#!/usr/bin/env bash
# Build and boot the desktop configuration in a QEMU VM (see modules/nixos/vm.nix).
#
#   ./vm.sh            build and start the VM
#   ./vm.sh --fresh    the same, but start from an empty VM disk
#
# The VM renders on the host GPU through virgl. QEMU's own window can't get a GL
# context on the NVIDIA driver, so it renders headless on a Mesa GPU (the
# Ryzen's integrated Radeon here) and is shown over VNC on this machine only:
#   vncviewer localhost:5905      (e.g. nix run nixpkgs#tigervnc -- localhost:5905)
#
# The VM's disk is desktop.qcow2 in this directory (git ignores it). A QEMU
# monitor socket, vm-monitor.sock, takes screenshots and key presses from
# scripts, e.g.: echo 'screendump shot.ppm' | socat - UNIX-CONNECT:vm-monitor.sock
# The VM's console (boot messages, then a login prompt) goes to vm-serial.log.
#
# SSH: ssh -p 2222 vizie@localhost, with the key in $VM_SSH_KEY (a .pub file,
# default ~/.ssh/id_ed25519.pub). The key is shared with this VM only.
set -euo pipefail
cd "$(dirname "$(readlink -f "$0")")"
here=$PWD

if [ "${1:-}" = --fresh ]; then
  rm -f desktop.qcow2
fi

# The first render node whose driver isn't nvidia.
rendernode=
for node in /dev/dri/renderD*; do
  driver=$(basename "$(readlink -f "/sys/class/drm/$(basename "$node")/device/driver")")
  if [ "$driver" != nvidia ]; then rendernode=$node; break; fi
done
if [ -z "$rendernode" ]; then
  echo "No non-NVIDIA GPU found for the VM to render on." >&2
  exit 1
fi

vm=$(nix build --no-link --print-out-paths .#nixosConfigurations.desktop.config.system.build.vm)

# Outside NixOS (e.g. on CachyOS, before the switch) there is no
# /run/opengl-driver for the Nix-built QEMU to find GPU drivers in; give it the
# Mesa from the same nixpkgs.
if [ ! -e /run/opengl-driver ]; then
  mesa=$(nix build --no-link --print-out-paths .#nixosConfigurations.desktop.pkgs.mesa)
  export GBM_BACKENDS_PATH=$mesa/lib/gbm
  export LIBGL_DRIVERS_PATH=$mesa/lib/dri
  export __EGL_VENDOR_LIBRARY_FILENAMES=$mesa/share/glvnd/egl_vendor.d/50_mesa.json
fi
export NIX_DISK_IMAGE="$here/desktop.qcow2"

# A temporary folder for the VM's sockets and /tmp/xchg, with the SSH key in it.
vmtmp=$(mktemp -d -t dot-vm.XXXXXXXX)
trap 'rm -rf -- "${vmtmp:?}"' EXIT
export TMPDIR=$vmtmp USE_TMPDIR=1
mkdir -p "$vmtmp/xchg"
key=${VM_SSH_KEY:-$HOME/.ssh/id_ed25519.pub}
if [ -f "$key" ]; then install -m644 "$key" "$vmtmp/xchg/authorized_keys"; fi
export QEMU_OPTS="${QEMU_OPTS:-} -display egl-headless,rendernode=$rendernode -vnc 127.0.0.1:5 -k no -monitor unix:$here/vm-monitor.sock,server,nowait -serial file:$here/vm-serial.log"
echo "VM starting; watch it with: vncviewer localhost:5905"
"$vm/bin/run-desktop-vm"