archive/zip: add AddFS method to zip Writer

The method AddFS can be used to add the contents of a fs.FS filesystem
to a zip archive.
This method walks the directory tree starting at the root of the filesystem
and adds each file to the archive.

Fixes: golang#54898

src/archive/zip/writer.go

@@ -11,6 +11,7 @@ import (
 	"hash"
 	"hash/crc32"
 	"io"
+	"io/fs"
 	"strings"
 	"unicode/utf8"
 )
@@ -495,6 +496,41 @@ func (w *Writer) RegisterCompressor(method uint16, comp Compressor) {
 	w.compressors[method] = comp
 }
 
+// AddFS adds the files from fs.FS to the archive.
+// It walks the directory tree starting at the root of the filesystem
+// adding each file to the zip using deflate while maintaining the directory structure.
+func (w *Writer) AddFS(fsys fs.FS) error {
+	return fs.WalkDir(fsys, ".", func(name string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			return nil
+		}
+		info, err := d.Info()
+		if err != nil {
+			return err
+		}
+		h, err := FileInfoHeader(info)
+		if err != nil {
+			return err
+		}
+		h.Name = name
+		h.Method = Deflate
+		fw, err := w.CreateHeader(h)
+		if err != nil {
+			return err
+		}
+		f, err := fsys.Open(name)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+		_, err = io.Copy(fw, f)
+		return err
+	})
+}
+
 func (w *Writer) compressor(method uint16) Compressor {
 	comp := w.compressors[method]
 	if comp == nil {

src/archive/zip/writer_test.go

@@ -14,8 +14,10 @@ import (
 	"io/fs"
 	"math/rand"
 	"os"
+	"reflect"
 	"strings"
 	"testing"
+	"testing/fstest"
 	"time"
 )
 
@@ -602,3 +604,41 @@ func BenchmarkCompressedZipGarbage(b *testing.B) {
 		}
 	})
 }
+
+func TestWriterAddFs(t *testing.T) {
+	expectedFiles := []string{
+		"file.go",
+		"subfolder/another.go",
+	}
+	fsys := fstest.MapFS{
+		"file.go":              {Data: []byte("hello")},
+		"subfolder/another.go": {Data: []byte("world")},
+	}
+	println(fsys)
+
+	buf := new(bytes.Buffer)
+	zw := NewWriter(buf)
+	if err := zw.AddFS(fsys); err != nil {
+		zw.Close()
+		t.Fatal(err)
+	}
+
+	if err := zw.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Test that we can get the files back from the archive
+	zr, err := NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var foundFiles []string
+	for _, file := range zr.File {
+		foundFiles = append(foundFiles, file.FileHeader.Name)
+	}
+
+	if !reflect.DeepEqual(expectedFiles, foundFiles) {
+		t.Fatalf("got %+v, want %+v",
+			foundFiles, expectedFiles)
+	}
+}