Perhaps just wrap each preparation command in an if block?
if { execline-cd data }
Will continue the script if execline-cd succeeds, and abort if it does not.
On Mon, Sep 5, 2022, 1:54 PM Saj Goonatilleke via skaware <
skaware@list.skarnet.org> wrote:
> Hello,
>
> Within the context of, say, an s6 run script that needs to do a little bit
> of
> prep work before execing into a server, it is often helpful to abort early
> if
> any of said prep work stumbles upon an unexpected problem.
>
> Maybe the filesystem is R/O. Or full.
> Maybe an operator has fat-fingered something we need.
> Who knows! Production is full of surprises.
>
> sh provides set -e, which is often satisfactory for simple prep scripts
> (if one is aware of its footguns).
>
> Is there a succinct, idiomatic set -e equivalent in the execline suite?
>
> I have included an example s6 run script below, which demonstrates the
> closest I
> have come so far. fgsigwait is my alternative take on foreground[1].
> fgsigwait
> behaviour differs in the following respects:
>
> * fgsigwait exits[2] if prog1 exits with non-zero status.
> foreground, by comparison, sets the ? environment variable and continues.
> (fgsigwait -! reverts to foreground-ish behaviour.)
>
> * fgsigwait forwards some trappable signals to its child. HUP, INT, etc.
> This is where its name comes from.
> (PID 1 is broken on some of our systems, which provided the initial
> motivation
> for this hack. fgsigwait always waits on prog1 after forwarding a
> signal.
> This, plus execline's execution model, gave me fine control over when
> stuff
> is forked -- and helped us avoid some Z process leaks. Anyhoo.)
>
> fgsigwait otherwise aims to be a syntactic execlineb drop-in for
> foreground in
> cases where these features are desired. Chaining stuff with fgsigwait
> emulates
> sh set -e without needing to repeatedly and explicitly test the ? env var.
>
> --- 8< ---
> #!/usr/bin/env -S execlineb -WP
>
> fdmove -c 2 1
>
> # slow-deathroll is a simple wrapper around
> # github.com/leahneukirchen/snooze
> # extends the s6 one-second restart penalty if we crash too soon
> fgsigwait { slow-deathroll }
>
> # Here is a simple example of some prep work.
> # We want to bail here if any of these things exit non-zero.
> fgsigwait {
> execline-cd data
> if -t { test ! -e live } fgsigwait { mkdir -p empty } ln -s empty live
> }
>
> # maybe a smidge more prep here...
>
> # all prep done. exec into our server.
> nsd -d -c nsd.conf
> --- >8 ---
>
> This does work OK, but, every time I reach for my little program, I can't
> shake
> the feeling that I am probably reinventing some wheel.
>
> fgsigwait was implemented in Go, as is personal tradition when something
> need be
> hacked up in a hurry on the clock. Spooling up the Go runtime eats a few
> extra
> milliseconds of time, and a few megabytes of memory. While not practically
> significant to us -- we don't deploy to tiny machines -- a small part of
> me was
> sad to add bloat. It would be nice if I could get rid of it.
>
> Is there a better way to abort?
>
> I suppose I could just fork sh, but inlining is pleasant when the prep is
> short.
>
> Thanks!
>
>
> [1]: https://skarnet.org/software/execline/foreground.html
> [2]: https://skarnet.org/software/execline/exitcodes.html
>
