MDEV-14485 Server hangs on startup in THD::init

Solve 3 way deadlock between plugin_initialiaze(), THD::init() and
mysql_sys_var_char().

The deadlock exists because of the lock order inversion between
LOCK_global_system_variables mutex and LOCK_system_variables_hash
read-write lock-

In this case, it is enough to change LOCK_system_variables_hash to prefer
reads to fix the deadlock, i.e change it to mysql_prlock_t

1 files changed, 4 insertions, 4 deletions

diff --git a/sql/set_var.cc b/sql/set_var.cc
index 07395e3e708..e96e636e3d3 100644
--- a/sql/set_var.cc
+++ b/sql/set_var.cc
@@ -595,10 +595,10 @@ int mysql_del_sys_var_chain(sys_var *first)
 {
   int result= 0;
 
-  mysql_rwlock_wrlock(&LOCK_system_variables_hash);
+  mysql_prlock_wrlock(&LOCK_system_variables_hash);
   for (sys_var *var= first; var; var= var->next)
     result|= my_hash_delete(&system_variable_hash, (uchar*) var);
-  mysql_rwlock_unlock(&LOCK_system_variables_hash);
+  mysql_prlock_unlock(&LOCK_system_variables_hash);
 
   return result;
 }
@@ -1067,7 +1067,7 @@ int fill_sysvars(THD *thd, TABLE_LIST *tables, COND *cond)
 
   cond= make_cond_for_info_schema(thd, cond, tables);
   thd->count_cuted_fields= CHECK_FIELD_WARN;
-  mysql_rwlock_rdlock(&LOCK_system_variables_hash);
+  mysql_prlock_rdlock(&LOCK_system_variables_hash);
 
   for (uint i= 0; i < system_variable_hash.records; i++)
   {
@@ -1229,7 +1229,7 @@ int fill_sysvars(THD *thd, TABLE_LIST *tables, COND *cond)
   }
   res= 0;
 end:
-  mysql_rwlock_unlock(&LOCK_system_variables_hash);
+  mysql_prlock_unlock(&LOCK_system_variables_hash);
   thd->count_cuted_fields= save_count_cuted_fields;
   return res;
 }