os: add Root.Readlink

For #67002

Change-Id: I532a5ffc02c7457796540db54fa2f5ddad86e4b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/658995
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

api/next/67002.txt

@@ -2,3 +2,4 @@ pkg os, method (*Root) Chmod(string, fs.FileMode) error #67002
 pkg os, method (*Root) Chown(string, int, int) error #67002
 pkg os, method (*Root) Chtimes(string, time.Time, time.Time) error #67002
 pkg os, method (*Root) Lchown(string, int, int) error #67002
+pkg os, method (*Root) Readlink(string) (string, error) #67002

doc/next/6-stdlib/99-minor/os/67002.md

@@ -4,3 +4,4 @@ The [os.Root] type supports the following additional methods:
   * [os.Root.Chown]
   * [os.Root.Chtimes]
   * [os.Root.Lchown]
+  * [os.Root.Readlink]

src/os/root.go

@@ -193,6 +193,12 @@ func (r *Root) Lstat(name string) (FileInfo, error) {
 	return rootStat(r, name, true)
 }
 
+// Readlink returns the destination of the named symbolic link in the root.
+// See [Readlink] for more details.
+func (r *Root) Readlink(name string) (string, error) {
+	return rootReadlink(r, name)
+}
+
 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,

src/os/root_noopenat.go

@@ -155,3 +155,14 @@ func rootRemove(r *Root, name string) error {
 	}
 	return nil
 }
+
+func rootReadlink(r *Root, name string) (string, error) {
+	if err := checkPathEscapesLstat(r, name); err != nil {
+		return "", &PathError{Op: "readlinkat", Path: name, Err: err}
+	}
+	name, err := Readlink(joinPath(r.root.name, name))
+	if err != nil {
+		return "", &PathError{Op: "readlinkat", Path: name, Err: underlyingError(err)}
+	}
+	return name, nil
+}

src/os/root_openat.go

@@ -118,6 +118,16 @@ func rootMkdir(r *Root, name string, perm FileMode) error {
 	return nil
 }
 
+func rootReadlink(r *Root, name string) (string, error) {
+	target, err := doInRoot(r, name, func(parent sysfdType, name string) (string, error) {
+		return readlinkat(parent, name)
+	})
+	if err != nil {
+		return "", &PathError{Op: "readlinkat", Path: name, Err: err}
+	}
+	return target, nil
+}
+
 func rootRemove(r *Root, name string) error {
 	_, err := doInRoot(r, name, func(parent sysfdType, name string) (struct{}, error) {
 		return struct{}{}, removeat(parent, name)

src/os/root_test.go

@@ -664,6 +664,35 @@ func TestRootLstat(t *testing.T) {
 	}
 }
 
+func TestRootReadlink(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 != "" {
+				// Readlink will read the final link, rather than following it.
+				wantError = false
+			} else {
+				// Readlink fails on non-link targets.
+				wantError = true
+			}
+
+			got, err := root.Readlink(test.open)
+			if errEndsTest(t, err, wantError, "root.Readlink(%q)", test.open) {
+				return
+			}
+
+			want, err := os.Readlink(filepath.Join(root.Name(), test.ltarget))
+			if err != nil {
+				t.Fatalf("os.Readlink(%q) = %v, want success", test.ltarget, err)
+			}
+			if got != want {
+				t.Errorf("root.Readlink(%q) = %q, want %q", test.open, got, want)
+			}
+		})
+	}
+}
+
 // A rootConsistencyTest is a test case comparing os.Root behavior with
 // the corresponding non-Root function.
 //
@@ -1063,6 +1092,18 @@ func TestRootConsistencyLstat(t *testing.T) {
 	}
 }
 
+func TestRootConsistencyReadlink(t *testing.T) {
+	for _, test := range rootConsistencyTestCases {
+		test.run(t, func(t *testing.T, path string, r *os.Root) (string, error) {
+			if r == nil {
+				return os.Readlink(path)
+			} else {
+				return r.Readlink(path)
+			}
+		})
+	}
+}
+
 func TestRootRenameAfterOpen(t *testing.T) {
 	switch runtime.GOOS {
 	case "windows":

src/os/root_windows.go

@@ -314,3 +314,12 @@ func chtimesat(dirfd syscall.Handle, name string, atime time.Time, mtime time.Ti
 	}
 	return syscall.SetFileTime(h, nil, &a, &w)
 }
+
+func readlinkat(dirfd syscall.Handle, name string) (string, error) {
+	fd, err := openat(dirfd, name, windows.O_OPEN_REPARSE, 0)
+	if err != nil {
+		return "", err
+	}
+	defer syscall.CloseHandle(fd)
+	return readReparseLinkHandle(fd)
+}