[Bug 277764] daemon(8): high CPU usage after stopping and continuing child process

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277764

            Bug ID: 277764
           Summary: daemon(8): high CPU usage after stopping and
                    continuing child process
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: 5mxnbm6q5@mozmail.com

Using daemon utility to execute a program (child) and then sending a stop
signal (ex. SIGSTOP) to the child process and afterwards sending the continue
signal (SIGCONT) can make the daemon process get stuck at 100% CPU usage.

Full setup:
create a script test.sh containing the following:
```
#!/bin/sh

while true; do echo hello; sleep 1; done
```
execute daemon utility: `daemon -f -p test.pid ./test.sh`
send stop signal: `pkill -STOP -F test.pid`
send continue signal: `pkill -CONT -F test.pid`
wait until test.sh executes `echo hello`
daemon process rises to 100% usage (single core)


As far as i can tell stopping and continuing the child process sends a SIGCHLD
signal and the daemon utility assumes that the child is closing. It then keeps
calling waitpid (which returns immediately, returning 0) inside an infinite
loop in the daemon_is_child_dead function and this causes the high CPU usage.
Also before entering daemon_is_child_dead function, the daemon process is
waiting / listening for child output in the listen_child function. That's why
the above script contains an echo so the daemon will exit that function and go
on to process the SIGCHLD event.

Maybe the fix could be to include WCONTINUED option and setting status output
parameter (currently set to NULL) when calling waitpid inside
daemon_is_child_dead and then checking if WIFCONTINUED(status) equals True and
in that case returning false?

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