented
-        return tableRPC.dumpArgMap();
+        return tableRPC.dumpArgMap(jsonRequest);
     }
 
     return tableRPC.help(strCommand, jsonRequest);
@@ -492,13 +492,18 @@ std::vector<std::string> CRPCTable::listCommands() const
     return commandList;
 }
 
-UniValue CRPCTable::dumpArgMap() const
+UniValue CRPCTable::dumpArgMap(const JSONRPCRequest& args_request) const
 {
+    JSONRPCRequest request(args_request);
+    request.mode = JSONRPCRequest::GET_ARGS;
+
     UniValue ret{UniValue::VARR};
     for (const auto& cmd : mapCommands) {
-        for (const auto& c : cmd.second) {
+        UniValue result;
-            const auto help = RpcMethodFnType(c->unique_id)();
+        if (ExecuteCommands(cmd.second, request, result)) {
-            help.AppendArgMap(ret);
+            for (const auto& values : result.getValues()) {
+                ret.push_back(values);
+            }
         }
     }
     return ret;

src/rpc/server.h

@@ -148,7 +148,7 @@ public:
     /**
      * Return all named arguments that need to be converted by the client from string to another JSON type
      */
-    UniValue dumpArgMap() const;
+    UniValue dumpArgMap(const JSONRPCRequest& request) const;
 
     /**
      * Appends a CRPCCommand to the dispatch table.

src/rpc/util.cpp

@@ -478,6 +478,9 @@ std::string RPCExamples::ToDescriptionString() const
 
 UniValue RPCHelpMan::HandleRequest(const JSONRPCRequest& request)
 {
+    if (request.mode == JSONRPCRequest::GET_ARGS) {
+        return GetArgMap();
+    }
     /*
      * Check if the given request is valid according to this command or if
      * the user is asking for help information, and throw help when appropriate.
@@ -561,8 +564,9 @@ std::string RPCHelpMan::ToString() const
     return ret;
 }
 
-void RPCHelpMan::AppendArgMap(UniValue& arr) const
+UniValue RPCHelpMan::GetArgMap() const
 {
+    UniValue arr{UniValue::VARR};
     for (int i{0}; i < int(m_args.size()); ++i) {
         const auto& arg = m_args.at(i);
         std::vector<std::string> arg_names;
@@ -577,6 +581,7 @@ void RPCHelpMan::AppendArgMap(UniValue& arr) const
             arr.push_back(map);
         }
     }
+    return arr;
 }
 
 std::string RPCArg::GetFirstName() const

src/rpc/util.h

@@ -337,8 +337,8 @@ public:
 
     UniValue HandleRequest(const JSONRPCRequest& request);
     std::string ToString() const;
-    /** Append the named args that need to be converted from string to another JSON type */
+    /** Return the named args that need to be converted from string to another JSON type */
-    void AppendArgMap(UniValue& arr) const;
+    UniValue GetArgMap() const;
     /** If the supplied number of args is neither too small nor too high */
     bool IsValidNumArgs(size_t num_args) const;
     std::vector<std::string> GetArgNames() const;