Oleksandr Byelkin 5fe1d7d076 MDEV-14526: MariaDB keeps crashing under load when query_cache_type is changed ...

The problem was in such scenario:
T1 - starts registering query and locked QC
T2 - starts disabling QC and wait for UNLOCK
T1 - unlock QC
T2 - disable QC and destroy signals without waiting for query unlock
T1 a) - not yet unlocked query in qc and crash on attempt to unlock because
        QC signals are destroyed
   b) if above was done before destruction, it execute end_of results first
      time at exit on after try_lock which see QC disables and return TRUE.
      But it do not reset query_cache_tls->first_query_block which lead to
      second call of end_of_result when diagnostic arena has already
      inappropriate status (not is_eof()).

Fix is:
  1) wait for all queries unlocked before destroying them by locking and
     unlocking
  2) remove query_cache_tls->first_query_block if QC disabled

2018-01-14 10:49:56 +01:00

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