Modify the cond_var usage in the bg_task.fetch_task()

[jd-zhang]

Issue migrated from trac ticket # 789

component: kunlun-storage | priority: major

2022-06-01 19:41:40: snow@zettadb.com created the issue

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Miss use the cond_var in the BG_task *fetch_task() may leads tricky mysqld crash.\\
mysqld: ../nptl/pthread_mutex_lock.c:81: __pthread_mutex_lock: Assertion `mutex->__data.__owner == 0' failed.\

(gdb) bt
#0 0x00007ff2927358d5 in pthread_kill () from /lib64/libpthread.so.0
#1 0x000000000119aa35 in handle_fatal_signal (sig=6) at /home/kunlun/debugbuild/percona-server/sql/signal_handler.cc:194
#2
#3 0x00007ff290cb637f in raise () from /lib64/libc.so.6
#4 0x00007ff290ca0db5 in abort () from /lib64/libc.so.6
#5 0x00007ff290ca0c89 in __assert_fail_base.cold.0 () from /lib64/libc.so.6
#6 0x00007ff290caea76 in __assert_fail () from /lib64/libc.so.6
#7 0x00007ff292730b61 in pthread_mutex_lock () from /lib64/libpthread.so.0
#8 0x000000000125b172 in Scoped_pthread_mutex::Scoped_pthread_mutex (mtx=..., this=) at /home/kunlun/debugbuild/percona-server/sql/bg_task_exec.cc:29
#9 BG_task::mark_done (errmsg=0x0, err=0, this=0x7ff1f3c32200) at /home/kunlun/debugbuild/percona-server/sql/bg_task_exec.cc:118
#10 BG_rotate_purge::execute (this=0x7ff1f3c32200) at /home/kunlun/debugbuild/percona-server/sql/bg_task_exec.cc:206
#11 0x000000000125a64b in bg_task_worker (param0=0x3fbab80 <bg_work_params>) at /home/kunlun/debugbuild/percona-server/sql/bg_task_exec.cc:331
#12 0x00007ff29272e14a in start_thread () from /lib64/libpthread.so.0
#13 0x00007ff290d7bdc3 in clone () from /lib64/libc.so.6