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Re: a83d6ee812b: Safe session_track_system_variables snapshot


Hi, Sergey!

Just one question.

Why do you need Session_sysvars_tracker::session_track_system_variables
if you have THD::system_variables::session_track_system_variables ?

On Mar 25, Sergey Vojtovich wrote:
> revision-id: a83d6ee812b (mariadb-10.3.12-94-ga83d6ee812b)
> parent(s): 3c6f17a130b
> author: Sergey Vojtovich <svoj@xxxxxxxxxxx>
> committer: Sergey Vojtovich <svoj@xxxxxxxxxxx>
> timestamp: 2019-03-19 23:40:08 +0400
> message:
> Safe session_track_system_variables snapshot
> When enabling system variables tracker, make a copy of
> global_system_variables.session_track_system_variables
> under LOCK_global_system_variables. This protects from concurrent variable
> updates and potential freed memory access, as well as from variable
> reconstruction (which was previously protected by LOCK_plugin).
> We can also use this copy as a session variable pointer, so that we don't
> have to allocate memory and reconstruct it every time it is referenced.
> For this very reason we don't need buffer_length stuff anymore.
> As well as don't we need to take LOCK_plugin early in ::enable().
> Unified ::parse_var_list() to acquire LOCK_plugin unconditionally.
> For no apparent reason it wasn't previously acquired for global
> variable update.
> No reason to access session_sysvars_tracker via get_tracker(), so access
> it directly instead.
> Part of MDEV-14984 - regression in connect performance

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