Merge bitcoin/bitcoin#30896: kernel: Move background load thread to node context

bc7900f33d kernel: Move background load thread to node context (TheCharlatan)

Pull request description:

  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.

ACKs for top commit:
  maflcko:
    ACK bc7900f33d 🔄
  achow101:
    ACK bc7900f33d
  ryanofsky:
    Code review ACK bc7900f33d. Nice cleanup
  jonatack:
    Light ACK bc7900f33d
  stickies-v:
    ACK bc7900f33d

Tree-SHA512: add9c4823731324e3db50f95e023e99d55db7cc75c69083ae7c9c2157e5540968caa6cf10674aa4901f91366b02ebb1ff18bb977fec0a46431e2196448958b9d

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;