default.nix

with import <nixpkgs> { };

let
  cutPoints = builtins.map (drv: drv.outPath) [
    nodejs
    python2
    python3
    coreutils
    openssl.out
    glibc
    bash
    gcc-unwrapped.lib
    bzip2.out
  ];

  mkContentsDrv = contents: symlinkJoin {
    name = "container-env";
    paths = (if builtins.isList contents then contents else [ contents ]);
  };

  mkLayers = contents: cutPoints: runCommand "closure-paths" {
    cutJSON = writeText "cuts" (builtins.toJSON cutPoints);
    passAsFile = [ "cutJSON" ];

    exportReferencesGraph.graph = contents;
    __structuredAttrs = true;
    PATH = "${coreutils}/bin:${python3}/bin";
    builder = builtins.toFile "builder" ''
      . .attrs.sh

      cat $cutJSON

      python3 ${./make_layers.py} --graph .attrs.json --cuts $cutJSON > outf
      mv outf ''${outputs[out]}
    '';
  } "";

  mkDockerImage = contents: let
    aggDrv = mkContentsDrv contents;
    layers = mkLayers aggDrv cutPoints;
  in layers;

  drv = (python3.withPackages(ps: [ ps.flake8 ]));

in mkDockerImage [ drv ]

layers.json

// Example of what the above could would output at the moment
{
  "/nix/store/s3nl02c26303mfjvaaxnc19f8ks9hc8c-python3-3.6.6": [
    "/nix/store/6v0w8pn91zqdm2ri7wgrlg3y16gqsbij-xz-5.2.4",
    "/nix/store/78d783wx27w2ql4pcdq5pxz372ajksgm-sqlite-3.24.0",
    "/nix/store/8myyhvyvb93kpdv3valpxb81n08a7mqj-readline-6.3p08",
    "/nix/store/8nalpyx6f8arkmh23wx5bc46p6xgqp64-expat-2.2.6",
    "/nix/store/bv6znzsv2qkbcwwa251dx7n5dshz3nr3-zlib-1.2.11",
    "/nix/store/jsps5n2s3bc5i466k5vvqg4wl5013xms-libffi-3.2.1",
    "/nix/store/s3nl02c26303mfjvaaxnc19f8ks9hc8c-python3-3.6.6",
    "/nix/store/sr2hxkjn4f3y17safsxy3nwf8iv55jyn-ncurses-6.1",
    "/nix/store/vc7rd7zlhmw8mv61sjx0x633dwzlmkww-gdbm-1.17"
  ],
  "/nix/store/2xdf4mgczzfjwj1fmcz0jjx7plvvcgsq-coreutils-8.29": [
    "/nix/store/2xdf4mgczzfjwj1fmcz0jjx7plvvcgsq-coreutils-8.29",
    "/nix/store/gzxckycv7v43aa1pf0manyva5p58nd4m-attr-2.4.47",
    "/nix/store/xbnwfn8dslrmq9w39k3a52bdvigwqx23-acl-2.2.52"
  ],
  "/nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27": [
    "/nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27"
  ],
  "/nix/store/czx8vkrb9jdgjyz8qfksh10vrnqa723l-bash-4.4-p23": [
    "/nix/store/czx8vkrb9jdgjyz8qfksh10vrnqa723l-bash-4.4-p23"
  ],
  "/nix/store/arrv4xka8hgd65hva6fhlhjkyv74zjp9-container-env": [
    "/nix/store/2d1ng0mak79qakfz91s9gzzbq9i6l5gd-python3.6-flake8-3.5.0",
    "/nix/store/4f1sgfkjmfvmw9lz213rfqr6zm8gp6h6-python3-3.6.6-env",
    "/nix/store/9x3vj2c1k0xn2cyfyvzs7j74a3plhck5-python3.6-setuptools-40.2.0",
    "/nix/store/arrv4xka8hgd65hva6fhlhjkyv74zjp9-container-env",
    "/nix/store/b5pfwxq3zlbg0403ngr3h39jr42ys521-python3.6-pycodestyle-2.4.0",
    "/nix/store/yhk6i2daplybp34ff5zlrfh341ykmap9-python3.6-mccabe-0.6.1",
    "/nix/store/z1c96zgjwm1dlil3h53yy0cyxh6g6k8h-python3.6-pyflakes-2.0.0"
  ],
  "/nix/store/gsgdj34vv5hh3iyyfm7yh69j6dq0br9q-openssl-1.0.2p": [
    "/nix/store/gsgdj34vv5hh3iyyfm7yh69j6dq0br9q-openssl-1.0.2p"
  ],
  "/nix/store/wwhp4klykgvd7xwl94izc8xxb9idf4wy-bzip2-1.0.6.0.1": [
    "/nix/store/wwhp4klykgvd7xwl94izc8xxb9idf4wy-bzip2-1.0.6.0.1"
  ]
}

make_layers.py

#!/usr/bin/env python
import argparse
import json
import sys


argparser = argparse.ArgumentParser()
argparser.add_argument(
    '--graph', type=argparse.FileType('r', encoding='UTF-8'),
    required=True)
argparser.add_argument(
    '--cuts', type=argparse.FileType('r', encoding='UTF-8'),
    required=True)


def get_layer(graph, key, cut_points, _layer=None):
    if _layer is None:
        _layer = set()
    node = graph[key]
    _layer.add(key)

    for ref in node.get('references', []):
        if ref == key or ref in cut_points:
            continue
        get_layer(graph, ref, cut_points, _layer=_layer)
    return _layer


def process_layers(graph_data, cut_points):
    leaf_path = graph_data['exportReferencesGraph']['graph']
    graph = {
        node['path']: node
        for node in graph_data['graph']
    }

    layer_keys = cut_points.intersection(set(graph.keys()))
    layer_keys.add(leaf_path)

    return {
        layer_key: get_layer(graph, layer_key, cut_points)
        for layer_key in layer_keys
    }


def find_duplicate_refs(layers):
    dups = set()
    for layer_key, layer in layers.items():
        for l_k, l in layers.items():
            if layer_key == l_k:
                continue
            dups.update(l.intersection(layer))
    return dups


class SetJSONEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, set):
            return sorted(obj)
        return json.JSONEncoder.default(self, obj)


if __name__ == '__main__':
    args = argparser.parse_args()
    graph_data = json.load(args.graph)
    cut_points = set(json.load(args.cuts))

    layers = process_layers(graph_data, cut_points)
    for dup in find_duplicate_refs(layers):
        sys.stderr.write(
            'Found path in multiple layers, consider adding cut: %s\n' % dup)

    print(json.dumps(layers, cls=SetJSONEncoder, indent=2))