Move scheduler/threadGroup into common-init instead of per-app

This resolves #12229 which pointed out a shutdown deadlock due to
scheduler/checkqueue having been shut down while network message
processing is still running.

src/init.cpp

@@ -155,7 +155,10 @@ public:
 static std::unique_ptr<CCoinsViewErrorCatcher> pcoinscatcher;
 static std::unique_ptr<ECCVerifyHandle> globalVerifyHandle;
 
-void Interrupt(boost::thread_group& threadGroup)
+static boost::thread_group threadGroup;
+static CScheduler scheduler;
+
+void Interrupt()
 {
     InterruptHTTPServer();
     InterruptHTTPRPC();
@@ -164,7 +167,6 @@ void Interrupt(boost::thread_group& threadGroup)
     InterruptTorControl();
     if (g_connman)
         g_connman->Interrupt();
-    threadGroup.interrupt_all();
 }
 
 void Shutdown()
@@ -199,6 +201,12 @@ void Shutdown()
     g_connman.reset();
 
     StopTorControl();
+
+    // After everything has been shut down, but before things get flushed, stop the
+    // CScheduler/checkqueue threadGroup
+    threadGroup.interrupt_all();
+    threadGroup.join_all();
+
     if (fDumpMempoolLater && gArgs.GetArg("-persistmempool", DEFAULT_PERSIST_MEMPOOL)) {
         DumpMempool();
     }
@@ -705,7 +713,7 @@ bool InitSanityCheck(void)
     return true;
 }
 
-bool AppInitServers(boost::thread_group& threadGroup)
+bool AppInitServers()
 {
     RPCServer::OnStarted(&OnRPCStarted);
     RPCServer::OnStopped(&OnRPCStopped);
@@ -1184,7 +1192,7 @@ bool AppInitLockDataDirectory()
     return true;
 }
 
-bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
+bool AppInitMain()
 {
     const CChainParams& chainparams = Params();
     // ********************************************************* Step 4a: application initialization
@@ -1251,7 +1259,7 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
     if (gArgs.GetBoolArg("-server", false))
     {
         uiInterface.InitMessage.connect(SetRPCWarmupStatus);
-        if (!AppInitServers(threadGroup))
+        if (!AppInitServers())
             return InitError(_("Unable to start HTTP server. See debug log for details."));
     }