first attempt

Signed-off-by: Xe <me@christine.website>

build-rootfs.sh

@@ -0,0 +1,35 @@
+source $stdenv/setup
+
+set -o pipefail
+
+# basic file system layout
+mkdir -p $out/etc $out/proc $out/sys $out/dev $out/run $out/tmp $out/var/tmp $out/var/lib $out/var/log
+
+# empty files to mount over with host's version
+touch $out/etc/resolv.conf $out/etc/machine-id
+
+# required for portable services
+cp ${osRelease} $out/etc/os-release
+
+
+# units must be copied to /etc/…
+mkdir -p $out/etc/systemd/system
+units=($units)
+unitNames=($unitNames)
+for ((n = 0; n < ${#units[*]}; n++)); do
+    unit=${units[$n]}
+    unitName=${unitNames[$n]}
+    cp $unit $out/etc/systemd/system/$unitName
+done
+
+
+# symlinks
+objects=($objects)
+targets=($targets)
+for ((n = 0; n < ${#objects[*]}; n++)); do
+    object=${objects[$n]}
+    target=${targets[$n]}
+
+    mkdir -p $(dirname $out/$target)
+    ln -s $object $out/$target
+done

default.nix

@@ -0,0 +1,82 @@
+{ pkgs, lib, stdenv }:
+/*
+  Create a systemd portable service image
+  https://systemd.io/PORTABLE_SERVICES/
+
+  Example:
+  …
+  pkgs.portableService {
+    name = "hello";
+    version = "2.4.3";
+    description = "hello portable";
+
+    units = [ hello-service ./files/hello.socket ];
+
+    symlinks = [
+      { object = "${pkgs.cacert}/etc/ssl"; symlink = "/etc/ssl"; }
+      { object = "${pkgs.bash}/bin/bash"; symlink = "/bin/sh"; }
+    ];
+  }
+  …
+*/
+{ name
+, version ? "dev"
+, description ? null
+, homepage ? null
+, units ? [ ]
+, symlinks ? [ ]
+, contents ? [ ]
+, squashfsTools ? pkgs.squashfsTools
+, squash-compression ? "xz -Xdict-size 100%"
+, squash-block-size ? "1M"
+}:
+
+let
+  image-name = "${name}_${version}";
+
+  os-release-params = lib.filterAttrs (n: v: v != null) {
+    PORTABLE_ID = name;
+    PORTABLE_PRETTY_NAME = description;
+    HOME_URL = homepage;
+    ID = "nixos";
+    PRETTY_NAME = "NixOS";
+    BUILD_ID = "rolling";
+  };
+
+  os-release = pkgs.writeText "os-release" (lib.generators.toKeyValue {} os-release-params);
+
+  getUnitName = u:
+    if lib.isDerivation u then u.name
+    else if builtins.isPath u then baseNameOf u
+    else throw "unit must be either derivation or path";
+
+  rootfs = stdenv.mkDerivation {
+    name = "rootfs";
+    builder = ./build-rootfs.sh;
+    inherit units;
+    unitNames = map getUnitName units;
+    osRelease = os-release;
+    objects = map (x: x.object) symlinks;
+    targets = map (x: x.symlink) symlinks;
+  };
+in
+
+stdenv.mkDerivation {
+  name = "${image-name}.raw";
+  nativeBuildInputs = [ squashfsTools ];
+
+  buildCommand = ''
+    closureInfo=${pkgs.closureInfo { rootPaths = contents ++ [rootfs]; }}
+
+    mkdir -p nix/store
+    for i in $(< $closureInfo/store-paths); do
+      cp -a "$i" "''${i:1}"
+    done
+
+    mksquashfs nix ${rootfs}/* $out \
+      -quiet -noappend \
+      -keep-as-directory \
+      -all-root -root-mode 755 \
+      -b ${squash-block-size} -comp ${squash-compression}
+  '';
+}

flake.nix

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+{
+  description = "Nix tools to help you build portable services";
+
+  outputs = { self, nixpkgs, utils }: {
+    overlay = final: prev: prev.callPackage ./. { };
+  };
+}