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|
// Package image implements logic for creating the image metadata
// (such as the image manifest and configuration).
package manifest
import (
"crypto/sha256"
"encoding/json"
"fmt"
"sort"
)
const (
// manifest constants
schemaVersion = 2
// media types
manifestType = "application/vnd.docker.distribution.manifest.v2+json"
layerType = "application/vnd.docker.image.rootfs.diff.tar"
configType = "application/vnd.docker.container.image.v1+json"
// image config constants
arch = "amd64"
os = "linux"
fsType = "layers"
)
type Entry struct {
MediaType string `json:"mediaType,omitempty"`
Size int64 `json:"size"`
Digest string `json:"digest"`
// This field is internal to Nixery and not part of the
// serialised entry.
MergeRating uint64 `json:"-"`
}
type manifest struct {
SchemaVersion int `json:"schemaVersion"`
MediaType string `json:"mediaType"`
Config Entry `json:"config"`
Layers []Entry `json:"layers"`
}
type imageConfig struct {
Architecture string `json:"architecture"`
OS string `json:"os"`
RootFS struct {
FSType string `json:"type"`
DiffIDs []string `json:"diff_ids"`
} `json:"rootfs"`
// sic! empty struct (rather than `null`) is required by the
// image metadata deserialiser in Kubernetes
Config struct{} `json:"config"`
}
// ConfigLayer represents the configuration layer to be included in
// the manifest, containing its JSON-serialised content and SHA256
// hash.
type ConfigLayer struct {
Config []byte
SHA256 string
}
// imageConfig creates an image configuration with the values set to
// the constant defaults.
//
// Outside of this module the image configuration is treated as an
// opaque blob and it is thus returned as an already serialised byte
// array and its SHA256-hash.
func configLayer(hashes []string) ConfigLayer {
c := imageConfig{}
c.Architecture = arch
c.OS = os
c.RootFS.FSType = fsType
c.RootFS.DiffIDs = hashes
j, _ := json.Marshal(c)
return ConfigLayer{
Config: j,
SHA256: fmt.Sprintf("%x", sha256.Sum256(j)),
}
}
// Manifest creates an image manifest from the specified layer entries
// and returns its JSON-serialised form as well as the configuration
// layer.
//
// Callers do not need to set the media type for the layer entries.
func Manifest(layers []Entry) (json.RawMessage, ConfigLayer) {
// Sort layers by their merge rating, from highest to lowest.
// This makes it likely for a contiguous chain of shared image
// layers to appear at the beginning of a layer.
//
// Due to moby/moby#38446 Docker considers the order of layers
// when deciding which layers to download again.
sort.Slice(layers, func(i, j int) bool {
return layers[i].MergeRating > layers[j].MergeRating
})
hashes := make([]string, len(layers))
for i, l := range layers {
l.MediaType = "application/vnd.docker.image.rootfs.diff.tar"
layers[i] = l
hashes[i] = l.Digest
}
c := configLayer(hashes)
m := manifest{
SchemaVersion: schemaVersion,
MediaType: manifestType,
Config: Entry{
MediaType: configType,
Size: int64(len(c.Config)),
Digest: "sha256:" + c.SHA256,
},
Layers: layers,
}
j, _ := json.Marshal(m)
return json.RawMessage(j), c
}
|
