run one off tasks in a docker swarm context

docker_task.sh

#!/usr/bin/env bash

set -euo pipefail

svc=$1

replicas () {
    docker service ls -f name=$svc --format '{{.Replicas}}'
}

# make sure it's at 0 running instances
[[ "$(replicas)" == "0/0" ]] || docker service scale $svc=0 > /dev/null

# scale it up, tail the logs
# todo we'll have to scale it to N=num_backend_replica once we go beyond a single backend instance
# and we're also in trouble once we go beyond 1 backend running per host, but that'll require more changes anyway
docker service scale $svc=1 -d
docker service logs $svc --raw --no-trunc -f --since 1s &

# wait until task starts running, then is not running any more, then kill the log tailing process
sleep 2
# DISCLAIMER: this is going to break for really short tasks, don't have a good solution yet
until [[ "$(replicas)" =~ ^"1/" ]]; do sleep 1; done
until [[ "$(replicas)" =~ ^"0/" ]]; do sleep 1; done
sleep 1 # make sure we got the logs
kill %1