Bug#25541037: MYSQL BUG ON DELETE

Description:- MyISAM table gets corrupted with concurrent
executions of INSERT, DELETE statements in a particular
sequence.

Analysis:- Due to the inappropriate manipulation of w_lock
and r_lock associated with a MyISAM table, there arises a
scenario where the table's state information becomes
invalid.

Fix:- A lock is introduced to resolve this issue.

storage/myisam/ha_myisam.cc

 /*
-   Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -1172,10 +1172,14 @@ int ha_myisam::repair(THD *thd, MI_CHECK &param, bool do_optimize)
     if (file->s->base.auto_key)
       update_auto_increment_key(&param, file, 1);
     if (optimize_done)
+    {
+      mysql_mutex_lock(&share->intern_lock);
       error = update_state_info(&param, file,
 				UPDATE_TIME | UPDATE_OPEN_COUNT |
 				(local_testflag &
 				 T_STATISTICS ? UPDATE_STAT : 0));
+      mysql_mutex_unlock(&share->intern_lock);
+    }
     info(HA_STATUS_NO_LOCK | HA_STATUS_TIME | HA_STATUS_VARIABLE |
 	 HA_STATUS_CONST);
     if (rows != file->state->records && ! (param.testflag & T_VERY_SILENT))

storage/myisam/mi_check.c

 /*
-   Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -4474,6 +4474,10 @@ int update_state_info(MI_CHECK *param, MI_INFO *info,uint update)
     int error;
     uint r_locks=share->r_locks,w_locks=share->w_locks;
     share->r_locks= share->w_locks= share->tot_locks= 0;
+
+    DBUG_EXECUTE_IF("simulate_incorrect_share_wlock_value",
+                    DEBUG_SYNC_C("after_share_wlock_set_to_0"););
+
     error=_mi_writeinfo(info,WRITEINFO_NO_UNLOCK);
     share->r_locks=r_locks;
     share->w_locks=w_locks;

storage/myisam/mi_locking.c

-/* Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -229,6 +229,10 @@ int mi_lock_database(MI_INFO *info, int lock_type)
       info->invalidator=info->s->invalidator;
       share->w_locks++;
       share->tot_locks++;
+
+      DBUG_EXECUTE_IF("simulate_incorrect_share_wlock_value",
+                      DEBUG_SYNC_C("after_share_wlock_increment"););
+
       info->s->in_use= list_add(info->s->in_use, &info->in_use);
       break;
     default: