BUG #16817: kill process cause postmaster hang

The following bug has been logged on the website:

Bug reference: 16817
Logged by: Bo Chen
Email address: bchen90(at)163(dot)com
PostgreSQL version: 11.8
Operating system: euleros v2r7 x86_64
Description:

Hi hackers

Recently we encountered a problem that after killed walwriter, we expect
the database can recover normally, but it not (the postmaster hang in the
stat of 'wait dead end'，and the archiver does't exit).
After analysis this problem, we found it could be a bug for a long time.
for archiver now use 'system' to call the configed archive command. For
'system' the linux programmer's manual describe the following 'During
execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT
will be ignored'.

So, when a child chrash, we now just SIGQUIT the archiver just one time,
while the archiver just execute 'system', SIGQUIT will be ignored, then the
posmaster hang in stat of 'wait dead end'.

For this porblem, we now added a SIGUSR2 for archiver after SIGQUIT for
HandleChildCrash. If there any other solution？

regards，ChenBo