programs: introduce a sane-sandboxed helper

not yet used, but will be soon

modules/programs/default.nix

@@ -348,6 +348,14 @@ in
         whether to ship programs which are uniquely slow to build.
       '';
     };
+    sane.sandboxHelper = mkOption {
+      type = types.package;
+      default = pkgs.callPackage ./sane-sandboxed.nix {};
+      description = ''
+        `sane-sandbox` package.
+        exposed to facilitate debugging, e.g. `nix build '.#hostConfigs.desko.sane.sandboxHelper'`
+      '';
+    };
   };
 
   config =
@@ -364,6 +372,9 @@ in
       };
     in lib.mkMerge [
       (take (sane-lib.mkTypedMerge take configs))
+      {
+        environment.systemPackages = [ config.sane.sandboxHelper ];
+      }
       {
         # expose the pkgs -- as available to the system -- as a build target.
         system.build.pkgs = pkgs;

modules/programs/sane-sandboxed

@@ -0,0 +1,77 @@
+#!@runtimeShell@
+
+test -n "$SANE_SANDBOX_DEBUG" && set -x
+
+_cli=()
+_rootPaths=()
+_homePaths=()
+_net=
+_dns=()
+_firejailFlags=()
+
+allowPath() {
+  # if the path is relative, add to _homePaths, else _rootPaths
+  if [ "${1:0:1}" = "/" ]; then
+    _rootPaths+=("$1")
+  else
+    _homePaths+=("$1")
+  fi
+}
+
+## parse CLI args into the variables declared above
+
+while [ "$#" -ne 0 ]; do
+  _arg="$1"
+  shift
+  case "$_arg" in
+    (--)
+      # rest of args are for the CLI
+      _cli+=$@
+      break
+      ;;
+    (--sane-sandbox-disable)
+      SANE_SANDBOX_DISABLE=1
+      ;;
+    (--sane-sandbox-firejail-arg)
+      _firejailFlags+=("$1")
+      shift
+      ;;
+    (--sane-sandbox-path)
+      allowPath "$1"
+      shift
+      ;;
+    (--sane-sandbox-net)
+      _net="$1"
+      shift
+      ;;
+    (--sane-sandbox-dns)
+      _dns+=("$1")
+      shift
+      ;;
+    (*)
+      _cli+=("$_arg")
+      ;;
+  esac
+done
+
+test -n "$SANE_SANDBOX_DISABLE" && exec "${_cli[@]}"
+
+## construct firejail flags from sane-sandbox flags
+
+for _path in "${_rootPaths[@]}"; do
+  _firejailFlags+=("--noblacklist=$_path" "--whitelist=$_path")
+done
+
+for _path in "${_homePaths[@]}"; do
+  _firejailFlags+=("--noblacklist="'${HOME}/'"$_path" "--whitelist="'${HOME}/'"$_path")
+done
+
+if [ -n "$_net" ]; then
+  _firejailFlags+=("--net=$_net")
+fi
+
+for _addr in "${_dns[@]}"; do
+  _firejailFlags+=("--dns=$_addr")
+done
+
+PATH="$PATH:@firejail@" exec firejail "${_firejailFlags[@]}" -- "${_cli[@]}"

modules/programs/sane-sandboxed.nix

@@ -0,0 +1,33 @@
+{ stdenv
+, runtimeShell
+, firejail
+}:
+
+stdenv.mkDerivation {
+  pname = "sane-sandboxed";
+  version = "0.1";
+  src = ./.;  #< TODO: should just be `./sane-sandboxed`
+
+  firejail = "${firejail}/bin/firejail";
+  inherit runtimeShell;
+  postPatch = ''
+    substituteAllInPlace sane-sandboxed
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    install -d "$out"
+    install -d "$out/bin"
+    install -m 755 sane-sandboxed $out/bin/sane-sandboxed
+    runHook postInstall
+  '';
+
+  meta = {
+    description = ''
+      helper program to run some other program in a sandbox.
+      factoring this out allows:
+      1. to abstract over the particular sandbox implementation (bwrap, firejail, ...).
+      2. to modify sandbox settings without forcing a rebuild of the sandboxed package.
+    '';
+  };
+}