txtar: add fs.FS support

Fixes #44158

From implementation copied with permission from
https://github.com/josharian/txtarfs/blob/main/txtarfs_test.go
golang/go#44158 (comment)

Co-authored-by: josharian

Author: earthboundkid

txtar/fs.go

@@ -0,0 +1,221 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.16
+
+package txtar
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+	"path"
+	"sort"
+	"strings"
+	"time"
+)
+
+var _ fs.FS = (*Archive)(nil)
+
+// Open implements fs.FS.
+func (a *Archive) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+
+	for _, f := range a.Files {
+		if f.Name == name {
+			return &openFile{f, 0}, nil
+		}
+	}
+	var list []fileInfo
+	var dirs = make(map[string]bool)
+	if name == "." {
+		for _, f := range a.Files {
+			i := strings.Index(f.Name, "/")
+			if i < 0 {
+				list = append(list, fileInfo{f, 0666})
+			} else {
+				dirs[f.Name[:i]] = true
+			}
+		}
+	} else {
+		prefix := name + "/"
+		for _, f := range a.Files {
+			if strings.HasPrefix(f.Name, prefix) {
+				felem := f.Name[len(prefix):]
+				i := strings.Index(felem, "/")
+				if i < 0 {
+					list = append(list, fileInfo{f, 0666})
+				} else {
+					dirs[f.Name[len(prefix):len(prefix)+i]] = true
+				}
+			}
+		}
+		// If there are no children of the name,
+		// then the directory is treated as not existing.
+		if list == nil && len(dirs) == 0 {
+			return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+		}
+	}
+	for name := range dirs {
+		list = append(list, fileInfo{File{Name: name}, fs.ModeDir | 0666})
+	}
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].File.Name < list[j].File.Name
+	})
+
+	return &openDir{name, fileInfo{File{Name: name}, fs.ModeDir | 0666}, list, 0}, nil
+}
+
+var _ fs.ReadFileFS = (*Archive)(nil)
+
+// ReadFile implements fs.ReadFileFS.
+func (a *Archive) ReadFile(name string) ([]byte, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	if name == "." {
+		return nil, &fs.PathError{Op: "read", Path: name, Err: errors.New("is a directory")}
+	}
+	prefix := name + "/"
+	for _, f := range a.Files {
+		if f.Name == name {
+			return f.Data, nil
+		}
+		// It's a directory
+		if strings.HasPrefix(f.Name, prefix) {
+			return nil, &fs.PathError{Op: "read", Path: name, Err: errors.New("is a directory")}
+		}
+	}
+	return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+}
+
+var _ fs.File = (*openFile)(nil)
+
+type openFile struct {
+	File
+	offset int64
+}
+
+func (o *openFile) Stat() (fs.FileInfo, error) { return fileInfo{o.File, 0666}, nil }
+
+func (o *openFile) Close() error { return nil }
+
+func (f *openFile) Read(b []byte) (int, error) {
+	if f.offset >= int64(len(f.File.Data)) {
+		return 0, io.EOF
+	}
+	if f.offset < 0 {
+		return 0, &fs.PathError{Op: "read", Path: f.File.Name, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.File.Data[f.offset:])
+	f.offset += int64(n)
+	return n, nil
+}
+
+func (f *openFile) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case 0:
+		// offset += 0
+	case 1:
+		offset += f.offset
+	case 2:
+		offset += int64(len(f.File.Data))
+	}
+	if offset < 0 || offset > int64(len(f.File.Data)) {
+		return 0, &fs.PathError{Op: "seek", Path: f.File.Name, Err: fs.ErrInvalid}
+	}
+	f.offset = offset
+	return offset, nil
+}
+
+func (f *openFile) ReadAt(b []byte, offset int64) (int, error) {
+	if offset < 0 || offset > int64(len(f.File.Data)) {
+		return 0, &fs.PathError{Op: "read", Path: f.File.Name, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.File.Data[offset:])
+	if n < len(b) {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+var _ fs.FileInfo = fileInfo{}
+
+type fileInfo struct {
+	File
+	m fs.FileMode
+}
+
+func (f fileInfo) Name() string               { return path.Base(f.File.Name) }
+func (f fileInfo) Size() int64                { return int64(len(f.File.Data)) }
+func (f fileInfo) Mode() fs.FileMode          { return f.m }
+func (f fileInfo) Type() fs.FileMode          { return f.m.Type() }
+func (f fileInfo) ModTime() time.Time         { return time.Time{} }
+func (f fileInfo) IsDir() bool                { return f.m.IsDir() }
+func (f fileInfo) Sys() interface{}           { return f.File }
+func (f fileInfo) Info() (fs.FileInfo, error) { return f, nil }
+
+type openDir struct {
+	path string
+	fileInfo
+	entry  []fileInfo
+	offset int
+}
+
+func (d *openDir) Stat() (fs.FileInfo, error) { return &d.fileInfo, nil }
+func (d *openDir) Close() error               { return nil }
+func (d *openDir) Read(b []byte) (int, error) {
+	return 0, &fs.PathError{Op: "read", Path: d.path, Err: errors.New("is a directory")}
+}
+
+func (d *openDir) ReadDir(count int) ([]fs.DirEntry, error) {
+	n := len(d.entry) - d.offset
+	if count > 0 && n > count {
+		n = count
+	}
+	if n == 0 && count > 0 {
+		return nil, io.EOF
+	}
+	list := make([]fs.DirEntry, n)
+	for i := range list {
+		list[i] = &d.entry[d.offset+i]
+	}
+	d.offset += n
+	return list, nil
+}
+
+// From constructs an Archive with the contents of fsys and an empty Comment.
+// Subsequent changes to fsys are not reflected in the returned archive.
+//
+// The transformation is lossy.
+// For example, because directories are implicit in txtar archives,
+// empty directories in fsys will be lost, and txtar does not represent file mode, mtime, or other file metadata.
+// From does not guarantee that a.File[i].Data contain no file marker lines.
+// See also warnings on Format.
+// In short, it is unwise to use txtar as a generic filesystem serialization mechanism.
+func From(fsys fs.FS) (*Archive, error) {
+	ar := new(Archive)
+	walkfn := func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			// Directories in txtar are implicit.
+			return nil
+		}
+		data, err := fs.ReadFile(fsys, path)
+		if err != nil {
+			return err
+		}
+		ar.Files = append(ar.Files, File{Name: path, Data: data})
+		return nil
+	}
+
+	if err := fs.WalkDir(fsys, ".", walkfn); err != nil {
+		return nil, err
+	}
+	return ar, nil
+}

txtar/fs_test.go

@@ -0,0 +1,93 @@
+// Copyright 2021 The Go 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 txtar
+
+import (
+	"io/fs"
+	"sort"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+func TestFS(t *testing.T) {
+	for _, tc := range []struct{ name, input, files string }{
+		{
+			name:  "empty",
+			input: ``,
+			files: "",
+		},
+		{
+			name: "one",
+			input: `
+-- one.txt --
+one
+`,
+			files: "one.txt",
+		},
+		{
+			name: "two",
+			input: `
+-- one.txt --
+one
+-- two.txt --
+two
+`,
+			files: "one.txt two.txt",
+		},
+		{
+			name: "subdirectories",
+			input: `
+-- one.txt --
+one
+-- 2/two.txt --
+two
+-- 2/3/three.txt --
+three
+-- 4/four.txt --
+three
+`,
+			files: "one.txt 2/two.txt 2/3/three.txt 4/four.txt",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			a := Parse([]byte(tc.input))
+			files := strings.Fields(tc.files)
+			if err := fstest.TestFS(a, files...); err != nil {
+				t.Fatal(err)
+			}
+			for _, name := range files {
+				for _, f := range a.Files {
+					if f.Name == name {
+						b, err := fs.ReadFile(a, name)
+						if err != nil {
+							t.Fatal(err)
+						}
+						if string(b) != string(f.Data) {
+							t.Fatalf("mismatched contents for %q", name)
+						}
+					}
+				}
+			}
+			a2, err := From(a)
+			if err != nil {
+				t.Fatalf("failed to write fsys for %v: %v", tc.name, err)
+			}
+
+			if in, out := normalized(a), normalized(a2); in != out {
+				t.Errorf("From round trip failed: %q != %q", in, out)
+			}
+
+		})
+	}
+}
+
+func normalized(a *Archive) string {
+	a.Comment = nil
+	sort.Slice(a.Files, func(i, j int) bool {
+		return a.Files[i].Name < a.Files[j].Name
+	})
+	return string(Format(a))
+}