MDEV-6156: Parallel replication incorrectly caches charset between worker threads

The previous patch for this bug was unfortunately completely wrong.

The purpose of cached_charset is to remember which character set we
have installed currently in the THD, so that in the common case where
charset does not change between queries, we do not need to update it
in the THD. Thus, it is important that the cached_charset field is
tightly coupled to the THD for which it handles caching.

Thus the right place to put cached_charset seems to be in the THD.
This patch introduces a field THD:system_thread_info where such info
in general can be placed without further inflating the THD with unused
data for other threads (THD is already far too big as it is). It then
moves the cached_charset into this slot for the SQL driver thread and
for the parallel replication worker threads.

The THD::rpl_filter field is also moved inside system_thread_info, to
keep the size of THD unchanged. Moving further fields in to reduce the
size of THD is a separate task, filed as MDEV-6164.

1 files changed, 10 insertions, 0 deletions

diff --git a/sql/rpl_mi.h b/sql/rpl_mi.h
index a136af38356..af739b1dad4 100644
--- a/sql/rpl_mi.h
+++ b/sql/rpl_mi.h
@@ -216,6 +216,16 @@ public:
   bool stop_all_slaves(THD *thd);
 };
 
+
+/*
+  The class rpl_io_thread_info is the THD::system_thread_info for the IO thread.
+*/
+class rpl_io_thread_info
+{
+public:
+};
+
+
 bool check_master_connection_name(LEX_STRING *name);
 void create_logfile_name_with_suffix(char *res_file_name, size_t length,
                              const char *info_file,