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Re: Possible race condition or NULL pointer triggered by OQGraph


Hi, Andrew!

On Jun 02, Andrew McDonnell wrote:
> Hi all
> (Cross-posted to oqgraph-developers, maria-developers)
> I am trying to track down a segfault apparently triggered by concurrent
> execution of queries through OQGraph - for context, see:
> https://mariadb.atlassian.net/browse/MDEV-6282
> I am however a bit confused as to what is going on, because at least some of
> the information leads me to suspect that there could be more going on here
> than just a simple race condition.
> --Immediate Cause--
> On the surface, a segfault appears to be caused in a method in sql_class.h,
> Statement::check_limit_rows_examined() dereferencing a NULL pointer, `lex`.
> The method check_limit_rows_examined() was called from sql_class.h method
> handler::increment_statistics() .

I don't see how thd->lex could be NULL there. It shouldn't be changed
concurrently on anything.

Can you actually repeat the crash?


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