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-rw-r--r--tools/nixery/server/builder/builder.go1
-rw-r--r--tools/nixery/server/layers/grouping.go8
-rw-r--r--tools/nixery/server/manifest/manifest.go15
3 files changed, 20 insertions, 4 deletions
diff --git a/tools/nixery/server/builder/builder.go b/tools/nixery/server/builder/builder.go
index d8cbbb8f21c9..614291e660c5 100644
--- a/tools/nixery/server/builder/builder.go
+++ b/tools/nixery/server/builder/builder.go
@@ -250,6 +250,7 @@ func prepareLayers(ctx context.Context, s *State, image *Image, result *ImageRes
 			if err != nil {
 				return nil, err
 			}
+			entry.MergeRating = l.MergeRating
 
 			go cacheLayer(ctx, s, l.Hash(), *entry)
 			entries = append(entries, *entry)
diff --git a/tools/nixery/server/layers/grouping.go b/tools/nixery/server/layers/grouping.go
index 07a9e0e230a5..9992cd3c13d6 100644
--- a/tools/nixery/server/layers/grouping.go
+++ b/tools/nixery/server/layers/grouping.go
@@ -141,7 +141,7 @@ type Popularity = map[string]int
 // build for the container image.
 type Layer struct {
 	Contents    []string `json:"contents"`
-	mergeRating uint64
+	MergeRating uint64
 }
 
 // Hash the contents of a layer to create a deterministic identifier that can be
@@ -153,7 +153,7 @@ func (l *Layer) Hash() string {
 
 func (a Layer) merge(b Layer) Layer {
 	a.Contents = append(a.Contents, b.Contents...)
-	a.mergeRating += b.mergeRating
+	a.MergeRating += b.MergeRating
 	return a
 }
 
@@ -291,7 +291,7 @@ func groupLayer(dt *flow.DominatorTree, root *closure) Layer {
 		// both the size and the popularity when making merge
 		// decisions, but there might be a smarter way to do
 		// it than a plain multiplication.
-		mergeRating: uint64(root.Popularity) * size,
+		MergeRating: uint64(root.Popularity) * size,
 	}
 }
 
@@ -309,7 +309,7 @@ func dominate(budget int, graph *simple.DirectedGraph) []Layer {
 	}
 
 	sort.Slice(layers, func(i, j int) bool {
-		return layers[i].mergeRating < layers[j].mergeRating
+		return layers[i].MergeRating < layers[j].MergeRating
 	})
 
 	if len(layers) > budget {
diff --git a/tools/nixery/server/manifest/manifest.go b/tools/nixery/server/manifest/manifest.go
index f777e3f585df..2f236178b65f 100644
--- a/tools/nixery/server/manifest/manifest.go
+++ b/tools/nixery/server/manifest/manifest.go
@@ -6,6 +6,7 @@ import (
 	"crypto/sha256"
 	"encoding/json"
 	"fmt"
+	"sort"
 )
 
 const (
@@ -27,6 +28,10 @@ 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 {
@@ -85,6 +90,16 @@ func configLayer(hashes []string) ConfigLayer {
 //
 // 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"