Bug#51591 deadlock in the plugins+status+variables

Potential deadlock situation involving LOCK_plugin,
LOCK_global_system_variables and LOCK_status.
      
This patch backports the fix from next-mr, unlocking
LOCK_plugin before calling plugin->init() and
add_status_vars().

sql/sql_plugin.cc

@@ -1006,9 +1006,14 @@ void plugin_unlock_list(THD *thd, plugin_ref *list, uint count)
 
 static int plugin_initialize(struct st_plugin_int *plugin)
 {
+  int ret= 1;
   DBUG_ENTER("plugin_initialize");
 
   safe_mutex_assert_owner(&LOCK_plugin);
+  uint state= plugin->state;
+  DBUG_ASSERT(state == PLUGIN_IS_UNINITIALIZED);
+
+  pthread_mutex_unlock(&LOCK_plugin);
   if (plugin_type_initialize[plugin->plugin->type])
   {
     if ((*plugin_type_initialize[plugin->plugin->type])(plugin))
@@ -1027,8 +1032,7 @@ static int plugin_initialize(struct st_plugin_int *plugin)
       goto err;
     }
   }
-
-  plugin->state= PLUGIN_IS_READY;
+  state= PLUGIN_IS_READY; // plugin->init() succeeded
 
   if (plugin->plugin->status_vars)
   {
@@ -1047,7 +1051,8 @@ static int plugin_initialize(struct st_plugin_int *plugin)
     if (add_status_vars(array)) // add_status_vars makes a copy
       goto err;
 #else
-    add_status_vars(plugin->plugin->status_vars); // add_status_vars makes a copy
+    if (add_status_vars(plugin->plugin->status_vars))
+      goto err;
 #endif /* FIX_LATER */
   }
 
@@ -1067,9 +1072,12 @@ static int plugin_initialize(struct st_plugin_int *plugin)
     }
   }
 
-  DBUG_RETURN(0);
+  ret= 0;
+
 err:
-  DBUG_RETURN(1);
+  pthread_mutex_lock(&LOCK_plugin);
+  plugin->state= state;
+  DBUG_RETURN(ret);
 }