Preventing killing of child processes?

[Sidney San Martín]

TN2083 explains that launchd kills child processes when the parent
exits, and suggests using setsid() as a workaround when a child needs
to stay alive.

I'm in that situation, (see
http://lists.macosforge.org/pipermail/launchd-dev/2009-September/000664.html
for details, I want to change the StartInterval of my job and use a
helper tool to un/reload myself from launchd), but I can't get my
helper tool into a state where it's not killed. Calling setsid() from
either the daemon (before launching the helper) or from the helper
tool itself returns -1 and it's killed off by launchd as soon as the
job is unloaded.

What's the right way to use setsid() (or another approach) here?
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[Dave Keck]

Calling setpgrp() from your child should prevent launchd from killing it.
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[Sidney San Martín]

That doesn't seem to be doing it. The child process still exits as
soon as it unloads the parent job from launchd.

On Tue, Oct 6, 2009 at 7:09 PM, Dave Keck <davekeck@...> wrote:
> Calling setpgrp() from your child should prevent launchd from killing it.
>
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[Dave Keck]

> That doesn't seem to be doing it. The child process still exits as
> soon as it unloads the parent job from launchd.

Hmm, I just did a controlled test, and it certainly does the trick for
me on 10.5.8. Here's what I did:

===== parent_process.c

#include <stdio.h>
#include <unistd.h>

int main(int argc, const char *argv[])
{
    if (!fork())
    {
        execl("/child_process", "/child_process");
        _exit(0);
    }
    sleep(10);
    return 0;
}

===== child_process.c

#include <stdio.h>
#include <unistd.h>

int main(int argc, const char *argv[])
{
    setpgrp();
    sleep(60);
    return 0;
}

===== com.parent.parent_process.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.parent.parent_process</string>
<key>ProgramArguments</key>
<array>
<string>/parent_process</string>
</array>
<key>RunAtLoad</key>
<true/>
</dict>
</plist>

=====

Compile parent_process.c and child_process.c, and put them and the
plist in your drive's root directory. Then:

    launchctl load /com.parent.parent_process.plist

You'll notice that the parent exits after 10 seconds, while the child
stays alive for a full minute. Likewise, if you unload the parent
early, the child still stays alive past the parent's death. Without
setpgrp(), the child dies with the parent. Does that help?
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[Sidney San Martín]

That did help — and as it turns out, my problem wasn't launchd, but a
missed SIGPIPE at the same time. I should have debugged it out more
before posting to the list.

Thanks, Dave.

On Fri, Oct 9, 2009 at 7:50 PM, Dave Keck <davekeck@...> wrote:

>> That doesn't seem to be doing it. The child process still exits as
>> soon as it unloads the parent job from launchd.
>
> Hmm, I just did a controlled test, and it certainly does the trick for
> me on 10.5.8. Here's what I did:
>
> ===== parent_process.c
>
> #include <stdio.h>
> #include <unistd.h>
>
> int main(int argc, const char *argv[])
> {
>    if (!fork())
>    {
>        execl("/child_process", "/child_process");
>        _exit(0);
>    }
>    sleep(10);
>    return 0;
> }
>
> ===== child_process.c
>
> #include <stdio.h>
> #include <unistd.h>
>
> int main(int argc, const char *argv[])
> {
>    setpgrp();
>    sleep(60);
>    return 0;
> }
>
> ===== com.parent.parent_process.plist
>
> <?xml version="1.0" encoding="UTF-8"?>
> <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
> "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
> <plist version="1.0">
> <dict>
> <key>Label</key>
> <string>com.parent.parent_process</string>
> <key>ProgramArguments</key>
> <array>
> <string>/parent_process</string>
> </array>
> <key>RunAtLoad</key>
> <true/>
> </dict>
> </plist>
>
> =====
>
> Compile parent_process.c and child_process.c, and put them and the
> plist in your drive's root directory. Then:
>
>    launchctl load /com.parent.parent_process.plist
>
> You'll notice that the parent exits after 10 seconds, while the child
> stays alive for a full minute. Likewise, if you unload the parent
> early, the child still stays alive past the parent's death. Without
> setpgrp(), the child dies with the parent. Does that help?
>

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