MDEV-29843 Do not use asynchronous log_write_upto() for system THDs

Non-blocking log_write_upto (MDEV-24341) was only designed  for the
client connections. Fix, so it is not be triggered for any system THD.

Previously, an incomplete solution only excluded Innodb purge THDs, but
not  the slave for example.

The hang in MDEV still remains somewhat a mystery though, it is not
immediately clear how exactly condition variable can become corrupted.
But it is clear that it can be avoided.

sql/sql_class.cc

@@ -5127,12 +5127,22 @@ void reset_thd(MYSQL_THD thd)
   before writing response to client, to provide durability
   guarantees, in other words, server can't send OK packet
   before modified data is durable in redo log.
-*/
-extern "C" void thd_increment_pending_ops(MYSQL_THD thd)
+
+  NOTE: system THD (those that are not associated with client
+        connection) do not allows async operations yet.
+
+  @param thd  a THD
+  @return thd
+  @retval nullptr   if this is system THD */
+extern "C" MYSQL_THD thd_increment_pending_ops(MYSQL_THD thd)
 {
+  if (!thd || thd->system_thread != NON_SYSTEM_THREAD)
+    return nullptr;
   thd->async_state.inc_pending_ops();
+  return thd;
 }
 
+
 /**
   This function can be used by plugin/engine to indicate
   end of async operation (such as end of group commit
@@ -5143,6 +5153,8 @@ extern "C" void thd_increment_pending_ops(MYSQL_THD thd)
 extern "C" void thd_decrement_pending_ops(MYSQL_THD thd)
 {
   DBUG_ASSERT(thd);
+  DBUG_ASSERT(thd->system_thread == NON_SYSTEM_THREAD);
+
   thd_async_state::enum_async_state state;
   if (thd->async_state.dec_pending_ops(&state) == 0)
   {

sql/sql_class.h

@@ -2586,9 +2586,6 @@ struct thd_async_state
   }
 };
 
-extern "C" void thd_increment_pending_ops(MYSQL_THD);
-extern "C" void thd_decrement_pending_ops(MYSQL_THD);
-
 
 /**
   @class THD

storage/innobase/handler/ha_innodb.cc

@@ -1823,15 +1823,6 @@ MYSQL_THD innobase_create_background_thd(const char* name)
 	return thd;
 }
 
-extern "C" void thd_increment_pending_ops(MYSQL_THD);
-
-THD *innodb_thd_increment_pending_ops(THD *thd)
-{
-  if (!thd || THDVAR(thd, background_thread))
-    return nullptr;
-  thd_increment_pending_ops(thd);
-  return thd;
-}
 
 /** Close opened tables, free memory, delete items for a MYSQL_THD.
 @param[in]	thd	MYSQL_THD to reset */

storage/innobase/include/ha_prototypes.h

@@ -199,13 +199,6 @@ but can be used for comparison.
 extern "C" unsigned long long thd_start_utime(const MYSQL_THD thd);
 
 
-/**
-  Indicate the start of an async operation in a foreground thread.
-@param thd          current_thd
-@return thd
-@retval nullptr   if this is not a foreground thread */
-THD *innodb_thd_increment_pending_ops(THD *thd);
-
 /** Determines the current SQL statement.
 Thread unsafe, can only be called from the thread owning the THD.
 @param[in]	thd	MySQL thread handle

storage/innobase/trx/trx0trx.cc

@@ -1134,8 +1134,8 @@ trx_finalize_for_fts(
 	trx->fts_trx = NULL;
 }
 
-
-extern "C" void thd_decrement_pending_ops(MYSQL_THD);
+extern "C" MYSQL_THD thd_increment_pending_ops(MYSQL_THD);
+extern "C" void  thd_decrement_pending_ops(MYSQL_THD);
 
 
 #include "../log/log0sync.h"
@@ -1168,7 +1168,7 @@ static void trx_flush_log_if_needed_low(lsn_t lsn, const trx_t *trx)
   }
 
   completion_callback cb;
-  if ((cb.m_param = innodb_thd_increment_pending_ops(trx->mysql_thd)))
+  if ((cb.m_param = thd_increment_pending_ops(trx->mysql_thd)))
   {
     cb.m_callback = (void (*)(void *)) thd_decrement_pending_ops;
     log_write_up_to(lsn, flush, false, &cb);