BUG#23526 - show table status reports incorrect values for MyISAM tables

This problem could happen when show table status get outdated copy
of TABLE object from table cache.

MyISAM updates state info when external_lock() method is called. Though
I_S does not lock a table to avoid deadlocks. If I_S opens a table which
is in a table cache it will likely get outdated state info copy.

In this case shared state copy is more recent than local copy. This problem
is fixed by correctly restoring myisam state info pointer back to original
value, that is to shared state.

Affects MyISAM only. No good deterministic test case for this fix.

include/thr_lock.h

@@ -122,6 +122,7 @@ typedef struct st_thr_lock {
   void (*get_status)(void*, int);	/* When one gets a lock */
   void (*copy_status)(void*,void*);
   void (*update_status)(void*);		/* Before release of write */
+  void (*restore_status)(void*);         /* Before release of read */
   my_bool (*check_status)(void *);
 } THR_LOCK;
 

myisam/mi_locking.c

@@ -326,6 +326,15 @@ void mi_update_status(void* param)
   }
 }
 
+
+void mi_restore_status(void *param)
+{
+  MI_INFO *info= (MI_INFO*) param;
+  info->state= &info->s->state.state;
+  info->append_insert_at_end= 0;
+}
+
+
 void mi_copy_status(void* to,void *from)
 {
   ((MI_INFO*) to)->state= &((MI_INFO*) from)->save_state;

myisam/mi_open.c

@@ -505,6 +505,7 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
 	share->lock.get_status=mi_get_status;
 	share->lock.copy_status=mi_copy_status;
 	share->lock.update_status=mi_update_status;
+        share->lock.restore_status= mi_restore_status;
 	share->lock.check_status=mi_check_status;
       }
     }

myisam/myisamdef.h

@@ -732,6 +732,7 @@ int mi_unique_comp(MI_UNIQUEDEF *def, const byte *a, const byte *b,
 		   my_bool null_are_equal);
 void mi_get_status(void* param, int concurrent_insert);
 void mi_update_status(void* param);
+void mi_restore_status(void* param);
 void mi_copy_status(void* to,void *from);
 my_bool mi_check_status(void* param);
 void mi_disable_non_unique_index(MI_INFO *info, ha_rows rows);

mysys/thr_lock.c

@@ -758,8 +758,16 @@ void thr_unlock(THR_LOCK_DATA *data)
   }
   else
     lock->write.last=data->prev;
-  if (lock_type >= TL_WRITE_CONCURRENT_INSERT && lock->update_status)
-    (*lock->update_status)(data->status_param);
+  if (lock_type >= TL_WRITE_CONCURRENT_INSERT)
+  {
+    if (lock->update_status)
+      (*lock->update_status)(data->status_param);
+  }
+  else
+  {
+    if (lock->restore_status)
+      (*lock->restore_status)(data->status_param);
+  }
   if (lock_type == TL_READ_NO_INSERT)
     lock->read_no_write_count--;
   data->type=TL_UNLOCK;				/* Mark unlocked */