Re: Cleaning up background processes

In article <slrnv3v08i.17e3.naddy@lorvorc.mips.inka.de>,
Christian Weisgerber  <naddy@mips.inka.de> wrote:

On 2024-05-06, Kenny McCormack <gazelle@shell.xmission.com> wrote:
>

Having said that, I think we are all making our own assumptions about what
the actual, underlying problem is.  Given that OP is not a newbie, it would
help a lot if he would clarify what exact situation he is dealing with,
rather than have us all guess (which is SOP when the poster *is* a newbie).

>
As part of a regression test suite, somebody wrote something like
this in a script:
>
       ./http-server &
       trap "kill %1" HUP INT QUIT PIPE TERM
       sleep 1 # server starts up
>
       [... Tests ...]
>
       kill %1
       wait %1 # wait for http-server
>
Using job control syntax in a non-interactive shell is already wrong,
although, to my surprise, it works as intended in common shells.

Thanks for the followup.  I agree that using job control syntax in a
(non-interactive) script is weird and to be avoided - even if it does "work".
It should be good enough to just store the value of $! and then use that
later on.

I also believe that there is no "silver bullet" solution to this problem.
There should be; it is shame that there isn't.  You really should not even
find yourself in the situation of having to hunt down and kill wayward
processes when something unexpected happens in a script, but the fact is
you do.

For an example of this, I give you: sshfs.  I love sshfs, but it is a fact
that if you try to sshfs-mount something that "isn't there" (for various
values of "isn't there"), it creates a mess that takes several steps to
undo (clean up).  I have successfully tackled this problem, but it is
non-trivial, which is not unexpected, considering how many moving parts are
involved in an sshfs mount operation.

I've been looking at how to replace this with a portable-ish,
reliable-ish solution.  An obvious step is to use a standard
background process reference $! instead of job control %1.
Less obvious is the question how to deal with signals that abort
the script, without leaving background processes hanging around.

Someone mentioned using prctl() and PR_DEATHSIG earlier.  I think that's a
good idea, albeit not universal and, of course, Linux-specific.  I think it
would work in your specific case of a wayward http server.

--
Faith doesn't give you the answers; it just stops you from asking the questions.