Set notifications m_tip_block in LoadChainTip()

Ensure KernelNotifications m_tip_block is set even if no new block arrives.

Additionally, have node init always wait for this to happen.

src/init.cpp

@@ -1770,7 +1770,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
     node.background_init_thread = std::thread(&util::TraceThread, "initload", [=, &chainman, &args, &node] {
         ScheduleBatchPriority();
-        // Import blocks
+        // Import blocks and ActivateBestChain()
         ImportBlocks(chainman, vImportFiles);
         if (args.GetBoolArg("-stopafterblockimport", DEFAULT_STOPAFTERBLOCKIMPORT)) {
             LogPrintf("Stopping after block import\n");
@@ -1793,8 +1793,18 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
     });
 
-    // Wait for genesis block to be processed
+    /*
-    if (WITH_LOCK(chainman.GetMutex(), return chainman.ActiveTip() == nullptr)) {
+     * Wait for genesis block to be processed. Typically kernel_notifications.m_tip_block
+     * has already been set by a call to LoadChainTip() in CompleteChainstateInitialization().
+     * But this is skipped if the chainstate doesn't exist yet or is being wiped:
+     *
+     * 1. first startup with an empty datadir
+     * 2. reindex
+     * 3. reindex-chainstate
+     *
+     * In these case it's connected by a call to ActivateBestChain() in the initload thread.
+     */
+    {
         WAIT_LOCK(kernel_notifications.m_tip_block_mutex, lock);
         kernel_notifications.m_tip_block_cv.wait(lock, [&]() EXCLUSIVE_LOCKS_REQUIRED(kernel_notifications.m_tip_block_mutex) {
             return !kernel_notifications.m_tip_block.IsNull() || ShutdownRequested(node);

src/interfaces/mining.h

@@ -75,8 +75,8 @@ public:
     virtual std::optional<BlockRef> getTip() = 0;
 
     /**
-     * Waits for the connected tip to change. If the tip was not connected on
+     * Waits for the connected tip to change. During node initialization, this will
-     * startup, this will wait.
+     * wait until the tip is connected.
      *
      * @param[in] current_tip block hash of the current chain tip. Function waits
      *                        for the chain tip to differ from this.

src/node/blockstorage.h

@@ -430,6 +430,7 @@ public:
     void CleanupBlockRevFiles() const;
 };
 
+// Calls ActivateBestChain() even if no blocks are imported.
 void ImportBlocks(ChainstateManager& chainman, std::span<const fs::path> import_paths);
 } // namespace node
 

src/node/kernel_notifications.h

@@ -56,9 +56,9 @@ public:
 
     Mutex m_tip_block_mutex;
     std::condition_variable m_tip_block_cv GUARDED_BY(m_tip_block_mutex);
-    //! The block for which the last blockTip notification was received for.
+    //! The block for which the last blockTip notification was received.
-    //! The initial ZERO means that no block has been connected yet, which may
+    //! It's first set when the tip is connected during node initialization.
-    //! be true even long after startup, until shutdown.
+    //! Might be unset during an early shutdown.
     uint256 m_tip_block GUARDED_BY(m_tip_block_mutex){uint256::ZERO};
 
 private:

src/validation.cpp

@@ -4721,6 +4721,13 @@ bool Chainstate::LoadChainTip()
               m_chain.Height(),
               FormatISO8601DateTime(tip->GetBlockTime()),
               GuessVerificationProgress(m_chainman.GetParams().TxData(), tip));
+
+    // Ensure KernelNotifications m_tip_block is set even if no new block arrives.
+    if (this->GetRole() != ChainstateRole::BACKGROUND) {
+        // Ignoring return value for now.
+        (void)m_chainman.GetNotifications().blockTip(GetSynchronizationState(/*init=*/true, m_chainman.m_blockman.m_blockfiles_indexed), *pindex);
+    }
+
     return true;
 }