Bug#25460 - High concurrency MyISAM access causes severe mysqld crash.

Decreased code duplication by calling memory mapping
function through mi_extra().

storage/myisam/mi_open.c

@@ -506,22 +506,6 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
       share->data_file_type = DYNAMIC_RECORD;
     my_afree((gptr) disk_cache);
     mi_setup_functions(share);
-    if (open_flags & HA_OPEN_MMAP)
-    {
-      info.s= share;
-      if (mi_dynmap_file(&info, share->state.state.data_file_length))
-      {
-        /* purecov: begin inspected */
-        /* Ignore if mmap fails. Use file I/O instead. */
-        DBUG_PRINT("warning", ("mmap failed: errno: %d", errno));
-        /* purecov: end */
-      }
-      else
-      {
-        share->file_read= mi_mmap_pread;
-        share->file_write= mi_mmap_pwrite;
-      }
-    }
     share->is_log_table= FALSE;
 #ifdef THREAD
     thr_lock_init(&share->lock);
@@ -552,6 +536,14 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
       }
     }
 #endif
+    /*
+      Memory mapping can only be requested after initializing intern_lock.
+    */
+    if (open_flags & HA_OPEN_MMAP)
+    {
+      info.s= share;
+      mi_extra(&info, HA_EXTRA_MMAP, 0);
+    }
   }
   else
   {