MDEV-33867 main.query_cache_debug fails with heap-use-after-free

What's happening: 1. Query_cache::insert() locks the QC and verifies that it's enabled 2. parallel thread tries to disable it. trylock fails (QC is locked) so the status becomes DISABLE_REQUEST 3. Query_cache::insert() calls Query_cache::write_result_data() which allocates a new block and unlocks the QC. 4. Query_cache::unlock() notices there are no more QC users and a pending DISABLE_REQUEST so it disables the QC and frees all the memory, including the new block that was just allocated 5. Query_cache::write_result_data() proceeds to write into the freed block Fix: change m_cache_status under a mutex. Approved by Oleksandr Byelkin <sanja@mariadb.com>

Author: vuvova

sql/sql_cache.cc

@@ -2530,14 +2530,9 @@ void Query_cache::destroy()
 
 void Query_cache::disable_query_cache(THD *thd)
 {
+ lock(thd);
  m_cache_status= DISABLE_REQUEST;
- /*
- If there is no requests in progress try to free buffer.
- try_lock(TRY) will exit immediately if there is lock.
- unlock() should free block.
- */
- if (m_requests_in_progress == 0 && !try_lock(thd, TRY))
- unlock();
+ unlock();
 }