Code review comment for lp:~laurynas-biveinis/percona-server/bp-split-5.6

A Jenkins run of the latest branch turned up bug 1224432. Logged a separate bug because I am not sure I'll manage to debug it during this MP cycle.

"...
2013-09-12 12:15:38 7ff450082700 InnoDB: Assertion failure in thread 140687291459328 in file buf0buf.cc line 3694
InnoDB: Failing assertion: buf_fix_count > 0
...

Which appears to be a race condition on buf_fix_count on a page that is a sentinel for buffer pool watch. How exactly this can happen is not clear to me currently. All the watch sentinel buf_fix_count changes happen under zip_mutex and a corresponding page_hash X lock. Further, buf_page_init_for_read() asserts buf_fix_count > 0 through buf_pool_watch_is_sentinel() at line 3647, thus this should have changed between 3647 and 3694, but the hash is X-latched throughout, even though the zip mutex is only acquired at 3670."