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package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"os"
"strings"
)
const dockerDbDir = "/var/lib/docker/image/btrfs"
const repoFile = dockerDbDir + "/repositories.json"
const cfgFile = "/etc/docker-btrfs.json"
func getImageTag() (string, string) {
buf, err := ioutil.ReadFile(cfgFile)
if err != nil {
panic(err)
}
var dat map[string]interface{}
err = json.Unmarshal(buf, &dat)
if err != nil {
panic(err)
}
return dat["docker_image"].(string), dat["docker_tag"].(string)
}
func getImageID(image string, tag string) string {
buf, err := ioutil.ReadFile(repoFile)
if err != nil {
panic(err)
}
var dat map[string]interface{}
err = json.Unmarshal(buf, &dat)
if err != nil {
panic(err)
}
repositories := dat["Repositories"].(map[string]interface{})
imageTags := repositories[image].(map[string]interface{})
return imageTags[image+":"+tag].(string)
}
func getDiffIDs(imgID string) []string {
imgPath := strings.Replace(imgID, ":", "/", 1)
buf, err := ioutil.ReadFile(dockerDbDir + "/imagedb/content/" + imgPath)
if err != nil {
panic(err)
}
var dat map[string]interface{}
err = json.Unmarshal(buf, &dat)
if err != nil {
panic(err)
}
rootfs := dat["rootfs"].(map[string]interface{})
diffIDsInterface := rootfs["diff_ids"].([]interface{})
diffIDsStr := make([]string, len(diffIDsInterface))
for i, v := range diffIDsInterface {
diffIDsStr[i] = strings.Replace(v.(string), "sha256:", "", 1)
}
return diffIDsStr
}
type node struct {
parent string
id string
diff string
cache string
}
func getNode(id string) node {
var node node
node.id = id
buf, err := ioutil.ReadFile(dockerDbDir + "/layerdb/sha256/" + id + "/cache-id")
if err != nil {
panic(err)
}
node.cache = strings.Replace(string(buf), "sha256:", "", 1)
buf, err = ioutil.ReadFile(dockerDbDir + "/layerdb/sha256/" + id + "/diff")
if err != nil {
panic(err)
}
node.diff = strings.Replace(string(buf), "sha256:", "", 1)
buf, err = ioutil.ReadFile(dockerDbDir + "/layerdb/sha256/" + id + "/parent")
if os.IsNotExist(err) {
node.parent = ""
} else if err != nil {
panic(err)
} else {
node.parent = strings.Replace(string(buf), "sha256:", "", 1)
}
return node
}
func readLayerInfo() map[string]node {
files, err := ioutil.ReadDir(dockerDbDir + "/layerdb/sha256/")
if err != nil {
panic(err)
}
nodes := make(map[string]node)
for i := range files {
id := files[i].Name()
nodes[id] = getNode(id)
}
return nodes
}
func searchForCacheID(diffIDs []string, layers map[string]node) string {
candidates := make([]string, 0, len(layers))
last := diffIDs[len(diffIDs)-1]
for i, v := range layers {
if v.diff == last {
sz := len(candidates)
candidates = candidates[:sz+1]
candidates[sz] = i
}
}
candidatesCurrent := make([]string, len(candidates))
for i, v := range candidates {
candidatesCurrent[i] = layers[v].parent
}
for i := len(diffIDs) - 2; i >= 0; i-- {
for j := 0; j < len(candidates); {
if layers[candidatesCurrent[j]].diff == diffIDs[i] {
candidatesCurrent[j] = layers[candidatesCurrent[j]].parent
j++
} else {
oldLen := len(candidates)
candidates[j] = candidates[oldLen-1]
candidatesCurrent[j] = candidatesCurrent[oldLen-1]
candidates = candidates[:oldLen-1]
candidatesCurrent = candidatesCurrent[:oldLen-1]
}
}
}
return layers[candidates[0]].cache
}
func main() {
image, tag := getImageTag()
imgID := getImageID(image, tag)
diffIDs := getDiffIDs(imgID)
layers := readLayerInfo()
cacheID := searchForCacheID(diffIDs, layers)
fmt.Print(cacheID)
}
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