MDEV-33209 Stack overflow in main.json_debug_nonembedded due to incorrect debug injection

In the JSON functions, the debug injection for stack overflows is
inaccurate and may cause actual stack overflows. Let us simply
inject stack overflow errors without actually relying on the ability
of check_stack_overrun() to do so.

Reviewed by: Rucha Deodhar

Author: dr-m

sql/item_jsonfunc.cc

@@ -20,20 +20,14 @@
 #include "item.h"
 #include "sql_parse.h" // For check_stack_overrun
 
-/*
-  Allocating memory and *also* using it (reading and
-  writing from it) because some build instructions cause
-  compiler to optimize out stack_used_up. Since alloca()
-  here depends on stack_used_up, it doesnt get executed
-  correctly and causes json_debug_nonembedded to fail
-  ( --error ER_STACK_OVERRUN_NEED_MORE does not occur).
-*/
-#define ALLOCATE_MEM_ON_STACK(A) do \
-                              { \
-                                uchar *array= (uchar*)alloca(A); \
-                                bzero(array, A); \
-                                my_checksum(0, array, A); \
-                              } while(0)
+#ifndef DBUG_OFF
+static int dbug_json_check_min_stack_requirement()
+{
+  my_error(ER_STACK_OVERRUN_NEED_MORE, MYF(ME_FATAL),
+           my_thread_stack_size, my_thread_stack_size, STACK_MIN_SIZE);
+  return 1;
+}
+#endif
 
 /*
   Compare ASCII string against the string with the specified
@@ -151,11 +145,8 @@ int json_path_parts_compare(
   int res, res2;
 
   DBUG_EXECUTE_IF("json_check_min_stack_requirement",
-                  {
-                    long arbitrary_var;
-                    long stack_used_up= (available_stack_size(current_thd->thread_stack, &arbitrary_var));
-                    ALLOCATE_MEM_ON_STACK(my_thread_stack_size-stack_used_up-STACK_MIN_SIZE);
-                  });
+                  return dbug_json_check_min_stack_requirement(););
+
   if (check_stack_overrun(current_thd, STACK_MIN_SIZE , NULL))
     return 1;
 
@@ -1210,11 +1201,7 @@ static int check_contains(json_engine_t *js, json_engine_t *value)
   json_engine_t loc_js;
   bool set_js;
   DBUG_EXECUTE_IF("json_check_min_stack_requirement",
-                  {
-                    long arbitrary_var;
-                    long stack_used_up= (available_stack_size(current_thd->thread_stack, &arbitrary_var));
-                    ALLOCATE_MEM_ON_STACK(my_thread_stack_size-stack_used_up-STACK_MIN_SIZE);
-                  });
+                  return dbug_json_check_min_stack_requirement(););
   if (check_stack_overrun(current_thd, STACK_MIN_SIZE , NULL))
     return 1;
 
@@ -2128,13 +2115,8 @@ String *Item_func_json_object::val_str(String *str)
 
 static int do_merge(String *str, json_engine_t *je1, json_engine_t *je2)
 {
-
   DBUG_EXECUTE_IF("json_check_min_stack_requirement",
-                  {
-                    long arbitrary_var;
-                    long stack_used_up= (available_stack_size(current_thd->thread_stack, &arbitrary_var));
-                    ALLOCATE_MEM_ON_STACK(my_thread_stack_size-stack_used_up-STACK_MIN_SIZE);
-                  });
+                  return dbug_json_check_min_stack_requirement(););
   if (check_stack_overrun(current_thd, STACK_MIN_SIZE , NULL))
     return 1;
 
@@ -2471,11 +2453,7 @@ static int do_merge_patch(String *str, json_engine_t *je1, json_engine_t *je2,
                           bool *empty_result)
 {
   DBUG_EXECUTE_IF("json_check_min_stack_requirement",
-                  {
-                    long arbitrary_var;
-                    long stack_used_up= (available_stack_size(current_thd->thread_stack, &arbitrary_var));
-                    ALLOCATE_MEM_ON_STACK(my_thread_stack_size-stack_used_up-STACK_MIN_SIZE);
-                  });
+                  return dbug_json_check_min_stack_requirement(););
   if (check_stack_overrun(current_thd, STACK_MIN_SIZE , NULL))
     return 1;