init: Delay RPC block notif until warmup finished

See added code comment for more details.

src/init.cpp

@@ -1777,7 +1777,17 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
     // ********************************************************* Step 13: finished
 
+    // At this point, the RPC is "started", but still in warmup, which means it
+    // cannot yet be called. Before we make it callable, we need to make sure
+    // that the RPC's view of the best block is valid and consistent with
+    // ChainstateManager's ActiveTip.
+    //
+    // If we do not do this, RPC's view of the best block will be height=0 and
+    // hash=0x0. This will lead to erroroneous responses for things like
+    // waitforblockheight.
+    RPCNotifyBlockChange(chainman.ActiveTip());
     SetRPCWarmupFinished();
+
     uiInterface.InitMessage(_("Done loading").translated);
 
     for (const auto& client : node.chain_clients) {

src/node/chainstate.cpp

@@ -5,7 +5,6 @@
 #include <node/chainstate.h>
 
 #include <chainparams.h> // for CChainParams
-#include <rpc/blockchain.h> // for RPCNotifyBlockChange
 #include <util/time.h> // for GetTime
 #include <node/blockstorage.h> // for CleanupBlockRevFiles, fHavePruned, fReindex
 #include <shutdown.h> // for ShutdownRequested
@@ -144,7 +143,6 @@ std::optional<ChainstateLoadVerifyError> VerifyLoadedChainstate(ChainstateManage
         for (CChainState* chainstate : chainman.GetAll()) {
             if (!is_coinsview_empty(chainstate)) {
                 const CBlockIndex* tip = chainstate->m_chain.Tip();
-                RPCNotifyBlockChange(tip);
                 if (tip && tip->nTime > GetTime() + MAX_FUTURE_BLOCK_TIME) {
                     return ChainstateLoadVerifyError::ERROR_BLOCK_FROM_FUTURE;
                 }