os: document Readlink behavior for relative links

Also provide a runnable example to illustrate that behavior.

This should help users to avoid the common mistake of expecting
os.Readlink to return an absolute path.

Fixes #57766.

Change-Id: I8f60aa111ebda0cae985758615019aaf26d5cb41
Reviewed-on: https://go-review.googlesource.com/c/go/+/546995
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/os/example_test.go

@@ -263,3 +263,57 @@ func ExampleMkdirAll() {
 		log.Fatal(err)
 	}
 }
+
+func ExampleReadlink() {
+	// First, we create a relative symlink to a file.
+	d, err := os.MkdirTemp("", "")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(d)
+	targetPath := filepath.Join(d, "hello.txt")
+	if err := os.WriteFile(targetPath, []byte("Hello, Gophers!"), 0644); err != nil {
+		log.Fatal(err)
+	}
+	linkPath := filepath.Join(d, "hello.link")
+	if err := os.Symlink("hello.txt", filepath.Join(d, "hello.link")); err != nil {
+		if errors.Is(err, errors.ErrUnsupported) {
+			// Allow the example to run on platforms that do not support symbolic links.
+			fmt.Printf("%s links to %s\n", filepath.Base(linkPath), "hello.txt")
+			return
+		}
+		log.Fatal(err)
+	}
+
+	// Readlink returns the relative path as passed to os.Symlink.
+	dst, err := os.Readlink(linkPath)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s links to %s\n", filepath.Base(linkPath), dst)
+
+	var dstAbs string
+	if filepath.IsAbs(dst) {
+		dstAbs = dst
+	} else {
+		// Symlink targets are relative to the directory containing the link.
+		dstAbs = filepath.Join(filepath.Dir(linkPath), dst)
+	}
+
+	// Check that the target is correct by comparing it with os.Stat
+	// on the original target path.
+	dstInfo, err := os.Stat(dstAbs)
+	if err != nil {
+		log.Fatal(err)
+	}
+	targetInfo, err := os.Stat(targetPath)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if !os.SameFile(dstInfo, targetInfo) {
+		log.Fatalf("link destination (%s) is not the same file as %s", dstAbs, targetPath)
+	}
+
+	// Output:
+	// hello.link links to hello.txt
+}

src/os/file.go

@@ -392,6 +392,15 @@ func Rename(oldpath, newpath string) error {
 	return rename(oldpath, newpath)
 }
 
+// Readlink returns the destination of the named symbolic link.
+// If there is an error, it will be of type *PathError.
+//
+// If the link destination is relative, Readlink returns the relative path
+// without resolving it to an absolute one.
+func Readlink(name string) (string, error) {
+	return readlink(name)
+}
+
 // Many functions in package syscall return a count of -1 instead of 0.
 // Using fixCount(call()) instead of call() corrects the count.
 func fixCount(n int, err error) (int, error) {

src/os/file_plan9.go

@@ -505,9 +505,7 @@ func Symlink(oldname, newname string) error {
 	return &LinkError{"symlink", oldname, newname, syscall.EPLAN9}
 }
 
-// Readlink returns the destination of the named symbolic link.
-// If there is an error, it will be of type *PathError.
-func Readlink(name string) (string, error) {
+func readlink(name string) (string, error) {
 	return "", &PathError{Op: "readlink", Path: name, Err: syscall.EPLAN9}
 }
 

src/os/file_unix.go

@@ -426,9 +426,7 @@ func Symlink(oldname, newname string) error {
 	return nil
 }
 
-// Readlink returns the destination of the named symbolic link.
-// If there is an error, it will be of type *PathError.
-func Readlink(name string) (string, error) {
+func readlink(name string) (string, error) {
 	for len := 128; ; len *= 2 {
 		b := make([]byte, len)
 		var (

src/os/file_windows.go

@@ -406,7 +406,7 @@ func normaliseLinkPath(path string) (string, error) {
 	return "", errors.New("GetFinalPathNameByHandle returned unexpected path: " + s)
 }
 
-func readlink(path string) (string, error) {
+func readReparseLink(path string) (string, error) {
 	h, err := openSymlink(path)
 	if err != nil {
 		return "", err
@@ -438,10 +438,8 @@ func readlink(path string) (string, error) {
 	}
 }
 
-// Readlink returns the destination of the named symbolic link.
-// If there is an error, it will be of type *PathError.
-func Readlink(name string) (string, error) {
-	s, err := readlink(fixLongPath(name))
+func readlink(name string) (string, error) {
+	s, err := readReparseLink(fixLongPath(name))
 	if err != nil {
 		return "", &PathError{Op: "readlink", Path: name, Err: err}
 	}