MDEV-23483: Set Galera SST thd as system thread

Revert change to MDL and set SST donor thread as a system thread.
Joiner thread was already a system thread.

sql/mdl.cc

@@ -25,7 +25,6 @@
 #include <mysql/psi/mysql_stage.h>
 #include "wsrep_mysqld.h"
 #include "wsrep_thd.h"
-#include "wsrep_sst.h"
 
 #ifdef HAVE_PSI_INTERFACE
 static PSI_mutex_key key_MDL_wait_LOCK_wait_status;
@@ -2138,17 +2137,10 @@ MDL_context::acquire_lock(MDL_request *mdl_request, double lock_wait_timeout)
     wait_status= m_wait.timed_wait(m_owner, &abs_shortwait, FALSE,
                                    mdl_request->key.get_wait_state_name());
 
-    THD* thd= m_owner->get_thd();
-
     if (wait_status != MDL_wait::EMPTY)
       break;
     /* Check if the client is gone while we were waiting. */
-    if (! thd_is_connected(thd))
-    {
-#if defined(WITH_WSREP) && !defined(EMBEDDED_LIBRARY)
-      // During SST client might not be connected
-      if (!wsrep_is_sst_progress())
-#endif
+    if (! thd_is_connected(m_owner->get_thd()))
     {
       /*
        * The client is disconnected. Don't wait forever:
@@ -2158,7 +2150,6 @@ MDL_context::acquire_lock(MDL_request *mdl_request, double lock_wait_timeout)
       wait_status= MDL_wait::TIMEOUT;
       break;
     }
-    }
 
     mysql_prlock_wrlock(&lock->m_rwlock);
     if (lock->needs_notification(ticket))

sql/wsrep_sst.cc

-/* Copyright 2008-2015 Codership Oy <http://www.codership.com>
+/* Copyright 2008-2020 Codership Oy <http://www.codership.com>
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -192,7 +192,6 @@ bool wsrep_before_SE()
 
 static bool            sst_complete = false;
 static bool            sst_needed   = false;
-static bool            sst_in_progress = false;
 
 #define WSREP_EXTEND_TIMEOUT_INTERVAL 30
 #define WSREP_TIMEDWAIT_SECONDS 10
@@ -1545,11 +1544,11 @@ static void* sst_donor_thread (void* a)
   wsrep_uuid_t  ret_uuid= WSREP_UUID_UNDEFINED;
   // seqno of complete SST
   wsrep_seqno_t ret_seqno= WSREP_SEQNO_UNDEFINED;
-  // SST is now in progress
-  sst_in_progress= true;
 
-  wsp::thd thd(FALSE); // we turn off wsrep_on for this THD so that it can
+  // We turn off wsrep_on for this THD so that it can
   // operate with wsrep_ready == OFF
+  // We also set this SST thread THD as system thread
+  wsp::thd thd(FALSE, true);
   wsp::process proc(arg->cmd, "r", arg->env);
 
   err= proc.error();
@@ -1648,8 +1647,6 @@ static void* sst_donor_thread (void* a)
   wsrep->sst_sent (wsrep, &state_id, -err);
   proc.wait();
 
-  sst_in_progress= false;
-
   return NULL;
 }
 
@@ -1824,8 +1821,3 @@ void wsrep_SE_initialized()
 {
   SE_initialized = true;
 }
-
-bool wsrep_is_sst_progress()
-{
-  return (sst_in_progress);
-}

sql/wsrep_utils.cc

@@ -413,7 +413,7 @@ process::wait ()
   return err_;
 }
 
-thd::thd (my_bool won) : init(), ptr(new THD(0))
+thd::thd (my_bool won, bool system_thread) : init(), ptr(new THD(0))
 {
   if (ptr)
   {
@@ -421,6 +421,8 @@ thd::thd (my_bool won) : init(), ptr(new THD(0))
     ptr->store_globals();
     ptr->variables.option_bits&= ~OPTION_BIN_LOG; // disable binlog
     ptr->variables.wsrep_on = won;
+    if (system_thread)
+      ptr->system_thread= SYSTEM_THREAD_GENERIC;
     ptr->security_ctx->master_access= ~(ulong)0;
     lex_start(ptr);
   }

sql/wsrep_utils.h

@@ -298,7 +298,7 @@ class thd
 
 public:
 
-  thd(my_bool wsrep_on);
+  thd(my_bool wsrep_on, bool system_thread=false);
   ~thd();
   THD* const ptr;
 };