MDEV-5816: Stored programs: validation of stored program statements

Introduced the new data member new_query_arena_is_set of the class sp_head.
This data member is used as a protection against double invocation of
the method restore_thd_mem_root that is called for restoration of the
current query arena. Previously, the data member sp_head::m_thd
is used for this goal but after support for re-compilation of a failed
stored routine statement has been added the data member sp_head::m_thd
can't be used for this goal. The reason is that on a statement re-compilation
after the method restore_thd_mem_root() is called the method
sp_head::add_instr() invoked to add a new instruction for just re-compiled
statement. The method sp_head::add_instr() de-references m_thd to access
the free_list data member. If m_thd was used as a guard against double
invocation it would result in a crash on dereferencing null pointer.

sql/sp_head.cc

@@ -532,6 +532,7 @@ void sp_head::destroy(sp_head *sp)
     MEM_ROOT own_root= sp->main_mem_root;
     DBUG_PRINT("info", ("mem_root %p moved to %p",
                         &sp->mem_root, &own_root));
+    sp->m_thd= nullptr;
     delete sp;
 
     free_root(&own_root, MYF(0));
@@ -567,6 +568,7 @@ sp_head::sp_head(MEM_ROOT *mem_root_arg, sp_package *parent,
    m_sp_cache_version(0),
    m_creation_ctx(0),
    unsafe_flags(0),
+   new_query_arena_is_set(false),
    m_created(0),
    m_modified(0),
    m_recursion_level(0),
@@ -2790,6 +2792,7 @@ sp_head::reset_thd_mem_root(THD *thd)
   free_list= thd->free_list; // Keep the old list
   thd->free_list= NULL; // Start a new one
   m_thd= thd;
+  new_query_arena_is_set= true;
   DBUG_VOID_RETURN;
 }
 
@@ -2809,7 +2812,7 @@ sp_head::restore_thd_mem_root(THD *thd)
    skip restoration of old arena/mem_root if this method has been
    already called for this routine.
   */
-  if (!m_thd)
+  if (!new_query_arena_is_set)
     DBUG_VOID_RETURN;
 
   Item *flist= free_list;	// The old list
@@ -2820,7 +2823,8 @@ sp_head::restore_thd_mem_root(THD *thd)
                       &mem_root, &thd->mem_root));
   thd->free_list= flist;        // Restore the old one
   thd->mem_root= m_thd_root;
-  m_thd= NULL;
+  new_query_arena_is_set= false;
+
   DBUG_VOID_RETURN;
 }
 

sql/sp_head.h

@@ -262,6 +262,7 @@ class sp_head :private Query_arena,
   */
   uint32 unsafe_flags;
 
+  bool new_query_arena_is_set;
 public:
   inline Stored_program_creation_ctx *get_creation_ctx()
   {