MDEV-39141 MariaDB crashes in THD::THD() due to misalignment

fix my_malloc() to return 16-aligned pointers

(type_assoc_array.sp-assoc-array-64bit prints changes in memory_used,
and my_malloc() wastes less memory now)

Author: vuvova

mysys/my_malloc.c

@@ -19,14 +19,23 @@
 #include "mysys_err.h"
 #include <m_string.h>
 
+/*
+  when values won't fit in the provided bits anymore, the options are:
+  * m_size has 2 unused bits because of alignment
+  * m_owner has 4 unused bits because of alignment
+  * if these 6 bits won't help either, HEADER_SIZE has to become 32
+*/
+#define HEADER_M_SIZE_WIDTH 48 /* good for up to 256TB in one malloc */
+#define HEADER_M_KEY_WIDTH  16
+#define HEADER_SIZE 16
+
 struct my_memory_header
 {
   PSI_thread *m_owner;
-  size_t m_size;
-  PSI_memory_key m_key;
+  ulonglong m_size:HEADER_M_SIZE_WIDTH;
+  uint m_key:HEADER_M_KEY_WIDTH;
 };
 typedef struct my_memory_header my_memory_header;
-#define HEADER_SIZE 24
 
 #define USER_TO_HEADER(P) ((my_memory_header*)((char *)(P) - HEADER_SIZE))
 #define HEADER_TO_USER(P) ((char*)(P) + HEADER_SIZE)
@@ -73,6 +82,7 @@ void *my_malloc(PSI_memory_key key, size_t size, myf my_flags)
   void *point;
   DBUG_ENTER("my_malloc");
   DBUG_PRINT("my",("size: %zu flags: %lu", size, my_flags));
+  DBUG_ASSERT(key < 1ULL << HEADER_M_KEY_WIDTH);
   compile_time_assert(sizeof(my_memory_header) <= HEADER_SIZE);
 
   if (!(my_flags & (MY_WME | MY_FAE)))
@@ -81,7 +91,7 @@ void *my_malloc(PSI_memory_key key, size_t size, myf my_flags)
   /* Safety */
   if (!size)
     size=1;
-  if (size > SIZE_T_MAX - 1024L*1024L*16L)           /* Wrong call */
+  if (size > 1ULL << HEADER_M_SIZE_WIDTH)           /* Wrong call */
     DBUG_RETURN(0);
 
   /* We have to align size as we store MY_THREAD_SPECIFIC flag in the LSB */

plugin/type_assoc_array/mysql-test/type_assoc_array/sp-assoc-array-64bit.result

@@ -44,15 +44,15 @@ END;
 /
 CALL p1;
 id	diff
-p0	176
+p0	152
 id	diff
-p1	376
+p1	328
 id	diff
-p2	2624
+p2	2552
 id	diff
-p2upd	2624
+p2upd	2552
 id	diff
-p2del	376
+p2del	328
 diff1
 0
 DROP PROCEDURE p1;