Merge #21035: Remove pointer cast in CRPCTable::dumpArgMap

9048c58e10 Remove pointer cast in CRPCTable::dumpArgMap (Russell Yanofsky)
14f3d9b908 refactor: Add RPC server ExecuteCommands function (Russell Yanofsky)
6158a6d397 refactor: Replace JSONRPCRequest fHelp field with mode field (Russell Yanofsky)

Pull request description:

  This change is needed to fix the `rpc_help.py` test failing in #10102: https://cirrus-ci.com/task/5469433013469184?command=ci#L2275

  The [`CRPCTable::dumpArgMap`](16b784d953/src/rpc/server.cpp (L492)) method currently works by casting RPC `unique_id` integer field to a function pointer, and then calling it. The `unique_id` field wasn't supposed to be used this way (it's meant to be used to detect RPC aliases) and as a result, this code segfaults in the `rpc_help.py` test in multiprocess PR #10102 because wallet RPC functions aren't directly accessible from the node process.

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

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/projects/10).

ACKs for top commit:
  MarcoFalke:
    re-ACK 9048c58e10 👑

Tree-SHA512: cd1a01c1daa5bde2c2455b63548371ee4cf39688313969ad2016d9a0fd4344102e3fd43034058f253364518e9632d57cf21abffad0d6a2c0c94b7a6921cbe615

src/rpc/util.cpp

@@ -459,6 +459,21 @@ std::string RPCExamples::ToDescriptionString() const
     return m_examples.empty() ? m_examples : "\nExamples:\n" + m_examples;
 }
 
+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.
+     */
+    if (request.mode == JSONRPCRequest::GET_HELP || !IsValidNumArgs(request.params.size())) {
+        throw std::runtime_error(ToString());
+    }
+    return m_fun(*this, request);
+}
+
 bool RPCHelpMan::IsValidNumArgs(size_t num_args) const
 {
     size_t num_required_args = 0;
@@ -532,8 +547,9 @@ 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;
@@ -548,6 +564,7 @@ void RPCHelpMan::AppendArgMap(UniValue& arr) const
             arr.push_back(map);
         }
     }
+    return arr;
 }
 
 std::string RPCArg::GetFirstName() const