default.nix

{
  buildPackages,
  cacert,
  callPackage,
  closureInfo,
  coreutils,
  docker,
  e2fsprogs,
  findutils,
  go,
  jq,
  jshon,
  lib,
  makeWrapper,
  moreutils,
  nix,
  pigz,
  rsync,
  runCommand,
  runtimeShell,
  shadow,
  skopeo,
  stdenv,
  storeDir ? builtins.storeDir,
  substituteAll,
  symlinkJoin,
  utillinux,
  vmTools,
  writeReferencesToFile,
  writeScript,
  writeText,
  writePython3,
  system,  # Note: This is the cross system we're compiling for
}:

# WARNING: this API is unstable and may be subject to backwards-incompatible changes in the future.
let

  mkDbExtraCommand = contents: let
    contentsList = if builtins.isList contents then contents else [ contents ];
  in ''
    echo "Generating the nix database..."
    echo "Warning: only the database of the deepest Nix layer is loaded."
    echo "         If you want to use nix commands in the container, it would"
    echo "         be better to only have one layer that contains a nix store."

    export NIX_REMOTE=local?root=$PWD
    # A user is required by nix
    # https://github.com/NixOS/nix/blob/9348f9291e5d9e4ba3c4347ea1b235640f54fd79/src/libutil/util.cc#L478
    export USER=nobody
    ${buildPackages.nix}/bin/nix-store --load-db < ${closureInfo {rootPaths = contentsList;}}/registration

    mkdir -p nix/var/nix/gcroots/docker/
    for i in ${lib.concatStringsSep " " contentsList}; do
    ln -s $i nix/var/nix/gcroots/docker/$(basename $i)
    done;
  '';

  # Map nixpkgs architecture to Docker notation
  # Reference: https://github.com/docker-library/official-images#architectures-other-than-amd64
  getArch = nixSystem: {
    aarch64-linux = "arm64v8";
    armv7l-linux = "arm32v7";
    x86_64-linux = "amd64";
    powerpc64le-linux = "ppc64le";
    i686-linux = "i386";
  }.${nixSystem} or "Can't map Nix system ${nixSystem} to Docker architecture notation. Please check that your input and your requested build are correct or update the mapping in Nixpkgs.";

in
rec {

  examples = callPackage ./examples.nix {
    inherit buildImage pullImage shadowSetup buildImageWithNixDb;
  };

  pullImage = let
    fixName = name: builtins.replaceStrings ["/" ":"] ["-" "-"] name;
  in
    { imageName
      # To find the digest of an image, you can use skopeo:
      # see doc/functions.xml
    , imageDigest
    , sha256
    , os ? "linux"
    , arch ? getArch system

      # This is used to set name to the pulled image
    , finalImageName ? imageName
      # This used to set a tag to the pulled image
    , finalImageTag ? "latest"

    , name ? fixName "docker-image-${finalImageName}-${finalImageTag}.tar"
    }:

    runCommand name {
      inherit imageDigest;
      imageName = finalImageName;
      imageTag = finalImageTag;
      impureEnvVars = stdenv.lib.fetchers.proxyImpureEnvVars;
      outputHashMode = "flat";
      outputHashAlgo = "sha256";
      outputHash = sha256;

      nativeBuildInputs = lib.singleton skopeo;
      SSL_CERT_FILE = "${cacert.out}/etc/ssl/certs/ca-bundle.crt";

      sourceURL = "docker://${imageName}@${imageDigest}";
      destNameTag = "${finalImageName}:${finalImageTag}";
    } ''
      skopeo --insecure-policy --tmpdir=$TMPDIR --override-os ${os} --override-arch ${arch} copy "$sourceURL" "docker-archive://$out:$destNameTag"
    '';

  # We need to sum layer.tar, not a directory, hence tarsum instead of nix-hash.
  # And we cannot untar it, because then we cannot preserve permissions ecc.
  tarsum = runCommand "tarsum" {
    nativeBuildInputs = [ go ];
  } ''
    mkdir tarsum
    cd tarsum

    cp ${./tarsum.go} tarsum.go
    export GOPATH=$(pwd)
    export GOCACHE="$TMPDIR/go-cache"
    mkdir -p src/github.com/docker/docker/pkg
    ln -sT ${docker.src}/components/engine/pkg/tarsum src/github.com/docker/docker/pkg/tarsum
    go build

    mkdir -p $out/bin

    cp tarsum $out/bin/
  '';

  # buildEnv creates symlinks to dirs, which is hard to edit inside the overlay VM
  mergeDrvs = {
    derivations,
    onlyDeps ? false
  }:
    runCommand "merge-drvs" {
      inherit derivations onlyDeps;
    } ''
      if [[ -n "$onlyDeps" ]]; then
        echo $derivations > $out
        exit 0
      fi

      mkdir $out
      for derivation in $derivations; do
        echo "Merging $derivation..."
        if [[ -d "$derivation" ]]; then
          # If it's a directory, copy all of its contents into $out.
          cp -drf --preserve=mode -f $derivation/* $out/
        else
          # Otherwise treat the derivation as a tarball and extract it
          # into $out.
          tar -C $out -xpf $drv || true
        fi
      done
    '';

  # Helper for setting up the base files for managing users and
  # groups, only if such files don't exist already. It is suitable for
  # being used in a runAsRoot script.
  shadowSetup = ''
    export PATH=${shadow}/bin:$PATH
    mkdir -p /etc/pam.d
    if [[ ! -f /etc/passwd ]]; then
      echo "root:x:0:0::/root:${runtimeShell}" > /etc/passwd
      echo "root:!x:::::::" > /etc/shadow
    fi
    if [[ ! -f /etc/group ]]; then
      echo "root:x:0:" > /etc/group
      echo "root:x::" > /etc/gshadow
    fi
    if [[ ! -f /etc/pam.d/other ]]; then
      cat > /etc/pam.d/other <<EOF
    account sufficient pam_unix.so
    auth sufficient pam_rootok.so
    password requisite pam_unix.so nullok sha512
    session required pam_unix.so
    EOF
    fi
    if [[ ! -f /etc/login.defs ]]; then
      touch /etc/login.defs
    fi
  '';

  # Run commands in a virtual machine.
  runWithOverlay = {
    name,
    fromImage ? null,
    fromImageName ? null,
    fromImageTag ? null,
    diskSize ? 1024,
    preMount ? "",
    postMount ? "",
    postUmount ? ""
  }:
  let
    result = vmTools.runInLinuxVM (
      runCommand name {
        preVM = vmTools.createEmptyImage {
          size = diskSize;
          fullName = "docker-run-disk";
        };
        inherit fromImage fromImageName fromImageTag;

        nativeBuildInputs = [ utillinux e2fsprogs jshon rsync jq ];
      } ''
      mkdir disk
      mkfs /dev/${vmTools.hd}
      mount /dev/${vmTools.hd} disk
      cd disk

      if [[ -n "$fromImage" ]]; then
        echo "Unpacking base image..."
        mkdir image
        tar -C image -xpf "$fromImage"

        if [[ -n "$fromImageName" ]] && [[ -n "$fromImageTag" ]]; then
          parentID="$(
            cat "image/manifest.json" |
              jq -r '.[] | select(.RepoTags | contains([$desiredTag])) | rtrimstr(".json")' \
                --arg desiredTag "$fromImageName:$fromImageTag"
          )"
        else
          echo "From-image name or tag wasn't set. Reading the first ID."
          parentID="$(cat "image/manifest.json" | jq -r '.[0].Config | rtrimstr(".json")')"
        fi

        cat ./image/manifest.json  | jq -r '.[0].Layers | .[]' > layer-list
      else
        touch layer-list
      fi

      # Unpack all of the parent layers into the image.
      lowerdir=""
      extractionID=0
      for layerTar in $(tac layer-list); do
        echo "Unpacking layer $layerTar"
        extractionID=$((extractionID + 1))

        mkdir -p image/$extractionID/layer
        tar -C image/$extractionID/layer -xpf image/$layerTar
        rm image/$layerTar

        find image/$extractionID/layer -name ".wh.*" -exec bash -c 'name="$(basename {}|sed "s/^.wh.//")"; mknod "$(dirname {})/$name" c 0 0; rm {}' \;

        # Get the next lower directory and continue the loop.
        lowerdir=image/$extractionID/layer''${lowerdir:+:}$lowerdir
      done

      mkdir work
      mkdir layer
      mkdir mnt

      ${lib.optionalString (preMount != "") ''
        # Execute pre-mount steps
        echo "Executing pre-mount steps..."
        ${preMount}
      ''}

      if [ -n "$lowerdir" ]; then
        mount -t overlay overlay -olowerdir=$lowerdir,workdir=work,upperdir=layer mnt
      else
        mount --bind layer mnt
      fi

      ${lib.optionalString (postMount != "") ''
        # Execute post-mount steps
        echo "Executing post-mount steps..."
        ${postMount}
      ''}

      umount mnt

      (
        cd layer
        cmd='name="$(basename {})"; touch "$(dirname {})/.wh.$name"; rm "{}"'
        find . -type c -exec bash -c "$cmd" \;
      )

      ${postUmount}
      '');
    in
    runCommand name {} ''
      mkdir -p $out
      cd ${result}
      cp layer.tar json VERSION $out
    '';

  exportImage = { name ? fromImage.name, fromImage, fromImageName ? null, fromImageTag ? null, diskSize ? 1024 }:
    runWithOverlay {
      inherit name fromImage fromImageName fromImageTag diskSize;

      postMount = ''
        echo "Packing raw image..."
        tar -C mnt --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" -cf $out .
      '';
    };


  # Create an executable shell script which has the coreutils in its
  # PATH. Since root scripts are executed in a blank environment, even
  # things like `ls` or `echo` will be missing.
  shellScript = name: text:
    writeScript name ''
      #!${runtimeShell}
      set -e
      export PATH=${coreutils}/bin:/bin
      ${text}
    '';

  # Create a "layer" (set of files).
  mkPureLayer = {
    # Name of the layer
    name,
    # JSON containing configuration and metadata for this layer.
    baseJson,
    # Files to add to the layer.
    contents ? null,
    # When copying the contents into the image, preserve symlinks to
    # directories (see `rsync -K`).  Otherwise, transform those symlinks
    # into directories.
    keepContentsDirlinks ? false,
    # Additional commands to run on the layer before it is tar'd up.
    extraCommands ? "", uid ? 0, gid ? 0
  }:
    runCommand "docker-layer-${name}" {
      inherit baseJson contents extraCommands;
      nativeBuildInputs = [ jshon rsync tarsum ];
    }
    ''
      mkdir layer
      if [[ -n "$contents" ]]; then
        echo "Adding contents..."
        for item in $contents; do
          echo "Adding $item"
          rsync -a${if keepContentsDirlinks then "K" else "k"} --chown=0:0 $item/ layer/
        done
      else
        echo "No contents to add to layer."
      fi

      chmod ug+w layer

      if [[ -n "$extraCommands" ]]; then
        (cd layer; eval "$extraCommands")
      fi

      # Tar up the layer and throw it into 'layer.tar'.
      echo "Packing layer..."
      mkdir $out
      tarhash=$(tar -C layer --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" --owner=${toString uid} --group=${toString gid} -cf - . | tee -p $out/layer.tar | tarsum)

      # Add a 'checksum' field to the JSON, with the value set to the
      # checksum of the tarball.
      cat ${baseJson} | jshon -s "$tarhash" -i checksum > $out/json

      # Indicate to docker that we're using schema version 1.0.
      echo -n "1.0" > $out/VERSION

      echo "Finished building layer '${name}'"
    '';

  # Make a "root" layer; required if we need to execute commands as a
  # privileged user on the image. The commands themselves will be
  # performed in a virtual machine sandbox.
  mkRootLayer = {
    # Name of the image.
    name,
    # Script to run as root. Bash.
    runAsRoot,
    # Files to add to the layer. If null, an empty layer will be created.
    contents ? null,
    # When copying the contents into the image, preserve symlinks to
    # directories (see `rsync -K`).  Otherwise, transform those symlinks
    # into directories.
    keepContentsDirlinks ? false,
    # JSON containing configuration and metadata for this layer.
    baseJson,
    # Existing image onto which to append the new layer.
    fromImage ? null,
    # Name of the image we're appending onto.
    fromImageName ? null,
    # Tag of the image we're appending onto.
    fromImageTag ? null,
    # How much disk to allocate for the temporary virtual machine.
    diskSize ? 1024,
    # Commands (bash) to run on the layer; these do not require sudo.
    extraCommands ? ""
  }:
    # Generate an executable script from the `runAsRoot` text.
    let
      runAsRootScript = shellScript "run-as-root.sh" runAsRoot;
      extraCommandsScript = shellScript "extra-commands.sh" extraCommands;
    in runWithOverlay {
      name = "docker-layer-${name}";

      inherit fromImage fromImageName fromImageTag diskSize;

      preMount = lib.optionalString (contents != null && contents != []) ''
        echo "Adding contents..."
        for item in ${toString contents}; do
          echo "Adding $item..."
          rsync -a${if keepContentsDirlinks then "K" else "k"} --chown=0:0 $item/ layer/
        done

        chmod ug+w layer
      '';

      postMount = ''
        mkdir -p mnt/{dev,proc,sys} mnt${storeDir}

        # Mount /dev, /sys and the nix store as shared folders.
        mount --rbind /dev mnt/dev
        mount --rbind /sys mnt/sys
        mount --rbind ${storeDir} mnt${storeDir}

        # Execute the run as root script. See 'man unshare' for
        # details on what's going on here; basically this command
        # means that the runAsRootScript will be executed in a nearly
        # completely isolated environment.
        unshare -imnpuf --mount-proc chroot mnt ${runAsRootScript}

        # Unmount directories and remove them.
        umount -R mnt/dev mnt/sys mnt${storeDir}
        rmdir --ignore-fail-on-non-empty \
          mnt/dev mnt/proc mnt/sys mnt${storeDir} \
          mnt$(dirname ${storeDir})
      '';

      postUmount = ''
        (cd layer; ${extraCommandsScript})

        echo "Packing layer..."
        mkdir -p $out
        tarhash=$(tar -C layer --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" -cf - . |
                    tee -p $out/layer.tar |
                    ${tarsum}/bin/tarsum)

        cat ${baseJson} | jshon -s "$tarhash" -i checksum > $out/json
        # Indicate to docker that we're using schema version 1.0.
        echo -n "1.0" > $out/VERSION

        echo "Finished building layer '${name}'"
      '';
    };

  buildLayeredImage = {name, ...}@args:
    let
      stream = streamLayeredImage args;
    in
      runCommand "${name}.tar.gz" {
        inherit (stream) imageName;
        passthru = { inherit (stream) imageTag; };
        nativeBuildInputs = [ pigz ];
      } "${stream} | pigz -nT > $out";

  # 1. extract the base image
  # 2. create the layer
  # 3. add layer deps to the layer itself, diffing with the base image
  # 4. compute the layer id
  # 5. put the layer in the image
  # 6. repack the image
  buildImage = args@{
    # Image name.
    name,
    # Image tag, when null then the nix output hash will be used.
    tag ? null,
    # Parent image, to append to.
    fromImage ? null,
    # Name of the parent image; will be read from the image otherwise.
    fromImageName ? null,
    # Tag of the parent image; will be read from the image otherwise.
    fromImageTag ? null,
    # Files to put on the image (a nix store path or list of paths).
    contents ? null,
    # When copying the contents into the image, preserve symlinks to
    # directories (see `rsync -K`).  Otherwise, transform those symlinks
    # into directories.
    keepContentsDirlinks ? false,
    # Docker config; e.g. what command to run on the container.
    config ? null,
    # Optional bash script to run on the files prior to fixturizing the layer.
    extraCommands ? "", uid ? 0, gid ? 0,
    # Optional bash script to run as root on the image when provisioning.
    runAsRoot ? null,
    # Size of the virtual machine disk to provision when building the image.
    diskSize ? 1024,
    # Time of creation of the image.
    created ? "1970-01-01T00:00:01Z",
  }:

    let
      baseName = baseNameOf name;

      # Create a JSON blob of the configuration. Set the date to unix zero.
      baseJson = let
          pure = writeText "${baseName}-config.json" (builtins.toJSON {
            inherit created config;
            architecture = getArch system;
            os = "linux";
          });
          impure = runCommand "${baseName}-config.json"
            { nativeBuildInputs = [ jq ]; }
            ''
               jq ".created = \"$(TZ=utc date --iso-8601="seconds")\"" ${pure} > $out
            '';
        in if created == "now" then impure else pure;

      layer =
        if runAsRoot == null
        then mkPureLayer {
          name = baseName;
          inherit baseJson contents keepContentsDirlinks extraCommands uid gid;
        } else mkRootLayer {
          name = baseName;
          inherit baseJson fromImage fromImageName fromImageTag
                  contents keepContentsDirlinks runAsRoot diskSize
                  extraCommands;
        };
      result = runCommand "docker-image-${baseName}.tar.gz" {
        nativeBuildInputs = [ jshon pigz coreutils findutils jq moreutils ];
        # Image name and tag must be lowercase
        imageName = lib.toLower name;
        imageTag = if tag == null then "" else lib.toLower tag;
        inherit fromImage baseJson;
        layerClosure = writeReferencesToFile layer;
        passthru.buildArgs = args;
        passthru.layer = layer;
        passthru.imageTag =
          if tag != null
            then lib.toLower tag
            else
              lib.head (lib.strings.splitString "-" (baseNameOf result.outPath));
        # Docker can't be made to run darwin binaries
        meta.badPlatforms = lib.platforms.darwin;
      } ''
        ${lib.optionalString (tag == null) ''
          outName="$(basename "$out")"
          outHash=$(echo "$outName" | cut -d - -f 1)

          imageTag=$outHash
        ''}

        # Print tar contents:
        # 1: Interpreted as relative to the root directory
        # 2: With no trailing slashes on directories
        # This is useful for ensuring that the output matches the
        # values generated by the "find" command
        ls_tar() {
          for f in $(tar -tf $1 | xargs realpath -ms --relative-to=.); do
            if [[ "$f" != "." ]]; then
              echo "/$f"
            fi
          done
        }

        mkdir image
        touch baseFiles
        baseEnvs='[]'
        if [[ -n "$fromImage" ]]; then
          echo "Unpacking base image..."
          tar -C image -xpf "$fromImage"

          # Store the layers and the environment variables from the base image
          cat ./image/manifest.json  | jq -r '.[0].Layers | .[]' > layer-list
          configName="$(cat ./image/manifest.json | jq -r '.[0].Config')"
          baseEnvs="$(cat "./image/$configName" | jq '.config.Env // []')"

          # Extract the parentID from the manifest
          if [[ -n "$fromImageName" ]] && [[ -n "$fromImageTag" ]]; then
            parentID="$(
              cat "image/manifest.json" |
                jq -r '.[] | select(.RepoTags | contains([$desiredTag])) | rtrimstr(".json")' \
                  --arg desiredTag "$fromImageName:$fromImageTag"
            )"
          else
            echo "From-image name or tag wasn't set. Reading the first ID."
            parentID="$(cat "image/manifest.json" | jq -r '.[0].Config | rtrimstr(".json")')"
          fi

          # Otherwise do not import the base image configuration and manifest
          chmod a+w image image/*.json
          rm -f image/*.json

          for l in image/*/layer.tar; do
            ls_tar $l >> baseFiles
          done
        else
          touch layer-list
        fi

        chmod -R ug+rw image

        mkdir temp
        cp ${layer}/* temp/
        chmod ug+w temp/*

        for dep in $(cat $layerClosure); do
          find $dep >> layerFiles
        done

        echo "Adding layer..."
        # Record the contents of the tarball with ls_tar.
        ls_tar temp/layer.tar >> baseFiles

        # Append nix/store directory to the layer so that when the layer is loaded in the
        # image /nix/store has read permissions for non-root users.
        # nix/store is added only if the layer has /nix/store paths in it.
        if [ $(wc -l < $layerClosure) -gt 1 ] && [ $(grep -c -e "^/nix/store$" baseFiles) -eq 0 ]; then
          mkdir -p nix/store
          chmod -R 555 nix
          echo "./nix" >> layerFiles
          echo "./nix/store" >> layerFiles
        fi

        # Get the files in the new layer which were *not* present in
        # the old layer, and record them as newFiles.
        comm <(sort -n baseFiles|uniq) \
             <(sort -n layerFiles|uniq|grep -v ${layer}) -1 -3 > newFiles
        # Append the new files to the layer.
        tar -rpf temp/layer.tar --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" \
          --owner=0 --group=0 --no-recursion --files-from newFiles

        echo "Adding meta..."

        # If we have a parentID, add it to the json metadata.
        if [[ -n "$parentID" ]]; then
          cat temp/json | jshon -s "$parentID" -i parent > tmpjson
          mv tmpjson temp/json
        fi

        # Take the sha256 sum of the generated json and use it as the layer ID.
        # Compute the size and add it to the json under the 'Size' field.
        layerID=$(sha256sum temp/json|cut -d ' ' -f 1)
        size=$(stat --printf="%s" temp/layer.tar)
        cat temp/json | jshon -s "$layerID" -i id -n $size -i Size > tmpjson
        mv tmpjson temp/json

        # Use the temp folder we've been working on to create a new image.
        mv temp image/$layerID

        # Add the new layer ID to the end of the layer list
        (
          cat layer-list
          # originally this used `sed -i "1i$layerID" layer-list`, but
          # would fail if layer-list was completely empty.
          echo "$layerID/layer.tar"
        ) | sponge layer-list

        # Create image json and image manifest
        imageJson=$(cat ${baseJson} | jq '.config.Env = $baseenv + .config.Env' --argjson baseenv "$baseEnvs")
        imageJson=$(echo "$imageJson" | jq ". + {\"rootfs\": {\"diff_ids\": [], \"type\": \"layers\"}}")
        manifestJson=$(jq -n "[{\"RepoTags\":[\"$imageName:$imageTag\"]}]")

        for layerTar in $(cat ./layer-list); do
          layerChecksum=$(sha256sum image/$layerTar | cut -d ' ' -f1)
          imageJson=$(echo "$imageJson" | jq ".history |= . + [{\"created\": \"$(jq -r .created ${baseJson})\"}]")
          # diff_ids order is from the bottom-most to top-most layer
          imageJson=$(echo "$imageJson" | jq ".rootfs.diff_ids |= . + [\"sha256:$layerChecksum\"]")
          manifestJson=$(echo "$manifestJson" | jq ".[0].Layers |= . + [\"$layerTar\"]")
        done

        imageJsonChecksum=$(echo "$imageJson" | sha256sum | cut -d ' ' -f1)
        echo "$imageJson" > "image/$imageJsonChecksum.json"
        manifestJson=$(echo "$manifestJson" | jq ".[0].Config = \"$imageJsonChecksum.json\"")
        echo "$manifestJson" > image/manifest.json

        # Store the json under the name image/repositories.
        jshon -n object \
          -n object -s "$layerID" -i "$imageTag" \
          -i "$imageName" > image/repositories

        # Make the image read-only.
        chmod -R a-w image

        echo "Cooking the image..."
        tar -C image --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" --owner=0 --group=0 --xform s:'^./':: -c . | pigz -nT > $out

        echo "Finished."
      '';

    in
    result;

  # Build an image and populate its nix database with the provided
  # contents. The main purpose is to be able to use nix commands in
  # the container.
  # Be careful since this doesn't work well with multilayer.
  buildImageWithNixDb = args@{ contents ? null, extraCommands ? "", ... }: (
    buildImage (args // {
      extraCommands = (mkDbExtraCommand contents) + extraCommands;
    })
  );

  buildLayeredImageWithNixDb = args@{ contents ? null, extraCommands ? "", ... }: (
    buildLayeredImage (args // {
      extraCommands = (mkDbExtraCommand contents) + extraCommands;
    })
  );

  streamLayeredImage = {
    # Image Name
    name,
    # Image tag, the Nix's output hash will be used if null
    tag ? null,
    # Files to put on the image (a nix store path or list of paths).
    contents ? [],
    # Docker config; e.g. what command to run on the container.
    config ? {},
    # Time of creation of the image. Passing "now" will make the
    # created date be the time of building.
    created ? "1970-01-01T00:00:01Z",
    # Optional bash script to run on the files prior to fixturizing the layer.
    extraCommands ? "",
    # We pick 100 to ensure there is plenty of room for extension. I
    # believe the actual maximum is 128.
    maxLayers ? 100
  }:
    assert
      (lib.assertMsg (maxLayers > 1)
      "the maxLayers argument of dockerTools.buildLayeredImage function must be greather than 1 (current value: ${toString maxLayers})");
    let
      streamScript = writePython3 "stream" {} ./stream_layered_image.py;
      baseJson = writeText "${name}-base.json" (builtins.toJSON {
         inherit config;
         architecture = getArch system;
         os = "linux";
      });

      contentsList = if builtins.isList contents then contents else [ contents ];

      # We store the customisation layer as a tarball, to make sure that
      # things like permissions set on 'extraCommands' are not overriden
      # by Nix. Then we precompute the sha256 for performance.
      customisationLayer = symlinkJoin {
        name = "${name}-customisation-layer";
        paths = contentsList;
        inherit extraCommands;
        postBuild = ''
          mv $out old_out
          (cd old_out; eval "$extraCommands" )

          mkdir $out

          tar \
            --owner 0 --group 0 --mtime "@$SOURCE_DATE_EPOCH" \
            --hard-dereference \
            -C old_out \
            -cf $out/layer.tar .

          sha256sum $out/layer.tar \
            | cut -f 1 -d ' ' \
            > $out/checksum
        '';
      };

      closureRoots = [ baseJson ] ++ contentsList;
      overallClosure = writeText "closure" (lib.concatStringsSep " " closureRoots);

      # These derivations are only created as implementation details of docker-tools,
      # so they'll be excluded from the created images.
      unnecessaryDrvs = [ baseJson overallClosure ];

      conf = runCommand "${name}-conf.json" {
        inherit maxLayers created;
        imageName = lib.toLower name;
        passthru.imageTag =
          if tag != null
            then tag
            else
              lib.head (lib.strings.splitString "-" (baseNameOf conf.outPath));
        paths = buildPackages.referencesByPopularity overallClosure;
        nativeBuildInputs = [ jq ];
      } ''
        ${if (tag == null) then ''
          outName="$(basename "$out")"
          outHash=$(echo "$outName" | cut -d - -f 1)

          imageTag=$outHash
        '' else ''
          imageTag="${tag}"
        ''}

        # convert "created" to iso format
        if [[ "$created" != "now" ]]; then
            created="$(date -Iseconds -d "$created")"
        fi

        paths() {
          cat $paths ${lib.concatMapStringsSep " "
                         (path: "| (grep -v ${path} || true)")
                         unnecessaryDrvs}
        }

        # Create $maxLayers worth of Docker Layers, one layer per store path
        # unless there are more paths than $maxLayers. In that case, create
        # $maxLayers-1 for the most popular layers, and smush the remainaing
        # store paths in to one final layer.
        #
        # The following code is fiddly w.r.t. ensuring every layer is
        # created, and that no paths are missed. If you change the
        # following lines, double-check that your code behaves properly
        # when the number of layers equals:
        #      maxLayers-1, maxLayers, and maxLayers+1, 0
        store_layers="$(
          paths |
            jq -sR '
              rtrimstr("\n") | split("\n")
                | (.[:$maxLayers-1] | map([.])) + [ .[$maxLayers-1:] ]
                | map(select(length > 0))
            ' \
              --argjson maxLayers "$(( maxLayers - 1 ))" # one layer will be taken up by the customisation layer
        )"

        cat ${baseJson} | jq '
          . + {
            "store_layers": $store_layers,
            "customisation_layer", $customisation_layer,
            "repo_tag": $repo_tag,
            "created": $created
          }
          ' --argjson store_layers "$store_layers" \
            --arg customisation_layer ${customisationLayer} \
            --arg repo_tag "$imageName:$imageTag" \
            --arg created "$created" |
          tee $out
      '';
      result = runCommand "stream-${name}" {
        inherit (conf) imageName;
        passthru = {
          inherit (conf) imageTag;

          # Distinguish tarballs and exes at the Nix level so functions that
          # take images can know in advance how the image is supposed to be used.
          isExe = true;
        };
        nativeBuildInputs = [ makeWrapper ];
      } ''
        makeWrapper ${streamScript} $out --add-flags ${conf}
      '';
    in result;
}