os: add Root.Stat and Root.Lstat

For #67002

Change-Id: I0903f45dbb4c44ea0280c340c96c5f3c3c0781be
Reviewed-on: https://go-review.googlesource.com/c/go/+/627475
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>

api/next/67002.txt

@@ -1,10 +1,12 @@
 pkg os, func OpenRoot(string) (*Root, error) #67002
 pkg os, method (*Root) Close() error #67002
 pkg os, method (*Root) Create(string) (*File, error) #67002
+pkg os, method (*Root) Lstat(string) (fs.FileInfo, error) #67002
 pkg os, method (*Root) Mkdir(string, fs.FileMode) error #67002
 pkg os, method (*Root) Name() string #67002
 pkg os, method (*Root) Open(string) (*File, error) #67002
 pkg os, method (*Root) OpenFile(string, int, fs.FileMode) (*File, error) #67002
 pkg os, method (*Root) OpenRoot(string) (*Root, error) #67002
 pkg os, method (*Root) Remove(string) error #67002
+pkg os, method (*Root) Stat(string) (fs.FileInfo, error) #67002
 pkg os, type Root struct #67002

src/internal/syscall/unix/at_wasip1.go

@@ -11,6 +11,7 @@ import (
 	"unsafe"
 )
 
+// The values of these constants are not part of the WASI API.
 const (
 	// UTIME_OMIT is the sentinel value to indicate that a time value should not
 	// be changed. It is useful for example to indicate for example with UtimesNano
@@ -18,7 +19,8 @@ const (
 	// Its value must match syscall/fs_wasip1.go
 	UTIME_OMIT = -0x2
 
-	AT_REMOVEDIR = 0x200
+	AT_REMOVEDIR        = 0x200
+	AT_SYMLINK_NOFOLLOW = 0x100
 )
 
 func Unlinkat(dirfd int, path string, flags int) error {
@@ -49,6 +51,24 @@ func Openat(dirfd int, path string, flags int, perm uint32) (int, error) {
 	return syscall.Openat(dirfd, path, flags, perm)
 }
 
+func Fstatat(dirfd int, path string, stat *syscall.Stat_t, flags int) error {
+	var filestatFlags uint32
+	if flags&AT_SYMLINK_NOFOLLOW == 0 {
+		filestatFlags |= syscall.LOOKUP_SYMLINK_FOLLOW
+	}
+	return errnoErr(path_filestat_get(
+		int32(dirfd),
+		uint32(filestatFlags),
+		unsafe.StringData(path),
+		size(len(path)),
+		unsafe.Pointer(stat),
+	))
+}
+
+//go:wasmimport wasi_snapshot_preview1 path_filestat_get
+//go:noescape
+func path_filestat_get(fd int32, flags uint32, path *byte, pathLen size, buf unsafe.Pointer) syscall.Errno
+
 func Readlinkat(dirfd int, path string, buf []byte) (int, error) {
 	var nwritten size
 	errno := path_readlink(

src/internal/syscall/windows/at_windows.go

@@ -19,6 +19,7 @@ import (
 const (
 	O_DIRECTORY    = 0x100000   // target must be a directory
 	O_NOFOLLOW_ANY = 0x20000000 // disallow symlinks anywhere in the path
+	O_OPEN_REPARSE = 0x40000000 // FILE_OPEN_REPARSE_POINT, used by Lstat
 )
 
 func Openat(dirfd syscall.Handle, name string, flag int, perm uint32) (_ syscall.Handle, e1 error) {
@@ -37,6 +38,10 @@ func Openat(dirfd syscall.Handle, name string, flag int, perm uint32) (_ syscall
 	case syscall.O_RDWR:
 		access = FILE_GENERIC_READ | FILE_GENERIC_WRITE
 		options |= FILE_NON_DIRECTORY_FILE
+	default:
+		// Stat opens files without requesting read or write permissions,
+		// but we still need to request SYNCHRONIZE.
+		access = SYNCHRONIZE
 	}
 	if flag&syscall.O_CREAT != 0 {
 		access |= FILE_GENERIC_WRITE
@@ -70,6 +75,10 @@ func Openat(dirfd syscall.Handle, name string, flag int, perm uint32) (_ syscall
 		return syscall.InvalidHandle, err
 	}
 
+	if flag&O_OPEN_REPARSE != 0 {
+		options |= FILE_OPEN_REPARSE_POINT
+	}
+
 	// We don't use FILE_OVERWRITE/FILE_OVERWRITE_IF, because when opening
 	// a file with FILE_ATTRIBUTE_READONLY these will replace an existing
 	// file with a new, read-only one.

src/internal/syscall/windows/syscall_windows.go

@@ -41,6 +41,7 @@ const (
 	ERROR_LOCK_FAILED            syscall.Errno = 167
 	ERROR_NO_TOKEN               syscall.Errno = 1008
 	ERROR_NO_UNICODE_TRANSLATION syscall.Errno = 1113
+	ERROR_CANT_ACCESS_FILE       syscall.Errno = 1920
 )
 
 const (

src/os/root.go

@@ -126,6 +126,22 @@ func (r *Root) Remove(name string) error {
 	return rootRemove(r, name)
 }
 
+// Stat returns a [FileInfo] describing the named file in the root.
+// See [Stat] for more details.
+func (r *Root) Stat(name string) (FileInfo, error) {
+	r.logStat(name)
+	return rootStat(r, name, false)
+}
+
+// Lstat returns a [FileInfo] describing the named file in the root.
+// If the file is a symbolic link, the returned FileInfo
+// describes the symbolic link.
+// See [Lstat] for more details.
+func (r *Root) Lstat(name string) (FileInfo, error) {
+	r.logStat(name)
+	return rootStat(r, name, true)
+}
+
 func (r *Root) logOpen(name string) {
 	if log := testlog.Logger(); log != nil {
 		// This won't be right if r's name has changed since it was opened,
@@ -134,6 +150,14 @@ func (r *Root) logOpen(name string) {
 	}
 }
 
+func (r *Root) logStat(name string) {
+	if log := testlog.Logger(); log != nil {
+		// This won't be right if r's name has changed since it was opened,
+		// but it's the best we can do.
+		log.Stat(joinPath(r.Name(), name))
+	}
+}
+
 // splitPathInRoot splits a path into components
 // and joins it with the given prefix and suffix.
 //

src/os/root_noopenat.go

@@ -75,6 +75,26 @@ func rootOpenFileNolog(r *Root, name string, flag int, perm FileMode) (*File, er
 	return f, nil
 }
 
+func rootStat(r *Root, name string, lstat bool) (FileInfo, error) {
+	var fi FileInfo
+	var err error
+	if lstat {
+		err = checkPathEscapesLstat(r, name)
+		if err == nil {
+			fi, err = Lstat(joinPath(r.root.name, name))
+		}
+	} else {
+		err = checkPathEscapes(r, name)
+		if err == nil {
+			fi, err = Stat(joinPath(r.root.name, name))
+		}
+	}
+	if err != nil {
+		return nil, &PathError{Op: "statat", Path: name, Err: underlyingError(err)}
+	}
+	return fi, nil
+}
+
 func rootMkdir(r *Root, name string, perm FileMode) error {
 	if err := checkPathEscapes(r, name); err != nil {
 		return &PathError{Op: "mkdirat", Path: name, Err: err}

src/os/root_test.go

@@ -509,6 +509,67 @@ func TestRootOpenFileAsRoot(t *testing.T) {
 	}
 }
 
+func TestRootStat(t *testing.T) {
+	for _, test := range rootTestCases {
+		test.run(t, func(t *testing.T, target string, root *os.Root) {
+			const content = "content"
+			if target != "" {
+				if err := os.WriteFile(target, []byte(content), 0o666); err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			fi, err := root.Stat(test.open)
+			if errEndsTest(t, err, test.wantError, "root.Stat(%q)", test.open) {
+				return
+			}
+			if got, want := fi.Name(), filepath.Base(test.open); got != want {
+				t.Errorf("root.Stat(%q).Name() = %q, want %q", test.open, got, want)
+			}
+			if got, want := fi.Size(), int64(len(content)); got != want {
+				t.Errorf("root.Stat(%q).Size() = %v, want %v", test.open, got, want)
+			}
+		})
+	}
+}
+
+func TestRootLstat(t *testing.T) {
+	for _, test := range rootTestCases {
+		test.run(t, func(t *testing.T, target string, root *os.Root) {
+			const content = "content"
+			wantError := test.wantError
+			if test.ltarget != "" {
+				// Lstat will stat the final link, rather than following it.
+				wantError = false
+			} else if target != "" {
+				if err := os.WriteFile(target, []byte(content), 0o666); err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			fi, err := root.Lstat(test.open)
+			if errEndsTest(t, err, wantError, "root.Stat(%q)", test.open) {
+				return
+			}
+			if got, want := fi.Name(), filepath.Base(test.open); got != want {
+				t.Errorf("root.Stat(%q).Name() = %q, want %q", test.open, got, want)
+			}
+			if test.ltarget == "" {
+				if got := fi.Mode(); got&os.ModeSymlink != 0 {
+					t.Errorf("root.Stat(%q).Mode() = %v, want non-symlink", test.open, got)
+				}
+				if got, want := fi.Size(), int64(len(content)); got != want {
+					t.Errorf("root.Stat(%q).Size() = %v, want %v", test.open, got, want)
+				}
+			} else {
+				if got := fi.Mode(); got&os.ModeSymlink == 0 {
+					t.Errorf("root.Stat(%q).Mode() = %v, want symlink", test.open, got)
+				}
+			}
+		})
+	}
+}
+
 // A rootConsistencyTest is a test case comparing os.Root behavior with
 // the corresponding non-Root function.
 //
@@ -599,6 +660,24 @@ var rootConsistencyTestCases = []rootConsistencyTest{{
 		"link => target",
 	},
 	open: "link/",
+}, {
+	name: "symlink slash dot",
+	fs: []string{
+		"target/file",
+		"link => target",
+	},
+	open: "link/.",
+}, {
+	name: "file symlink slash",
+	fs: []string{
+		"target",
+		"link => target",
+	},
+	open: "link/",
+	detailedErrorMismatch: func(t *testing.T) bool {
+		// os.Create returns ENOTDIR or EISDIR depending on the platform.
+		return runtime.GOOS == "js"
+	},
 }, {
 	name: "unresolved symlink",
 	fs: []string{
@@ -817,6 +896,42 @@ func TestRootConsistencyRemove(t *testing.T) {
 	}
 }
 
+func TestRootConsistencyStat(t *testing.T) {
+	for _, test := range rootConsistencyTestCases {
+		test.run(t, func(t *testing.T, path string, r *os.Root) (string, error) {
+			var fi os.FileInfo
+			var err error
+			if r == nil {
+				fi, err = os.Stat(path)
+			} else {
+				fi, err = r.Stat(path)
+			}
+			if err != nil {
+				return "", err
+			}
+			return fmt.Sprintf("name:%q size:%v mode:%v isdir:%v", fi.Name(), fi.Size(), fi.Mode(), fi.IsDir()), nil
+		})
+	}
+}
+
+func TestRootConsistencyLstat(t *testing.T) {
+	for _, test := range rootConsistencyTestCases {
+		test.run(t, func(t *testing.T, path string, r *os.Root) (string, error) {
+			var fi os.FileInfo
+			var err error
+			if r == nil {
+				fi, err = os.Lstat(path)
+			} else {
+				fi, err = r.Lstat(path)
+			}
+			if err != nil {
+				return "", err
+			}
+			return fmt.Sprintf("name:%q size:%v mode:%v isdir:%v", fi.Name(), fi.Size(), fi.Mode(), fi.IsDir()), nil
+		})
+	}
+}
+
 func TestRootRenameAfterOpen(t *testing.T) {
 	switch runtime.GOOS {
 	case "windows":

src/os/root_unix.go

@@ -113,6 +113,24 @@ func rootOpenDir(parent int, name string) (int, error) {
 	return fd, err
 }
 
+func rootStat(r *Root, name string, lstat bool) (FileInfo, error) {
+	fi, err := doInRoot(r, name, func(parent sysfdType, n string) (FileInfo, error) {
+		var fs fileStat
+		if err := unix.Fstatat(parent, n, &fs.sys, unix.AT_SYMLINK_NOFOLLOW); err != nil {
+			return nil, err
+		}
+		fillFileStatFromSys(&fs, name)
+		if !lstat && fs.Mode()&ModeSymlink != 0 {
+			return nil, checkSymlink(parent, n, syscall.ELOOP)
+		}
+		return &fs, nil
+	})
+	if err != nil {
+		return nil, &PathError{Op: "statat", Path: name, Err: err}
+	}
+	return fi, nil
+}
+
 func mkdirat(fd int, name string, perm FileMode) error {
 	return ignoringEINTR(func() error {
 		return unix.Mkdirat(fd, name, syscallMode(perm))

src/os/root_windows.go

@@ -198,6 +198,40 @@ func rootOpenDir(parent syscall.Handle, name string) (syscall.Handle, error) {
 	return h, err
 }
 
+func rootStat(r *Root, name string, lstat bool) (FileInfo, error) {
+	if len(name) > 0 && IsPathSeparator(name[len(name)-1]) {
+		// When a filename ends with a path separator,
+		// Lstat behaves like Stat.
+		//
+		// This behavior is not based on a principled decision here,
+		// merely the empirical evidence that Lstat behaves this way.
+		lstat = false
+	}
+	fi, err := doInRoot(r, name, func(parent syscall.Handle, n string) (FileInfo, error) {
+		fd, err := openat(parent, n, windows.O_OPEN_REPARSE, 0)
+		if err != nil {
+			return nil, err
+		}
+		defer syscall.CloseHandle(fd)
+		fi, err := statHandle(name, fd)
+		if err != nil {
+			return nil, err
+		}
+		if !lstat && fi.(*fileStat).isReparseTagNameSurrogate() {
+			link, err := readReparseLinkHandle(fd)
+			if err != nil {
+				return nil, err
+			}
+			return nil, errSymlink(link)
+		}
+		return fi, nil
+	})
+	if err != nil {
+		return nil, &PathError{Op: "statat", Path: name, Err: err}
+	}
+	return fi, nil
+}
+
 func mkdirat(dirfd syscall.Handle, name string, perm FileMode) error {
 	return windows.Mkdirat(dirfd, name, syscallMode(perm))
 }

src/os/stat_wasip1.go

@@ -32,6 +32,15 @@ func fillFileStatFromSys(fs *fileStat, name string) {
 	case syscall.FILETYPE_SYMBOLIC_LINK:
 		fs.mode |= ModeSymlink
 	}
+
+	// WASI does not support unix-like permissions, but Go programs are likely
+	// to expect the permission bits to not be zero so we set defaults to help
+	// avoid breaking applications that are migrating to WASM.
+	if fs.sys.Filetype == syscall.FILETYPE_DIRECTORY {
+		fs.mode |= 0700
+	} else {
+		fs.mode |= 0600
+	}
 }
 
 // For testing.

src/syscall/types_windows.go

@@ -48,6 +48,7 @@ const (
 	O_CLOEXEC      = 0x80000
 	o_DIRECTORY    = 0x100000   // used by internal/syscall/windows
 	o_NOFOLLOW_ANY = 0x20000000 // used by internal/syscall/windows
+	o_OPEN_REPARSE = 0x40000000 // used by internal/syscall/windows
 )
 
 const (