Merge #18740: Remove g_rpc_node global

b3f7f375ef refactor: Remove g_rpc_node global (Russell Yanofsky)
ccb5059ee8 scripted-diff: Remove g_rpc_node references (Russell Yanofsky)
6fca33b2ed refactor: Pass NodeContext to RPC and REST methods through util::Ref (Russell Yanofsky)
691c817b34 Add util::Ref class as temporary alternative for c++17 std::any (Russell Yanofsky)

Pull request description:

  This PR removes the `g_rpc_node` global, to get same benefits we see removing other globals and make RPC code more testable, modular, and reusable.

  This uses a hybrid of the approaches suggested in #17548. Instead of using `std::any`, which isn't available in c++11, or `void*`, which isn't type safe, it uses a small new `util::Ref` helper class, which acts like a simplified `std::any` that only holds references, not values.

  Motivation for writing this was to provide an simpler alternative to #18647 by Harris Brakmić (brakmic) which avoids some shortcomings of that PR (https://github.com/bitcoin/bitcoin/pull/18647#issuecomment-617878826)

ACKs for top commit:
  MarcoFalke:
    re-ACK b3f7f375ef, only change is adding back const and more tests 🚾
  ajtowns:
    ACK b3f7f375ef

Tree-SHA512: 56292268a001bdbe34d641db1180c215351503966ff451e55cc96c9137f1d262225d7d7733de9c9da7ce7d7a4b34213a98c2476266b58c89dbbb0f3cb5aa5d70

src/init.cpp

@@ -784,16 +784,16 @@ static bool InitSanityCheck()
     return true;
 }
 
-static bool AppInitServers()
+static bool AppInitServers(const util::Ref& context)
 {
     RPCServer::OnStarted(&OnRPCStarted);
     RPCServer::OnStopped(&OnRPCStopped);
     if (!InitHTTPServer())
         return false;
     StartRPC();
-    if (!StartHTTPRPC())
+    if (!StartHTTPRPC(context))
         return false;
-    if (gArgs.GetBoolArg("-rest", DEFAULT_REST_ENABLE)) StartREST();
+    if (gArgs.GetBoolArg("-rest", DEFAULT_REST_ENABLE)) StartREST(context);
     StartHTTPServer();
     return true;
 }
@@ -1238,7 +1238,7 @@ bool AppInitLockDataDirectory()
     return true;
 }
 
-bool AppInitMain(NodeContext& node)
+bool AppInitMain(const util::Ref& context, NodeContext& node)
 {
     const CChainParams& chainparams = Params();
     // ********************************************************* Step 4a: application initialization
@@ -1340,7 +1340,6 @@ bool AppInitMain(NodeContext& node)
     for (const auto& client : node.chain_clients) {
         client->registerRpcs();
     }
-    g_rpc_node = &node;
 #if ENABLE_ZMQ
     RegisterZMQRPCCommands(tableRPC);
 #endif
@@ -1353,7 +1352,7 @@ bool AppInitMain(NodeContext& node)
     if (gArgs.GetBoolArg("-server", false))
     {
         uiInterface.InitMessage_connect(SetRPCWarmupStatus);
-        if (!AppInitServers())
+        if (!AppInitServers(context))
             return InitError(_("Unable to start HTTP server. See debug log for details."));
     }