Re: Working on shell for perp/s6/etc., need advice re logging

From: Colin Booth <>
Date: Tue, 28 Jul 2015 08:58:14 -0700

On Tue, Jul 28, 2015 at 12:07 AM, Ido Perlmuter <> wrote:
> Hello there.
> Currently, I've implemented the "fg" command in a pretty dumb way: I read
> the service's rc.log file for perp or log/run file for s6, and take the
> last argument to tinylog or s6-log to figure out where log files are
> stored. Of course, this is a very bad way to do this, since I'm assuming
> the services are using tinylog/s6-log, and I'm using a regular expression
> that could easily miss.
You're not going to be able to get a separate view of stdout and
stderr since you've redirected those to the same place. As long as
that isn't an issue it should be pretty easy to solve this the right
way instead of using strace, funky interstitial logging scripts,
parsing the log script, etc. Assuming you're on linux and the output
file descriptor is stable, "the right way" is to use procfs to
directly read against the file descriptor that the logger is
outputting to. I don't know which file descriptors tinylog uses but
s6-log uses fd 4 for its file.

The below is for s6-log, something similar is doable with perp:
Use s6-svstat to find out the logger pid, parse the output for the
process id, then readlink /proc/$loggerpid/fd/4 to get the logger
output location (or, if you're feeling lazy, just tail the fd
directly). Caveats: you'll need account access to read
$svcdir/$svc/log/supervise as well as /proc/$pid/fd, and the current
fd 4 will stop being useful when a file rotation happens.

The first permission issue can be solved by doctoring the permissions
to the supervise directory beforehand (711 is safe and
supervise/status is mode 444 already), the second permission issue
cannot be solved that way since procfs resists chmod attempts. With
correct sudoers access to a wrapper program much of the privileged
access can be done safely by non-privileged folks, though standard
"limited" sudo access caveats apply (don't allow untrusted people
update the script in sounce control, etc). Either way though, this
method is cleaner than parsing the log/run script and is less prone to
misinformation (if the run script changes to point elsewhere but
s6-log hasn't restared, parsing the script will fail).
> I'm looking for some advice how reading the service's stdout/stderr streams
> could be done in a more fool proof, general way. The only way I know to tap
> into a process' output streams is via strace, but that means the user will
> have to install it, and run it as root, so that's not good.
Strace can be run as non-root, straces limitations are the same as any
other program: you need access to the calling user account. It's not
quite as limited as you're thinking but still a bad solution since any
time you're calling ptrace on a program you're adjusting its internal
state a little bit, and some badly written programs don't respond well
to that. Having strace installed everywhere isn't a bad idea for other
reasons, but using it for log reading is.

More genenerally speaking, unless your goal is to write a full
middleware translator between supervisorctl and process supervisors,
some portions (like the logging part) are going to be really fragile.
s6-log uses fd 4 for current, svlogd uses fd 6, but there's nothing
stopping a developer from using logger(1) (or something more esoteric
[0]) in their log/run script at which point all that "find the true
destination of the log stream" stuff is totally moot.

Also, fully expect people to do stupid things: I have both runit and
s6 active on my workstation, and until recently had a few services
supervised under s6 but logging via either svlogd (runit) or multilog
(daemontools). Not that any of those are bad, but you can't rely on
people to use the "right" logger with a supervision system unless you
go the route of supervisord and don't offer that choice in the first

I'd say write the management compatibility layer (which it sounds like
you already have), and then use the energy you'd spend fighting unix
pipes on teaching people how to make the most out of perp.


[0] Most of the services at work don't log anything locally and
instead send data down a zmq socket to N log brokers that forward
along to their subscribers. It's pretty heavy-weight, really slick,
and mostly functional.

"If the doors of perception were cleansed every thing would appear to
man as it is, infinite. For man has closed himself up, till he sees
all things thru' narrow chinks of his cavern."
  --  William Blake
Received on Tue Jul 28 2015 - 15:58:14 UTC

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