MDEV-10010 - Recursive call to mysql_rwlock_rdlock for LOCK_system_variables_hash

Avoid recursive LOCK_system_variables_hash acquisition in
intern_sys_var_ptr() by pre-syncing dynamic session variables.

sql/sql_plugin.cc

@@ -2955,31 +2955,11 @@ static st_bookmark *register_var(const char *plugin, const char *name,
   return result;
 }
 
-/*
-  returns a pointer to the memory which holds the thd-local variable or
-  a pointer to the global variable if thd==null.
-  If required, will sync with global variables if the requested variable
-  has not yet been allocated in the current thread.
-*/
-static uchar *intern_sys_var_ptr(THD* thd, int offset, bool global_lock)
-{
-  DBUG_ENTER("intern_sys_var_ptr");
-  DBUG_ASSERT(offset >= 0);
-  DBUG_ASSERT((uint)offset <= global_system_variables.dynamic_variables_head);
-
-  if (!thd)
-    DBUG_RETURN((uchar*) global_system_variables.dynamic_variables_ptr + offset);
 
-  /*
-    dynamic_variables_head points to the largest valid offset
-  */
-  if (!thd->variables.dynamic_variables_ptr ||
-      (uint)offset > thd->variables.dynamic_variables_head)
-  {
+void sync_dynamic_session_variables(THD* thd, bool global_lock)
+{
   uint idx;
 
-    mysql_rwlock_rdlock(&LOCK_system_variables_hash);
-
   thd->variables.dynamic_variables_ptr= (char*)
     my_realloc(thd->variables.dynamic_variables_ptr,
                global_variables_dynamic_size,
@@ -3036,7 +3016,32 @@ static uchar *intern_sys_var_ptr(THD* thd, int offset, bool global_lock)
          global_system_variables.dynamic_variables_head;
   thd->variables.dynamic_variables_size=
          global_system_variables.dynamic_variables_size;
+}
 
+
+/*
+  returns a pointer to the memory which holds the thd-local variable or
+  a pointer to the global variable if thd==null.
+  If required, will sync with global variables if the requested variable
+  has not yet been allocated in the current thread.
+*/
+static uchar *intern_sys_var_ptr(THD* thd, int offset, bool global_lock)
+{
+  DBUG_ENTER("intern_sys_var_ptr");
+  DBUG_ASSERT(offset >= 0);
+  DBUG_ASSERT((uint)offset <= global_system_variables.dynamic_variables_head);
+
+  if (!thd)
+    DBUG_RETURN((uchar*) global_system_variables.dynamic_variables_ptr + offset);
+
+  /*
+    dynamic_variables_head points to the largest valid offset
+  */
+  if (!thd->variables.dynamic_variables_ptr ||
+      (uint)offset > thd->variables.dynamic_variables_head)
+  {
+    mysql_rwlock_rdlock(&LOCK_system_variables_hash);
+    sync_dynamic_session_variables(thd, global_lock);
     mysql_rwlock_unlock(&LOCK_system_variables_hash);
   }
   DBUG_RETURN((uchar*)thd->variables.dynamic_variables_ptr + offset);

sql/sql_plugin.h

@@ -190,4 +190,5 @@ extern bool plugin_foreach_with_mask(THD *thd, plugin_foreach_func *func,
 extern bool plugin_dl_foreach(THD *thd, const LEX_STRING *dl,
                               plugin_foreach_func *func, void *arg);
 
+extern void sync_dynamic_session_variables(THD* thd, bool global_lock);
 #endif

sql/sql_show.cc

@@ -7209,6 +7209,17 @@ int fill_variables(THD *thd, TABLE_LIST *tables, COND *cond)
   COND *partial_cond= make_cond_for_info_schema(thd, cond, tables);
 
   mysql_rwlock_rdlock(&LOCK_system_variables_hash);
+
+  /*
+    Avoid recursive LOCK_system_variables_hash acquisition in
+    intern_sys_var_ptr() by pre-syncing dynamic session variables.
+  */
+  if (scope == OPT_SESSION &&
+      (!thd->variables.dynamic_variables_ptr ||
+       global_system_variables.dynamic_variables_head >
+       thd->variables.dynamic_variables_head))
+    sync_dynamic_session_variables(thd, true);
+
   res= show_status_array(thd, wild, enumerate_sys_vars(thd, sorted_vars, scope),
                          scope, NULL, "", tables->table,
                          upper_case_names, partial_cond);