high load

MySQL server could crash if two or more threads would initiate query
cache resize at the moments very close in time.

The problem was introduced with the fix of bug 21051 in 5.0 and 5.1:
simultaneous query cache resizes would wait for the first one in
progress, but then each thread would try to finish the operation,
accessing the data that was already reset (attempt to dereference
'bins' pointer, which may be NULL already).

The solution is to check after synchronization if another thread has
done the reset already (test 'query_cache_size > 0' again).

No test case is provided because the bug is a subject to a race.


sql/sql_cache.cc:
  We release 'structure_guard_mutex' in flush_cache(), so after the
  call we check if another thread had reset the cache before us.