os: add File.Chdir support

We should really be using syscall.Fchdir here, but this is a fix to get
Go 1.24 working.

src/os/file.go

@@ -318,6 +318,17 @@ func (f *File) Chmod(mode FileMode) (err error) {
 	return
 }
 
+// Chdir changes the current working directory to the file, which must be a
+// directory. If there is an error, it will be of type *PathError.
+func (f *File) Chdir() (err error) {
+	if f.handle == nil {
+		err = ErrClosed
+	} else {
+		err = f.chdir()
+	}
+	return
+}
+
 // LinkError records an error during a link or symlink or rename system call and
 // the paths that caused it.
 type LinkError struct {

src/os/file_other.go

@@ -159,3 +159,7 @@ func (f *File) Truncate(size int64) (err error) {
 func (f *File) chmod(mode FileMode) error {
 	return ErrUnsupported
 }
+
+func (f *File) chdir() error {
+	return ErrNotImplemented
+}

src/os/file_unix.go

@@ -166,6 +166,22 @@ func (f *File) chmod(mode FileMode) error {
 	return nil
 }
 
+func (f *File) chdir() error {
+	if f.handle == nil {
+		return ErrClosed
+	}
+
+	// TODO: use syscall.Fchdir instead
+	longName := fixLongPath(f.name)
+	e := ignoringEINTR(func() error {
+		return syscall.Chdir(longName)
+	})
+	if e != nil {
+		return &PathError{Op: "chdir", Path: f.name, Err: e}
+	}
+	return nil
+}
+
 // ReadAt reads up to len(b) bytes from the File starting at the given absolute offset.
 // It returns the number of bytes read and any error encountered, possibly io.EOF.
 // At end of file, Pread returns 0, io.EOF.

src/os/file_windows.go

@@ -146,3 +146,7 @@ func isWindowsNulName(name string) bool {
 func (f *File) chmod(mode FileMode) error {
 	return ErrNotImplemented
 }
+
+func (f *File) chdir() error {
+	return ErrNotImplemented
+}