#! /usr/bin/env bash

# Generate systemd .service file for QEMU/KVM virtual machines.
#
# Normally the machines are run from a dedicated user account with its home
# directory containing all the relevant files (management scripts, images,
# configurations, and sockets). However, this can be overriden with the
# following options
#
# --home <dir>
#   The virtual machines "home" directory. If unspecified, the user's home
#   directory is assumed.
#
# --bin <dir>
#   The virtual machines management scripts directory. If unspecified,
#   <home>/bin is assumed. If specified as relative, then assumed relative
#   to <home>.
#
# --etc <dir>
#   The virtual machines configuration files directory. If unspecified,
#   <home>/etc is assumed. If specified as relative, then assumed relative
#   to <home>.
#
# --var <dir>
#   The virtual machines image files directory. If unspecified, <home>/var is
#   assumed. If specified as relative, then assumed relative to <home>.
#
# --run <dir>
#   The virtual machines sockets directory. If unspecified, <home>/run is
#   assumed. If specified as relative, then assumed relative to <home>.
#
# If <user> is unspecified, the current user is assumed. If <group> is
# unspecified, the user's primary group is assumed.
#
usage="usage: $0 [<options>] [<user>] [<group>]"

owd="$(pwd)"
trap "{ cd '$owd'; exit 1; }" ERR
set -o errtrace # Trap in functions.

function info () { echo "$*" 1>&2; }
function error () { info "$*"; exit 1; }

home=
bin=
etc=
var=
run=

while [ "$#" -gt 0 ]; do
  case "$1" in
    --home)
      shift
      home="$1"
      shift
      ;;
    --bin)
      shift
      bin="$1"
      shift
      ;;
    --etc)
      shift
      etc="$1"
      shift
      ;;
    --var)
      shift
      var="$1"
      shift
      ;;
    --run)
      shift
      run="$1"
      shift
      ;;
    *)
      break
      ;;
  esac
done

user="$1"

if [ -z "$user" ]; then
  user="$(id -un)"
fi

group="$2"

if [ -z "$group" ]; then
  group="$(id -un "$user")"
fi

if [ -z "$home" ]; then
  home="$(eval echo ~$user)"
fi

function complete_dir () # <default> <home> <dir>
{
  local r
  if [ -z "$3" ]; then
    r="$2/$1"
  elif [ "${3:0:1}" != "/" ]; then
    r="$2/$3"
  else
    r="$3"
  fi
  echo "$(realpath --no-symlinks --canonicalize-missing "$r")"
}

bin="$(complete_dir bin "$home" "$bin")"
etc="$(complete_dir etc "$home" "$etc")"
var="$(complete_dir var "$home" "$var")"
run="$(complete_dir run "$home" "$run")"

name="vm-$user"
file="$name@.service"

# Thinks that must be \-escaped:
#
# - $ (including in comments)
# - \ (e.g., in line continuations)
#
cat <<EOF >"$file"
# $file -- QEMU/KVM machine service template for systemd
#
# user:  $user
# group: $group
# bin:   $bin
# etc:   $etc
# var:   $var
# run:   $run
#
# To install:
#
# sudo cp $file /etc/systemd/system/
# sudo chmod 644 /etc/systemd/system/$file
#
# cp ... $var/<machine>.img
# nano   $etc/<machine>.conf # Specify RAM, CPU, TAP, MAC, etc.
#
# sudo systemctl start  $name@<machine>
# sudo systemctl status $name@<machine>
# login-machine $run/<machine>-con.sock
# sudo systemctl stop   $name@<machine>
#
# sudo systemctl enable $name@<machine>

[Unit]
Description=QEMU/KVM virtual machine %I

Wants=network-online.target
#After=network-online.target
After=multi-user.target

[Service]
User=$user
Group=$user
UMask=0007
WorkingDirectory=~

Environment=CPU=1
Environment=RAM=2G

# These MUST be specific in EnvironmentFile!
#
#Environment=TAP=
#Environment=MAC=

# Note that using variable expansion in EnvironmentFile does not work (at
# least not with systemd 229).
#
EnvironmentFile=$etc/%i.conf

# Note that the first word of ExecStart cannot contain variable expansions.
#
ExecStart=$bin/vm-start \\
  --cpu \${CPU} \\
  --ram \${RAM} \\
  --tap \${TAP} \\
  --mac \${MAC} \\
  --monitor $run/%i-mon.sock \\
  --console $run/%i-con.sock \\
  $var/%i.img

ExecStop=$bin/vm-stop $run/%i-mon.sock

# Make systemd wait for ExecStop completion.
#
KillMode=none
TimeoutStopSec=60

[Install]
WantedBy=multi-user.target
EOF

info "generated $file for"
info "  user:  $user"
info "  group: $group"
info "  bin:   $bin"
info "  etc:   $etc"
info "  var:   $var"
info "  run:   $run"