archive/tar: add AddFS method to Writer

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

Fixes: golang#58000

api/next/58000.txt

@@ -0,0 +1 @@
+pkg archive/tar, method (*Writer) AddFS(fsys fs.FS) error #58000

src/archive/tar/writer.go

@@ -7,6 +7,7 @@ package tar
 import (
 	"fmt"
 	"io"
+	"io/fs"
 	"path"
 	"sort"
 	"strings"
@@ -403,6 +404,37 @@ func (tw *Writer) writeRawHeader(blk *block, size int64, flag byte) error {
 	return nil
 }
 
+// AddFS walks the directory tree provided by fs.FS adding each file to the tar archive.
+func (tw *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
+		if err := tw.WriteHeader(h); err != nil {
+			return err
+		}
+		f, err := fsys.Open(name)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+		_, err = io.Copy(tw, f)
+		return err
+	})
+}
+
 // splitUSTARPath splits a path according to USTAR prefix and suffix rules.
 // If the path is not splittable, then it will return ("", "", false).
 func splitUSTARPath(name string) (prefix, suffix string, ok bool) {

src/archive/tar/writer_test.go

@@ -15,6 +15,7 @@ import (
 	"sort"
 	"strings"
 	"testing"
+	"testing/fstest"
 	"testing/iotest"
 	"time"
 )
@@ -1333,3 +1334,38 @@ func TestFileWriter(t *testing.T) {
 		}
 	}
 }
+
+func TestWriterAddFs(t *testing.T) {
+	expectedFiles := []string{
+		"file.go",
+		"subfolder/another.go",
+	}
+	fsys := fstest.MapFS{
+		"file.go":              {},
+		"subfolder/another.go": {},
+	}
+	var buf bytes.Buffer
+	tw := NewWriter(&buf)
+	if err := tw.AddFS(fsys); err != nil {
+		t.Fatal(err)
+	}
+
+	// Test that we can get the files back from the archive
+	tr := NewReader(&buf)
+	var foundFiles []string
+	for {
+		hdr, err := tr.Next()
+		if err == io.EOF {
+			break // End of archive
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		foundFiles = append(foundFiles, hdr.Name)
+	}
+
+	if !reflect.DeepEqual(expectedFiles, foundFiles) {
+		t.Fatalf("got %+v, want %+v",
+			foundFiles, expectedFiles)
+	}
+}