mkctr: build the files layer in a deterministic order

mkctr.go

@@ -568,7 +568,10 @@ func layerFromFiles(logf logf, files map[string]string, layerMediaType types.Med
 		dirs[dir] = true
 		return nil
 	}
-	for src, dst := range files {
+	// Visit sources in sorted order so the same inputs always produce the
+	// same layer.
+	for _, src := range slices.Sorted(maps.Keys(files)) {
+		dst := files[src]
 		err := filepath.WalkDir(src, func(srcWalk string, d fs.DirEntry, err error) error {
 			path := strings.TrimPrefix(srcWalk, src)
 			dstWalk := filepath.Join(dst, path)

mkctr_test.go

@@ -0,0 +1,45 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/google/go-containerregistry/pkg/v1/types"
+)
+
+func TestLayerFromFilesReproducible(t *testing.T) {
+	dir := t.TempDir()
+	files := map[string]string{}
+	for _, name := range []string{"a", "b", "c", "d", "e"} {
+		src := filepath.Join(dir, name)
+		if err := os.WriteFile(src, []byte(name), 0o644); err != nil {
+			t.Fatal(err)
+		}
+		files[src] = "/usr/local/bin/" + name
+	}
+	logf := func(string, ...any) {}
+
+	var first string
+	for i := range 30 {
+		layer, err := layerFromFiles(logf, files, types.OCILayer)
+		if err != nil {
+			t.Fatal(err)
+		}
+		d, err := layer.DiffID()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if i == 0 {
+			first = d.String()
+			continue
+		}
+		if d.String() != first {
+			t.Fatalf("build %d produced layer %s; build 0 produced %s; want identical layers for identical inputs", i, d, first)
+		}
+	}
+}