Partial documentation due to time constraints. Will improve over time.
Also removed a redundant parameter link_idx from
spider_get_sys_tables_connect_info().
And deleted some commented out code.
We remove the call to update spider persistent table stats (sts/crd)
in spider_free_share(). This prevents spider from opening and closing
further tables during close(), which fixes the following issues:
MDEV-28739: ha_spider::close() is called during tdc_start_shutdown(),
which is called after query_cache_destroy(). Closing the sts/crd Aria
tables will trigger a call to Query_cache::invalidate_table(), which
will attempt to use the query cache mutex structure_guard_mutex
destroyed previously.
MDEV-29421: during ha_spider::close(), spider_free_share() could
trigger another spider_free_share() through updating sts/crd table,
because open_table() calls tc_add_table(), which could trigger another
ha_spider::close()...
Since spider sts/crd system tables are only updated here, there's no
use for these tables any more, and we remove all uses of these tables
too.
The removal should not cause any performance issue, as in memory
spider table stats are only updated based on a time
interval (spider_sts_interval and spider_crd_interval), which defaults
to 10 seconds. It should not affect accuracy either, due to the
infrequency of server restart. And inaccurate stats are not a problem
for optimizer anyway.
To be on the safe side, we defer the removal of the spider sts/crd
tables themselves to future.
Extracted out common subroutines, gave more meaningful names etc,
added comments etc.
Also:
- Documented active servers load balancing reads, and other fields in
SPIDER_SHARE etc.
- Removed commented out code
- Documented and refactored self-reference check
- Removed some unnecessary functions
- Renamed unhelpful roop_count
- Refactored spider_get_{sts,crd}, where we turn get_type into an enum
- Cleaned up spider_mbase_handler::show_table_status() and
spider_mbase_handler::show_index()
