kernel: Move background load thread to node context

The thread handle is never used by the ChainstateManager, so move it out
and into the node context. Users of the kernel library now no longer
have to manually join the thread when destructing the ChainstateManager.

src/bitcoin-chainstate.cpp

@@ -283,8 +283,6 @@ int main(int argc, char* argv[])
 epilogue:
     // Without this precise shutdown sequence, there will be a lot of nullptr
     // dereferencing and UB.
-    if (chainman.m_thread_load.joinable()) chainman.m_thread_load.join();
-
     validation_signals.FlushBackgroundCallbacks();
     {
         LOCK(cs_main);

src/init.cpp

@@ -298,7 +298,7 @@ void Shutdown(NodeContext& node)
 
     StopTorControl();
 
-    if (node.chainman && node.chainman->m_thread_load.joinable()) node.chainman->m_thread_load.join();
+    if (node.background_init_thread.joinable()) node.background_init_thread.join();
     // After everything has been shut down, but before things get flushed, stop the
     // the scheduler. After this point, SyncWithValidationInterfaceQueue() should not be called anymore
     // as this would prevent the shutdown from completing.
@@ -1789,7 +1789,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         vImportFiles.push_back(fs::PathFromString(strFile));
     }
 
-    chainman.m_thread_load = std::thread(&util::TraceThread, "initload", [=, &chainman, &args, &node] {
+    node.background_init_thread = std::thread(&util::TraceThread, "initload", [=, &chainman, &args, &node] {
         ScheduleBatchPriority();
         // Import blocks
         ImportBlocks(chainman, vImportFiles);

src/node/context.h

@@ -9,6 +9,7 @@
 #include <cstdlib>
 #include <functional>
 #include <memory>
+#include <thread>
 #include <vector>
 
 class ArgsManager;
@@ -86,6 +87,7 @@ struct NodeContext {
     std::atomic<int> exit_status{EXIT_SUCCESS};
     //! Manages all the node warnings
     std::unique_ptr<node::Warnings> warnings;
+    std::thread background_init_thread;
 
     //! Declare default constructor and destructor that are not inline, so code
     //! instantiating the NodeContext struct doesn't need to #include class

src/validation.h

@@ -42,7 +42,6 @@
 #include <span>
 #include <stdint.h>
 #include <string>
-#include <thread>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -1008,7 +1007,6 @@ public:
 
     const util::SignalInterrupt& m_interrupt;
     const Options m_options;
-    std::thread m_thread_load;
     //! A single BlockManager instance is shared across each constructed
     //! chainstate to avoid duplicating block metadata.
     node::BlockManager m_blockman;