cleanup THR_KEY_mysys

read TLS with my_thread_var
write TLS with set_mysys_var()

my_thread_var is no longer __attribute__ ((const)): this attribute
is simply incorrect here. Read gcc manual for more information.
sql/threadpool_generic.cc fails with that attribute.

include/my_pthread.h

@@ -729,13 +729,14 @@ struct st_my_thread_var
 #endif
 };
 
-extern struct st_my_thread_var *_my_thread_var(void) __attribute__ ((const));
+struct st_my_thread_var *_my_thread_var(void);
 extern void **my_thread_var_dbug(void);
 extern safe_mutex_t **my_thread_var_mutex_in_use(void);
 extern uint my_thread_end_wait_time;
 extern my_bool safe_mutex_deadlock_detector;
 #define my_thread_var (_my_thread_var())
 #define my_errno my_thread_var->thr_errno
+int set_mysys_var(struct st_my_thread_var *mysys_var);
 /*
   Keep track of shutdown,signal, and main threads so that my_end() will not
   report errors with them

mysys/my_thr_init.c

@@ -141,7 +141,7 @@ void my_thread_global_reinit(void)
   my_thread_destroy_internal_mutex();
   my_thread_init_internal_mutex();
 
-  tmp= my_pthread_getspecific(struct st_my_thread_var*, THR_KEY_mysys);
+  tmp= my_thread_var;
   DBUG_ASSERT(tmp);
 
   my_thread_destory_thr_mutex(tmp);
@@ -279,7 +279,7 @@ my_bool my_thread_init(void)
   fprintf(stderr,"my_thread_init(): pthread_self: %p\n", pthread_self());
 #endif  
 
-  if (my_pthread_getspecific(struct st_my_thread_var *,THR_KEY_mysys))
+  if (my_thread_var)
   {
 #ifdef EXTRA_DEBUG_THREADS
     fprintf(stderr,"my_thread_init() called more than once in thread 0x%lx\n",
@@ -297,7 +297,7 @@ my_bool my_thread_init(void)
     error= 1;
     goto end;
   }
-  pthread_setspecific(THR_KEY_mysys,tmp);
+  set_mysys_var(tmp);
   tmp->pthread_self= pthread_self();
   my_thread_init_thr_mutex(tmp);
 
@@ -334,7 +334,7 @@ my_bool my_thread_init(void)
 void my_thread_end(void)
 {
   struct st_my_thread_var *tmp;
-  tmp= my_pthread_getspecific(struct st_my_thread_var*,THR_KEY_mysys);
+  tmp= my_thread_var;
 
 #ifdef EXTRA_DEBUG_THREADS
   fprintf(stderr,"my_thread_end(): tmp: %p  pthread_self: %p  thread_id: %ld\n",
@@ -357,7 +357,7 @@ void my_thread_end(void)
     as the key is used by DBUG.
   */
   DBUG_POP();
-  pthread_setspecific(THR_KEY_mysys,0);
+  set_mysys_var(NULL);
 
   if (tmp && tmp->init)
   {
@@ -441,7 +441,7 @@ extern void **my_thread_var_dbug()
   struct st_my_thread_var *tmp;
   if (!my_thread_global_init_done)
     return NULL;
-  tmp= my_pthread_getspecific(struct st_my_thread_var*,THR_KEY_mysys);
+  tmp= my_thread_var;
   return tmp && tmp->init ? &tmp->dbug : 0;
 }
 #endif /* DBUG_OFF */
@@ -453,7 +453,7 @@ safe_mutex_t **my_thread_var_mutex_in_use()
   struct st_my_thread_var *tmp;
   if (!my_thread_global_init_done)
     return NULL;
-  tmp=  my_pthread_getspecific(struct st_my_thread_var*,THR_KEY_mysys);
+  tmp= my_thread_var;
   return tmp ? &tmp->mutex_in_use : 0;
 }
 

sql/threadpool_common.cc

@@ -45,7 +45,6 @@ static void  threadpool_remove_connection(THD *thd);
 static int   threadpool_process_request(THD *thd);
 static THD*  threadpool_add_connection(CONNECT *connect, void *scheduler_data);
 
-extern "C" pthread_key(struct st_my_thread_var*, THR_KEY_mysys);
 extern bool do_command(THD*);
 
 static inline TP_connection *get_TP_connection(THD *thd)
@@ -87,7 +86,7 @@ struct Worker_thread_context
 #ifdef HAVE_PSI_THREAD_INTERFACE
     psi_thread = PSI_THREAD_CALL(get_thread)();
 #endif
-    mysys_var= (st_my_thread_var *)pthread_getspecific(THR_KEY_mysys);
+    mysys_var= my_thread_var;
   }
 
   void restore()
@@ -95,7 +94,7 @@ struct Worker_thread_context
 #ifdef HAVE_PSI_THREAD_INTERFACE
     PSI_THREAD_CALL(set_thread)(psi_thread);
 #endif
-    pthread_setspecific(THR_KEY_mysys,mysys_var);
+    set_mysys_var(mysys_var);
     pthread_setspecific(THR_THD, 0);
   }
 };
@@ -141,7 +140,7 @@ static inline void set_thd_idle(THD *thd)
 */
 static void thread_attach(THD* thd)
 {
-  pthread_setspecific(THR_KEY_mysys,thd->mysys_var);
+  set_mysys_var(thd->mysys_var);
   thd->thread_stack=(char*)&thd;
   thd->store_globals();
 #ifdef HAVE_PSI_THREAD_INTERFACE
@@ -228,9 +227,9 @@ static THD* threadpool_add_connection(CONNECT *connect, void *scheduler_data)
     Store them in THD.
   */
 
-  pthread_setspecific(THR_KEY_mysys, 0);
+  set_mysys_var(NULL);
   my_thread_init();
-  st_my_thread_var* mysys_var= (st_my_thread_var *)pthread_getspecific(THR_KEY_mysys);
+  st_my_thread_var* mysys_var= my_thread_var;
   if (!mysys_var ||!(thd= connect->create_thd(NULL)))
   {
     /* Out of memory? */