io/fs: add Sub

Sub provides a convenient way to refer to a subdirectory
automatically in future operations, like Unix's chdir(2).

The CL also includes updates to fstest to check Sub implementations.

As part of updating fstest, I changed the meaning of TestFS's
expected list to introduce a special case: if you list no expected files,
that means the FS must be empty. In general it's OK not to list all
the expected files, but if you list none, that's almost certainly a
mistake - if your FS were broken and empty, you wouldn't find out.
Making no expected files mean "must be empty" makes the mistake
less likely - if your file system ever worked, then your test will keep
it working.

That change found a testing bug: embedtest was making exactly
that mistake.

Fixes #42322.

Change-Id: I63fd4aa866b30061a0e51ca9a1927e576d6ec41e
Reviewed-on: https://go-review.googlesource.com/c/go/+/274856
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

4 files changed, 210 insertions, 2 deletions

diff --git a/src/io/fs/readdir_test.go b/src/io/fs/readdir_test.go
index 46a4bc2788..405bfa67ca 100644
--- a/src/io/fs/readdir_test.go
+++ b/src/io/fs/readdir_test.go
@@ -16,12 +16,12 @@ func (readDirOnly) Open(name string) (File, error) { return nil, ErrNotExist }
 func TestReadDir(t *testing.T) {
 	check := func(desc string, dirs []DirEntry, err error) {
 		t.Helper()
-		if err != nil || len(dirs) != 1 || dirs[0].Name() != "hello.txt" {
+		if err != nil || len(dirs) != 2 || dirs[0].Name() != "hello.txt" || dirs[1].Name() != "sub" {
 			var names []string
 			for _, d := range dirs {
 				names = append(names, d.Name())
 			}
-			t.Errorf("ReadDir(%s) = %v, %v, want %v, nil", desc, names, err, []string{"hello.txt"})
+			t.Errorf("ReadDir(%s) = %v, %v, want %v, nil", desc, names, err, []string{"hello.txt", "sub"})
 		}
 	}
 
@@ -32,4 +32,12 @@ func TestReadDir(t *testing.T) {
 	// Test that ReadDir uses Open when the method is not present.
 	dirs, err = ReadDir(openOnly{testFsys}, ".")
 	check("openOnly", dirs, err)
+
+	// Test that ReadDir on Sub of . works (sub_test checks non-trivial subs).
+	sub, err := Sub(testFsys, ".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dirs, err = ReadDir(sub, ".")
+	check("sub(.)", dirs, err)
 }
diff --git a/src/io/fs/readfile_test.go b/src/io/fs/readfile_test.go
index 0afa334ace..07219c1445 100644
--- a/src/io/fs/readfile_test.go
+++ b/src/io/fs/readfile_test.go
@@ -18,6 +18,12 @@ var testFsys = fstest.MapFS{
 		ModTime: time.Now(),
 		Sys:     &sysValue,
 	},
+	"sub/goodbye.txt": {
+		Data:    []byte("goodbye, world"),
+		Mode:    0456,
+		ModTime: time.Now(),
+		Sys:     &sysValue,
+	},
 }
 
 var sysValue int
@@ -40,4 +46,14 @@ func TestReadFile(t *testing.T) {
 	if string(data) != "hello, world" || err != nil {
 		t.Fatalf(`ReadFile(openOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
 	}
+
+	// Test that ReadFile on Sub of . works (sub_test checks non-trivial subs).
+	sub, err := Sub(testFsys, ".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	data, err = ReadFile(sub, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(sub(.), "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
 }
diff --git a/src/io/fs/sub.go b/src/io/fs/sub.go
new file mode 100644
index 0000000000..381f409504
--- /dev/null
+++ b/src/io/fs/sub.go
@@ -0,0 +1,127 @@
+// Copyright 2020 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 fs
+
+import (
+	"errors"
+	"path"
+)
+
+// A SubFS is a file system with a Sub method.
+type SubFS interface {
+	FS
+
+	// Sub returns an FS corresponding to the subtree rooted at dir.
+	Sub(dir string) (FS, error)
+}
+
+// Sub returns an FS corresponding to the subtree rooted at fsys's dir.
+//
+// If fs implements SubFS, Sub calls returns fsys.Sub(dir).
+// Otherwise, if dir is ".", Sub returns fsys unchanged.
+// Otherwise, Sub returns a new FS implementation sub that,
+// in effect, implements sub.Open(dir) as fsys.Open(path.Join(dir, name)).
+// The implementation also translates calls to ReadDir, ReadFile, and Glob appropriately.
+//
+// Note that Sub(os.DirFS("/"), "prefix") is equivalent to os.DirFS("/prefix")
+// and that neither of them guarantees to avoid operating system
+// accesses outside "/prefix", because the implementation of os.DirFS
+// does not check for symbolic links inside "/prefix" that point to
+// other directories. That is, os.DirFS is not a general substitute for a
+// chroot-style security mechanism, and Sub does not change that fact.
+func Sub(fsys FS, dir string) (FS, error) {
+	if !ValidPath(dir) {
+		return nil, &PathError{Op: "sub", Path: dir, Err: errors.New("invalid name")}
+	}
+	if dir == "." {
+		return fsys, nil
+	}
+	if fsys, ok := fsys.(SubFS); ok {
+		return fsys.Sub(dir)
+	}
+	return &subFS{fsys, dir}, nil
+}
+
+type subFS struct {
+	fsys FS
+	dir  string
+}
+
+// fullName maps name to the fully-qualified name dir/name.
+func (f *subFS) fullName(op string, name string) (string, error) {
+	if !ValidPath(name) {
+		return "", &PathError{Op: op, Path: name, Err: errors.New("invalid name")}
+	}
+	return path.Join(f.dir, name), nil
+}
+
+// shorten maps name, which should start with f.dir, back to the suffix after f.dir.
+func (f *subFS) shorten(name string) (rel string, ok bool) {
+	if name == f.dir {
+		return ".", true
+	}
+	if len(name) >= len(f.dir)+2 && name[len(f.dir)] == '/' && name[:len(f.dir)] == f.dir {
+		return name[len(f.dir)+1:], true
+	}
+	return "", false
+}
+
+// fixErr shortens any reported names in PathErrors by stripping dir.
+func (f *subFS) fixErr(err error) error {
+	if e, ok := err.(*PathError); ok {
+		if short, ok := f.shorten(e.Path); ok {
+			e.Path = short
+		}
+	}
+	return err
+}
+
+func (f *subFS) Open(name string) (File, error) {
+	full, err := f.fullName("open", name)
+	if err != nil {
+		return nil, err
+	}
+	file, err := f.fsys.Open(full)
+	return file, f.fixErr(err)
+}
+
+func (f *subFS) ReadDir(name string) ([]DirEntry, error) {
+	full, err := f.fullName("open", name)
+	if err != nil {
+		return nil, err
+	}
+	dir, err := ReadDir(f.fsys, full)
+	return dir, f.fixErr(err)
+}
+
+func (f *subFS) ReadFile(name string) ([]byte, error) {
+	full, err := f.fullName("open", name)
+	if err != nil {
+		return nil, err
+	}
+	data, err := ReadFile(f.fsys, full)
+	return data, f.fixErr(err)
+}
+
+func (f *subFS) Glob(pattern string) ([]string, error) {
+	// Check pattern is well-formed.
+	if _, err := path.Match(pattern, ""); err != nil {
+		return nil, err
+	}
+	if pattern == "." {
+		return []string{"."}, nil
+	}
+
+	full := f.dir + "/" + pattern
+	list, err := Glob(f.fsys, full)
+	for i, name := range list {
+		name, ok := f.shorten(name)
+		if !ok {
+			return nil, errors.New("invalid result from inner fsys Glob: " + name + " not in " + f.dir) // can't use fmt in this package
+		}
+		list[i] = name
+	}
+	return list, f.fixErr(err)
+}
diff --git a/src/io/fs/sub_test.go b/src/io/fs/sub_test.go
new file mode 100644
index 0000000000..451b0efb02
--- /dev/null
+++ b/src/io/fs/sub_test.go
@@ -0,0 +1,57 @@
+// Copyright 2020 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 fs_test
+
+import (
+	. "io/fs"
+	"testing"
+)
+
+type subOnly struct{ SubFS }
+
+func (subOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestSub(t *testing.T) {
+	check := func(desc string, sub FS, err error) {
+		t.Helper()
+		if err != nil {
+			t.Errorf("Sub(sub): %v", err)
+			return
+		}
+		data, err := ReadFile(sub, "goodbye.txt")
+		if string(data) != "goodbye, world" || err != nil {
+			t.Errorf(`ReadFile(%s, "goodbye.txt" = %q, %v, want %q, nil`, desc, string(data), err, "goodbye, world")
+		}
+
+		dirs, err := ReadDir(sub, ".")
+		if err != nil || len(dirs) != 1 || dirs[0].Name() != "goodbye.txt" {
+			var names []string
+			for _, d := range dirs {
+				names = append(names, d.Name())
+			}
+			t.Errorf(`ReadDir(%s, ".") = %v, %v, want %v, nil`, desc, names, err, []string{"goodbye.txt"})
+		}
+	}
+
+	// Test that Sub uses the method when present.
+	sub, err := Sub(subOnly{testFsys}, "sub")
+	check("subOnly", sub, err)
+
+	// Test that Sub uses Open when the method is not present.
+	sub, err = Sub(openOnly{testFsys}, "sub")
+	check("openOnly", sub, err)
+
+	_, err = sub.Open("nonexist")
+	if err == nil {
+		t.Fatal("Open(nonexist): succeeded")
+	}
+	pe, ok := err.(*PathError)
+	if !ok {
+		t.Fatalf("Open(nonexist): error is %T, want *PathError", err)
+	}
+	if pe.Path != "nonexist" {
+		t.Fatalf("Open(nonexist): err.Path = %q, want %q", pe.Path, "nonexist")
+	}
+}