Change cache key calculation to be more reproducible.

Before we were using the full image digest, but that contains a timestamp. Now
we only use the layers themselves and the image config (env vars, etc.).

Also fix a bug in unpacking the layers themselves. mtimes can change during unpacking,
so set them all once at the end.

integration/dockerfiles/Dockerfile_test_cache

@@ -16,7 +16,14 @@
 # If the image is built twice, /date should be the same in both images
 # if the cache is implemented correctly
 
-FROM gcr.io/google-appengine/debian9@sha256:1d6a9a6d106bd795098f60f4abb7083626354fa6735e81743c7f8cfca11259f0
-RUN date > /date
-COPY context/foo /foo
-RUN echo hey
+FROM alpine as base_stage
+
+RUN mkdir foo && echo base_stage > foo/base
+
+FROM base_stage as cached_stage
+
+RUN echo cached_stage > foo/cache
+
+FROM cached_stage as bug_stage
+
+RUN echo bug_stage > foo/bug

pkg/executor/build.go

@@ -132,6 +132,7 @@ func (s *stageBuilder) optimize(compositeKey CompositeCache, cfg v1.Config, cmds
 			img, err := layerCache.RetrieveLayer(ck)
 			if err != nil {
 				logrus.Infof("No cached layer found for cmd %s", command.String())
+				logrus.Debugf("Key missing was: %s", compositeKey.Key())
 				break
 			}
 
@@ -153,7 +154,11 @@ func (s *stageBuilder) optimize(compositeKey CompositeCache, cfg v1.Config, cmds
 
 func (s *stageBuilder) build() error {
 	// Set the initial cache key to be the base image digest, the build args and the SrcContext.
-	compositeKey := NewCompositeCache(s.baseImageDigest)
+	dgst, err := util.ReproducibleDigest(s.image)
+	if err != nil {
+		return err
+	}
+	compositeKey := NewCompositeCache(dgst)
 	compositeKey.AddKey(s.opts.BuildArgs...)
 
 	cmds := []commands.DockerCommand{}

pkg/util/fs_util.go

@@ -75,7 +75,10 @@ func GetFSFromImage(root string, img v1.Image) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	extractedFiles := []string{}
+
+	// Store a map of files to their mtime. We need to set mtimes in a second pass because creating files
+	// can change the mtime of a directory.
+	extractedFiles := map[string]time.Time{}
 
 	for i, l := range layers {
 		logrus.Debugf("Extracting layer %d", i)
@@ -106,10 +109,17 @@ func GetFSFromImage(root string, img v1.Image) ([]string, error) {
 			if err := extractFile(root, hdr, tr); err != nil {
 				return nil, err
 			}
-			extractedFiles = append(extractedFiles, filepath.Join(root, filepath.Clean(hdr.Name)))
+			extractedFiles[filepath.Join(root, filepath.Clean(hdr.Name))] = hdr.ModTime
 		}
 	}
-	return extractedFiles, nil
+
+	fileNames := []string{}
+	for f, t := range extractedFiles {
+		fileNames = append(fileNames, f)
+		os.Chtimes(f, time.Time{}, t)
+	}
+
+	return fileNames, nil
 }
 
 // DeleteFilesystem deletes the extracted image file system
@@ -272,6 +282,7 @@ func extractFile(dest string, hdr *tar.Header, tr io.Reader) error {
 			return err
 		}
 	}
+
 	return nil
 }
 
@@ -382,7 +393,8 @@ func RelativeFiles(fp string, root string) ([]string, error) {
 }
 
 // ParentDirectories returns a list of paths to all parent directories
-// Ex. /some/temp/dir -> [/, /some, /some/temp, /some/temp/dir]
+// Ex. /some/temp/dir -> [/some, /some/temp, /some/temp/dir]
+// This purposefully excludes the /.
 func ParentDirectories(path string) []string {
 	path = filepath.Clean(path)
 	dirs := strings.Split(path, "/")

pkg/util/tar_util.go

@@ -54,6 +54,7 @@ func (t *Tar) Close() {
 
 // AddFileToTar adds the file at path p to the tar
 func (t *Tar) AddFileToTar(p string) error {
+	logrus.Debugf("Adding file %s to tar", p)
 	i, err := os.Lstat(p)
 	if err != nil {
 		return fmt.Errorf("Failed to get file info for %s: %s", p, err)

pkg/util/util.go

@@ -20,11 +20,13 @@ import (
 	"crypto/md5"
 	"crypto/sha256"
 	"encoding/hex"
+	"encoding/json"
 	"io"
 	"os"
 	"strconv"
 	"syscall"
 
+	"github.com/google/go-containerregistry/pkg/v1"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 )
@@ -127,3 +129,29 @@ func SHA256(r io.Reader) (string, error) {
 	}
 	return hex.EncodeToString(hasher.Sum(make([]byte, 0, hasher.Size()))), nil
 }
+
+type ReproducibleManifest struct {
+	Layers []v1.Descriptor
+	Config v1.Config
+}
+
+func ReproducibleDigest(img v1.Image) (string, error) {
+	mfst, err := img.Manifest()
+	if err != nil {
+		return "", err
+	}
+	cfg, err := img.ConfigFile()
+	if err != nil {
+		return "", err
+	}
+	rm := ReproducibleManifest{
+		Layers: mfst.Layers,
+		Config: cfg.Config,
+	}
+
+	b, err := json.Marshal(rm)
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}