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391 lines
12 KiB
391 lines
12 KiB
"""
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This script generates a Docker image from a set of store paths. Uses
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Docker Image Specification v1.2 as reference [1].
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It expects a JSON file with the following properties and writes the
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image as an uncompressed tarball to stdout:
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* "architecture", "config", "os", "created", "repo_tag" correspond to
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the fields with the same name on the image spec [2].
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* "created" can be "now".
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* "created" is also used as mtime for files added to the image.
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* "store_layers" is a list of layers in ascending order, where each
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layer is the list of store paths to include in that layer.
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The main challenge for this script to create the final image in a
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streaming fashion, without dumping any intermediate data to disk
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for performance.
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A docker image has each layer contents archived as separate tarballs,
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and they later all get enveloped into a single big tarball in a
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content addressed fashion. However, because how "tar" format works,
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we have to know about the name (which includes the checksum in our
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case) and the size of the tarball before we can start adding it to the
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outer tarball. We achieve that by creating the layer tarballs twice;
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on the first iteration we calculate the file size and the checksum,
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and on the second one we actually stream the contents. 'add_layer_dir'
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function does all this.
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[1]: https://github.com/moby/moby/blob/master/image/spec/v1.2.md
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[2]: https://github.com/moby/moby/blob/4fb59c20a4fb54f944fe170d0ff1d00eb4a24d6f/image/spec/v1.2.md#image-json-field-descriptions
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""" # noqa: E501
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import io
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import os
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import re
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import sys
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import json
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import hashlib
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import pathlib
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import tarfile
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import itertools
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import threading
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from datetime import datetime, timezone
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from collections import namedtuple
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def archive_paths_to(obj, paths, mtime):
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"""
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Writes the given store paths as a tar file to the given stream.
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obj: Stream to write to. Should have a 'write' method.
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paths: List of store paths.
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"""
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# gettarinfo makes the paths relative, this makes them
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# absolute again
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def append_root(ti):
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ti.name = "/" + ti.name
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return ti
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def apply_filters(ti):
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ti.mtime = mtime
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ti.uid = 0
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ti.gid = 0
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ti.uname = "root"
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ti.gname = "root"
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return ti
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def nix_root(ti):
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ti.mode = 0o0555 # r-xr-xr-x
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return ti
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def dir(path):
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ti = tarfile.TarInfo(path)
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ti.type = tarfile.DIRTYPE
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return ti
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with tarfile.open(fileobj=obj, mode="w|") as tar:
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# To be consistent with the docker utilities, we need to have
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# these directories first when building layer tarballs.
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tar.addfile(apply_filters(nix_root(dir("/nix"))))
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tar.addfile(apply_filters(nix_root(dir("/nix/store"))))
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for path in paths:
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path = pathlib.Path(path)
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if path.is_symlink():
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files = [path]
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else:
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files = itertools.chain([path], path.rglob("*"))
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for filename in sorted(files):
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ti = append_root(tar.gettarinfo(filename))
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# copy hardlinks as regular files
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if ti.islnk():
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ti.type = tarfile.REGTYPE
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ti.linkname = ""
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ti.size = filename.stat().st_size
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ti = apply_filters(ti)
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if ti.isfile():
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with open(filename, "rb") as f:
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tar.addfile(ti, f)
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else:
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tar.addfile(ti)
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class ExtractChecksum:
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"""
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A writable stream which only calculates the final file size and
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sha256sum, while discarding the actual contents.
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"""
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def __init__(self):
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self._digest = hashlib.sha256()
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self._size = 0
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def write(self, data):
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self._digest.update(data)
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self._size += len(data)
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def extract(self):
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"""
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Returns: Hex-encoded sha256sum and size as a tuple.
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"""
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return (self._digest.hexdigest(), self._size)
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FromImage = namedtuple("FromImage", ["tar", "manifest_json", "image_json"])
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# Some metadata for a layer
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LayerInfo = namedtuple("LayerInfo", ["size", "checksum", "path", "paths"])
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def load_from_image(from_image_str):
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"""
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Loads the given base image, if any.
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from_image_str: Path to the base image archive.
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Returns: A 'FromImage' object with references to the loaded base image,
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or 'None' if no base image was provided.
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"""
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if from_image_str is None:
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return None
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base_tar = tarfile.open(from_image_str)
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manifest_json_tarinfo = base_tar.getmember("manifest.json")
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with base_tar.extractfile(manifest_json_tarinfo) as f:
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manifest_json = json.load(f)
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image_json_tarinfo = base_tar.getmember(manifest_json[0]["Config"])
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with base_tar.extractfile(image_json_tarinfo) as f:
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image_json = json.load(f)
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return FromImage(base_tar, manifest_json, image_json)
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def add_base_layers(tar, from_image):
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"""
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Adds the layers from the given base image to the final image.
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tar: 'tarfile.TarFile' object for new layers to be added to.
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from_image: 'FromImage' object with references to the loaded base image.
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"""
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if from_image is None:
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print("No 'fromImage' provided", file=sys.stderr)
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return []
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layers = from_image.manifest_json[0]["Layers"]
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checksums = from_image.image_json["rootfs"]["diff_ids"]
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layers_checksums = zip(layers, checksums)
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for num, (layer, checksum) in enumerate(layers_checksums, start=1):
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layer_tarinfo = from_image.tar.getmember(layer)
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checksum = re.sub(r"^sha256:", "", checksum)
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tar.addfile(layer_tarinfo, from_image.tar.extractfile(layer_tarinfo))
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path = layer_tarinfo.path
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size = layer_tarinfo.size
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print("Adding base layer", num, "from", path, file=sys.stderr)
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yield LayerInfo(size=size, checksum=checksum, path=path, paths=[path])
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from_image.tar.close()
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def overlay_base_config(from_image, final_config):
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"""
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Overlays the final image 'config' JSON on top of selected defaults from the
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base image 'config' JSON.
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from_image: 'FromImage' object with references to the loaded base image.
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final_config: 'dict' object of the final image 'config' JSON.
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"""
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if from_image is None:
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return final_config
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base_config = from_image.image_json["config"]
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# Preserve environment from base image
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final_env = base_config.get("Env", []) + final_config.get("Env", [])
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if final_env:
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# Resolve duplicates (last one wins) and format back as list
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resolved_env = {entry.split("=", 1)[0]: entry for entry in final_env}
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final_config["Env"] = list(resolved_env.values())
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return final_config
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def add_layer_dir(tar, paths, store_dir, mtime):
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"""
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Appends given store paths to a TarFile object as a new layer.
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tar: 'tarfile.TarFile' object for the new layer to be added to.
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paths: List of store paths.
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store_dir: the root directory of the nix store
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mtime: 'mtime' of the added files and the layer tarball.
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Should be an integer representing a POSIX time.
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Returns: A 'LayerInfo' object containing some metadata of
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the layer added.
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"""
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invalid_paths = [i for i in paths if not i.startswith(store_dir)]
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assert len(invalid_paths) == 0, \
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f"Expecting absolute paths from {store_dir}, but got: {invalid_paths}"
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# First, calculate the tarball checksum and the size.
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extract_checksum = ExtractChecksum()
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archive_paths_to(
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extract_checksum,
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paths,
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mtime=mtime,
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)
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(checksum, size) = extract_checksum.extract()
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path = f"{checksum}/layer.tar"
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layer_tarinfo = tarfile.TarInfo(path)
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layer_tarinfo.size = size
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layer_tarinfo.mtime = mtime
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# Then actually stream the contents to the outer tarball.
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read_fd, write_fd = os.pipe()
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with open(read_fd, "rb") as read, open(write_fd, "wb") as write:
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def producer():
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archive_paths_to(
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write,
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paths,
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mtime=mtime,
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)
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write.close()
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# Closing the write end of the fifo also closes the read end,
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# so we don't need to wait until this thread is finished.
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#
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# Any exception from the thread will get printed by the default
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# exception handler, and the 'addfile' call will fail since it
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# won't be able to read required amount of bytes.
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threading.Thread(target=producer).start()
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tar.addfile(layer_tarinfo, read)
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return LayerInfo(size=size, checksum=checksum, path=path, paths=paths)
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def add_customisation_layer(target_tar, customisation_layer, mtime):
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"""
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Adds the customisation layer as a new layer. This is layer is structured
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differently; given store path has the 'layer.tar' and corresponding
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sha256sum ready.
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tar: 'tarfile.TarFile' object for the new layer to be added to.
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customisation_layer: Path containing the layer archive.
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mtime: 'mtime' of the added layer tarball.
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"""
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checksum_path = os.path.join(customisation_layer, "checksum")
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with open(checksum_path) as f:
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checksum = f.read().strip()
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assert len(checksum) == 64, f"Invalid sha256 at ${checksum_path}."
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layer_path = os.path.join(customisation_layer, "layer.tar")
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path = f"{checksum}/layer.tar"
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tarinfo = target_tar.gettarinfo(layer_path)
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tarinfo.name = path
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tarinfo.mtime = mtime
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with open(layer_path, "rb") as f:
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target_tar.addfile(tarinfo, f)
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return LayerInfo(
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size=None,
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checksum=checksum,
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path=path,
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paths=[customisation_layer]
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)
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def add_bytes(tar, path, content, mtime):
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"""
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Adds a file to the tarball with given path and contents.
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tar: 'tarfile.TarFile' object.
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path: Path of the file as a string.
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content: Contents of the file.
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mtime: 'mtime' of the file. Should be an integer representing a POSIX time.
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"""
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assert type(content) is bytes
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ti = tarfile.TarInfo(path)
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ti.size = len(content)
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ti.mtime = mtime
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tar.addfile(ti, io.BytesIO(content))
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def main():
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with open(sys.argv[1], "r") as f:
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conf = json.load(f)
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created = (
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datetime.now(tz=timezone.utc)
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if conf["created"] == "now"
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else datetime.fromisoformat(conf["created"])
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)
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mtime = int(created.timestamp())
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store_dir = conf["store_dir"]
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from_image = load_from_image(conf["from_image"])
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with tarfile.open(mode="w|", fileobj=sys.stdout.buffer) as tar:
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layers = []
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layers.extend(add_base_layers(tar, from_image))
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start = len(layers) + 1
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for num, store_layer in enumerate(conf["store_layers"], start=start):
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print("Creating layer", num, "from paths:", store_layer,
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file=sys.stderr)
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info = add_layer_dir(tar, store_layer, store_dir, mtime=mtime)
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layers.append(info)
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print("Creating layer", len(layers) + 1, "with customisation...",
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file=sys.stderr)
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layers.append(
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add_customisation_layer(
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tar,
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conf["customisation_layer"],
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mtime=mtime
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)
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)
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print("Adding manifests...", file=sys.stderr)
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image_json = {
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"created": datetime.isoformat(created),
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"architecture": conf["architecture"],
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"os": "linux",
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"config": overlay_base_config(from_image, conf["config"]),
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"rootfs": {
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"diff_ids": [f"sha256:{layer.checksum}" for layer in layers],
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"type": "layers",
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},
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"history": [
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{
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"created": datetime.isoformat(created),
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"comment": f"store paths: {layer.paths}"
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}
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for layer in layers
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],
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}
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image_json = json.dumps(image_json, indent=4).encode("utf-8")
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image_json_checksum = hashlib.sha256(image_json).hexdigest()
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image_json_path = f"{image_json_checksum}.json"
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add_bytes(tar, image_json_path, image_json, mtime=mtime)
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manifest_json = [
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{
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"Config": image_json_path,
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"RepoTags": [conf["repo_tag"]],
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"Layers": [layer.path for layer in layers],
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}
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]
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manifest_json = json.dumps(manifest_json, indent=4).encode("utf-8")
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add_bytes(tar, "manifest.json", manifest_json, mtime=mtime)
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print("Done.", file=sys.stderr)
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if __name__ == "__main__":
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main()
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