archive/tar: add AddFS method to Writer

api/next/58000.txt

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

src/archive/tar/writer.go

@@ -5,8 +5,10 @@
 package tar
 
 import (
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"path"
 	"sort"
 	"strings"
@@ -403,6 +405,43 @@ func (tw *Writer) writeRawHeader(blk *block, size int64, flag byte) error {
 	return nil
 }
 
+// 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 tar archive while maintaining the directory structure.
+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
+		}
+		// TODO(#49580): Handle symlinks when fs.ReadLinkFS is available.
+		if !info.Mode().IsRegular() {
+			return errors.New("tar: cannot add non-regular file")
+		}
+		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

@@ -9,12 +9,14 @@ import (
 	"encoding/hex"
 	"errors"
 	"io"
+	"io/fs"
 	"os"
 	"path"
 	"reflect"
 	"sort"
 	"strings"
 	"testing"
+	"testing/fstest"
 	"testing/iotest"
 	"time"
 )
@@ -1333,3 +1335,67 @@ func TestFileWriter(t *testing.T) {
 		}
 	}
 }
+
+func TestWriterAddFS(t *testing.T) {
+	fsys := fstest.MapFS{
+		"file.go":              {Data: []byte("hello")},
+		"subfolder/another.go": {Data: []byte("world")},
+	}
+	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)
+
+	entries, err := fsys.ReadDir(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var curfname string
+	for _, entry := range entries {
+		curfname = entry.Name()
+		if entry.IsDir() {
+			curfname += "/"
+			continue
+		}
+		hdr, err := tr.Next()
+		if err == io.EOF {
+			break // End of archive
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		data, err := io.ReadAll(tr)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if hdr.Name != curfname {
+			t.Fatalf("got filename %v, want %v",
+				curfname, hdr.Name)
+		}
+
+		origdata := fsys[curfname].Data
+		if string(data) != string(origdata) {
+			t.Fatalf("got file content %v, want %v",
+				data, origdata)
+		}
+	}
+}
+
+func TestWriterAddFSNonRegularFiles(t *testing.T) {
+	fsys := fstest.MapFS{
+		"device":  {Data: []byte("hello"), Mode: 0755 | fs.ModeDevice},
+		"symlink": {Data: []byte("world"), Mode: 0755 | fs.ModeSymlink},
+	}
+	var buf bytes.Buffer
+	tw := NewWriter(&buf)
+	if err := tw.AddFS(fsys); err == nil {
+		t.Fatal("expected error, got nil")
+	}
+}