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1da6682373
Image layers in manifests are now sorted in a stable (descending) order based on their merge rating, meaning that layers more likely to be shared between images come first. The reason for this change is Docker's handling of image layers on overlayfs2: Images are condensed into a single representation on disk after downloading. Due to this Docker will constantly redownload all layers that are applied in a different order in different images (layer order matters in imperatively created images), based on something it calls the 'ChainID'. Sorting the layers this way raises the likelihood of a long chain of matching layers at the beginning of an image. This relates to #39.
127 lines
3.2 KiB
Go
127 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"
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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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// This field is 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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}
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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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l.MediaType = "application/vnd.docker.image.rootfs.diff.tar"
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layers[i] = l
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hashes[i] = l.Digest
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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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