Don't fear the reaper

Or more appropriately, don't fear the subreaper: prctl(PR_SET_CHILD_SUBREAPER,..)

Imagine that a program spawns some resource intensive daemons, and when your program ends you want the daemons to stop as well? Using the Linux specific option to prctl and /proc/pid/stat one can reap and terminate the remaining orphans after your program halts.

Example usage

In the following example, the example-daemon.sh script daemonizes an infinitely sleeping process and spits out its pid. (source)

$ ./example-daemon.sh
21715
$ grep PPid /proc/21715/status
PPid: 1
$ 
$ reaper ./example-daemon.sh
21727
$ stat /proc/21727
stat: cannot stat '/proc/21727': No such file or directory
$ 
$ RUST_LOG=reaper=info reaper ./example-daemon.sh
INFO:reaper: spawned child (pid=21733): ./example-daemon.sh
INFO:reaper: child terminated (pid=21733,exit=0), continuing to reap its orphans
INFO:reaper: sending SIGTERM to orphan (pid=21739)
INFO:reaper: 1 orphan(s) reaped
INFO:reaper: exiting with status=0
21739
$ 
$ timeout --signal=SIGINT 2s ./lingering-parent-with-daemon.sh
21749
$ grep PPid /proc/21749/status
PPid: 1
$ 
$ RUST_LOG=reaper=info timeout --signal=SIGINT 2s reaper ./lingering-parent-with-daemon.sh
INFO:reaper: spawned child (pid=21779): ./lingering-parent-with-daemon.sh
INFO:reaper: child terminated (pid=21779,sig=SIGINT), continuing to reap its orphans
INFO:reaper: sending SIGTERM to orphan (pid=21785)
INFO:reaper: 1 orphan(s) reaped
INFO:reaper: child received signal SIGINT
21785