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https://code.tvl.fyi/depot.git:/tools/nixery.git
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f57a2baf92
Docker expects hashes of compressed tarballs in the manifest (as these are used to fetch from the content-addressable layer store), but for some reason it expects hashes in the configuration layer to be of uncompressed tarballs. To achieve this an additional SHA256 hash is calculcated while creating the layer tarballs, but before passing them to the gzip writer. In the current constellation the symlink layer is first compressed and then decompressed again to calculate its hash. This can be refactored in a future change.
129 lines
3.2 KiB
Go
129 lines
3.2 KiB
Go
// Package image implements logic for creating the image metadata
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// (such as the image manifest and configuration).
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package manifest
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import (
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"crypto/sha256"
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"encoding/json"
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"fmt"
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"sort"
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)
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const (
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// manifest constants
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schemaVersion = 2
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// media types
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manifestType = "application/vnd.docker.distribution.manifest.v2+json"
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layerType = "application/vnd.docker.image.rootfs.diff.tar.gzip"
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configType = "application/vnd.docker.container.image.v1+json"
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// image config constants
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arch = "amd64"
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os = "linux"
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fsType = "layers"
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)
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type Entry struct {
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MediaType string `json:"mediaType,omitempty"`
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Size int64 `json:"size"`
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Digest string `json:"digest"`
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// These fields are internal to Nixery and not part of the
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// serialised entry.
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MergeRating uint64 `json:"-"`
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TarHash string `json:",omitempty"`
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}
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type manifest struct {
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SchemaVersion int `json:"schemaVersion"`
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MediaType string `json:"mediaType"`
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Config Entry `json:"config"`
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Layers []Entry `json:"layers"`
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}
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type imageConfig struct {
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Architecture string `json:"architecture"`
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OS string `json:"os"`
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RootFS struct {
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FSType string `json:"type"`
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DiffIDs []string `json:"diff_ids"`
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} `json:"rootfs"`
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// sic! empty struct (rather than `null`) is required by the
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// image metadata deserialiser in Kubernetes
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Config struct{} `json:"config"`
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}
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// ConfigLayer represents the configuration layer to be included in
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// the manifest, containing its JSON-serialised content and SHA256
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// hash.
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type ConfigLayer struct {
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Config []byte
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SHA256 string
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}
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// imageConfig creates an image configuration with the values set to
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// the constant defaults.
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//
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// Outside of this module the image configuration is treated as an
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// opaque blob and it is thus returned as an already serialised byte
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// array and its SHA256-hash.
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func configLayer(hashes []string) ConfigLayer {
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c := imageConfig{}
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c.Architecture = arch
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c.OS = os
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c.RootFS.FSType = fsType
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c.RootFS.DiffIDs = hashes
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j, _ := json.Marshal(c)
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return ConfigLayer{
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Config: j,
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SHA256: fmt.Sprintf("%x", sha256.Sum256(j)),
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}
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}
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// Manifest creates an image manifest from the specified layer entries
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// and returns its JSON-serialised form as well as the configuration
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// layer.
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//
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// Callers do not need to set the media type for the layer entries.
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func Manifest(layers []Entry) (json.RawMessage, ConfigLayer) {
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// Sort layers by their merge rating, from highest to lowest.
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// This makes it likely for a contiguous chain of shared image
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// layers to appear at the beginning of a layer.
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//
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// Due to moby/moby#38446 Docker considers the order of layers
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// when deciding which layers to download again.
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sort.Slice(layers, func(i, j int) bool {
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return layers[i].MergeRating > layers[j].MergeRating
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})
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hashes := make([]string, len(layers))
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for i, l := range layers {
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hashes[i] = l.TarHash
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l.MediaType = layerType
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l.TarHash = ""
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layers[i] = l
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}
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c := configLayer(hashes)
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m := manifest{
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SchemaVersion: schemaVersion,
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MediaType: manifestType,
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Config: Entry{
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MediaType: configType,
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Size: int64(len(c.Config)),
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Digest: "sha256:" + c.SHA256,
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},
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Layers: layers,
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}
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j, _ := json.Marshal(m)
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return json.RawMessage(j), c
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}
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