如下所示，即使执行pg_ctl --mode immediate stop，仍然出现pg主进程无法停止的情况。因为walsender进程似乎一直无法停止。

2022-04-09 09:58:52.707936C @ postmaster 00000[2022-03-29 11:23:45 UTC] 0 [69880] LOG: received fast shutdown request
2022-04-09 09:59:20.104209C [unknown] zjh@postgres ::1(46696) walsender 57P03[2022-04-09 09:59:20 UTC] 0 [105097] FATAL: the database system is shutting down
2022-04-09 09:59:50.114885C [unknown] zjh@postgres ::1(46982) walsender 57P03[2022-04-09 09:59:50 UTC] 0 [105229] FATAL: the database system is shutting down
2022-04-09 10:01:20.569748C [unknown] zjh@postgres ::1(47824) walsender 57P03[2022-04-09 10:01:20 UTC] 0 [105720] FATAL: the database system is shutting down
2022-04-09 10:01:50.574433C [unknown] zjh@postgres ::1(48110) walsender 57P03[2022-04-09 10:01:50 UTC] 0 [105842] FATAL: the database system is shutting down
2022-04-09 10:01:57.128295C [unknown] zjh@postgres [local] client backend 57P03[2022-04-09 10:01:57 UTC] 0 [105880] FATAL: the database system is shutting down
2022-04-09 10:02:20.580586C [unknown] zjh@postgres ::1(48392) walsender 57P03[2022-04-09 10:02:20 UTC] 0 [106038] FATAL: the database system is shutting down
2022-04-09 10:02:50.584208C [unknown] zjh@postgres ::1(48676) walsender 57P03[2022-04-09 10:02:50 UTC] 0 [106177] FATAL: the database system is shutting down
2022-04-09 10:03:20.590836C [unknown] zjh@postgres ::1(48958) walsender 57P03[2022-04-09 10:03:20 UTC] 0 [106349] FATAL: the database system is shutting down
2022-04-09 10:03:30.541345C @ postmaster 00000[2022-03-29 11:23:45 UTC] 0 [69880] LOG: received immediate shutdown request
2022-04-09 10:03:50.595911C [unknown] zjh@postgres ::1(49244) walsender 57P03[2022-04-09 10:03:50 UTC] 0 [106474] FATAL: the database system is shutting down
2022-04-09 10:04:20.605568C [unknown] zjh@postgres ::1(49526) walsender 57P03[2022-04-09 10:04:20 UTC] 0 [106635] FATAL: the database system is shutting down

看进程情况：

[zjh@hs-10-20-30-193 log]$ ps axu | grep zjh | grep lightdb
zjh 102008 0.0 0.0 0 0 ? Zs 09:50 0:00 [lightdb] <defunct>
zjh 105400 0.0 0.1 6652464 455388 ? Ss 10:00 0:00 /home/zjh/lightdb-x-dev-server/bin/lightdb -D data
zjh 105405 0.0 0.0 163532 1620 ? Ss 10:00 0:00 lightdb: logger
zjh 105407 0.0 0.0 6652464 1660 ? Ss 10:00 0:00 lightdb: checkpointer
zjh 105408 0.1 0.0 6654008 52308 ? Ss 10:00 0:00 lightdb: background writer
zjh 105409 0.0 0.0 6652464 133092 ? Ss 10:00 0:00 lightdb: walwriter
zjh 105410 0.0 0.0 6656544 2676 ? Ss 10:00 0:00 lightdb: autovacuum launcher
zjh 105411 0.0 0.0 165788 1720 ? Ss 10:00 0:00 lightdb: stats collector
zjh 105412 0.0 0.0 6657516 5284 ? Ss 10:00 0:00 lightdb: pg_cron launcher
zjh 105413 0.0 0.0 6655048 4272 ? Ss 10:00 0:00 lightdb: pg_wait_sampling collector
zjh 105414 0.2 0.0 6656512 2684 ? Ss 10:00 0:01 lightdb: logical replication launcher

经整体查看，一个backend进程处于debug模式，但是已经收到信号QUIT了，但是gdb尚未断开。断开后，defunct（僵尸）进程就没有了。