mkctr: return an error instead of panicking on a missing --files source

mkctr.go

@@ -570,9 +570,14 @@ func layerFromFiles(logf logf, files map[string]string, layerMediaType types.Med
 	}
 	for src, dst := range files {
 		err := filepath.WalkDir(src, func(srcWalk string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
 			path := strings.TrimPrefix(srcWalk, src)
 			dstWalk := filepath.Join(dst, path)
-			writeDir(filepath.Dir(dstWalk))
+			if err := writeDir(filepath.Dir(dstWalk)); err != nil {
+				return err
+			}
 			if d.IsDir() {
 				return writeDir(dstWalk)
 			}

mkctr_test.go

@@ -0,0 +1,64 @@
+// 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 (
+	"archive/tar"
+	"io"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/google/go-containerregistry/pkg/v1/types"
+)
+
+func TestLayerFromFilesMissingSource(t *testing.T) {
+	missing := filepath.Join(t.TempDir(), "does-not-exist")
+	files := map[string]string{missing: "/app/file"}
+	logf := func(string, ...any) {}
+
+	_, err := layerFromFiles(logf, files, types.OCILayer)
+	if err == nil {
+		t.Fatal("layerFromFiles succeeded for a source path that does not exist")
+	}
+	if !strings.Contains(err.Error(), "does-not-exist") {
+		t.Errorf("error = %q; want it to name the missing path", err)
+	}
+}
+
+func TestLayerFromFilesCopiesFiles(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "a.txt"), []byte("hello"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	logf := func(string, ...any) {}
+
+	layer, err := layerFromFiles(logf, map[string]string{dir: "/app"}, types.OCILayer)
+	if err != nil {
+		t.Fatal(err)
+	}
+	rc, err := layer.Uncompressed()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rc.Close()
+	var got []string
+	tr := tar.NewReader(rc)
+	for {
+		h, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		got = append(got, h.Name)
+	}
+	if !slices.Contains(got, "/app/a.txt") {
+		t.Errorf("layer entries = %q; want /app/a.txt", got)
+	}
+}