fs.go

// Copyright 2016 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"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"runtime"
	"syscall"
)

// RenameWithFallback attempts to rename a file or directory, but falls back to
// copying in the event of a cross-device link error. If the fallback copy
// succeeds, src is still removed, emulating normal rename behavior.
func RenameWithFallback(src, dst string) error {
	_, err := os.Stat(src)
	if err != nil {
		return fmt.Errorf("cannot stat %s: %w", src, err)
	}

	err = os.Rename(src, dst)
	if err == nil {
		return nil
	}

	return renameFallback(err, src, dst)
}

// renameByCopy attempts to rename a file or directory by copying it to the
// destination and then removing the src thus emulating the rename behavior.
func renameByCopy(src, dst string) error {
	var cerr error
	if dir, _ := IsDir(src); dir {
		cerr = CopyDir(src, dst)
		if cerr != nil {
			cerr = fmt.Errorf("copying directory failed: %w", cerr)
		}
	} else {
		cerr = copyFile(src, dst)
		if cerr != nil {
			cerr = fmt.Errorf("copying file failed: %w", cerr)
		}
	}

	if cerr != nil {
		return fmt.Errorf("rename fallback failed: cannot rename %s to %s: %w", src, dst, cerr)
	}

	if err := os.RemoveAll(src); err != nil {
		return fmt.Errorf("cannot delete %s: %w", src, err)
	}

	return nil
}

var (
	errSrcNotDir = errors.New("source is not a directory")
	errDstExist  = errors.New("destination already exists")
)

// CopyDir recursively copies a directory tree, attempting to preserve permissions.
// Source directory must exist, destination directory must *not* exist.
func CopyDir(src, dst string) error {
	src = filepath.Clean(src)
	dst = filepath.Clean(dst)

	// We use os.Lstat() here to ensure we don't fall in a loop where a symlink
	// actually links to a one of its parent directories.
	fi, err := os.Lstat(src)
	if err != nil {
		return err
	}
	if !fi.IsDir() {
		return errSrcNotDir
	}

	_, err = os.Stat(dst)
	if err != nil && !os.IsNotExist(err) {
		return err
	}
	if err == nil {
		return errDstExist
	}

	if err = os.MkdirAll(dst, fi.Mode()); err != nil {
		return fmt.Errorf("cannot mkdir %s: %w", dst, err)
	}

	entries, err := os.ReadDir(src)
	if err != nil {
		return fmt.Errorf("cannot read directory %s: %w", dst, err)
	}

	for _, entry := range entries {
		srcPath := filepath.Join(src, entry.Name())
		dstPath := filepath.Join(dst, entry.Name())

		if entry.IsDir() {
			if err = CopyDir(srcPath, dstPath); err != nil {
				return fmt.Errorf("copying directory failed: %w", err)
			}
		} else {
			// This will include symlinks, which is what we want when
			// copying things.
			if err = copyFile(srcPath, dstPath); err != nil {
				return fmt.Errorf("copying file failed: %w", err)
			}
		}
	}

	return nil
}

// copyFile copies the contents of the file named src to the file named
// by dst. The file will be created if it does not already exist. If the
// destination file exists, all its contents will be replaced by the contents
// of the source file. The file mode will be copied from the source.
func copyFile(src, dst string) (err error) {
	if sym, err := IsSymlink(src); err != nil {
		return fmt.Errorf("symlink check failed: %w", err)
	} else if sym {
		if err := cloneSymlink(src, dst); err != nil {
			if runtime.GOOS == "windows" {
				// If cloning the symlink fails on Windows because the user
				// does not have the required privileges, ignore the error and
				// fall back to copying the file contents.
				//
				// ERROR_PRIVILEGE_NOT_HELD is 1314 (0x522):
				// https://msdn.microsoft.com/en-us/library/windows/desktop/ms681385(v=vs.85).aspx
				if lerr, ok := err.(*os.LinkError); ok && lerr.Err != syscall.Errno(1314) {
					return err
				}
			} else {
				return err
			}
		} else {
			return nil
		}
	}

	in, err := os.Open(src)
	if err != nil {
		return
	}
	defer in.Close()

	out, err := os.Create(dst)
	if err != nil {
		return
	}

	if _, err = io.Copy(out, in); err != nil {
		out.Close()
		return
	}

	// Check for write errors on Close
	if err = out.Close(); err != nil {
		return
	}

	si, err := os.Stat(src)
	if err != nil {
		return
	}

	// Temporary fix for Go < 1.9
	//
	// See: https://github.com/golang/dep/issues/774
	// and https://github.com/golang/go/issues/20829
	if runtime.GOOS == "windows" {
		dst = fixLongPath(dst)
	}
	err = os.Chmod(dst, si.Mode())

	return
}

// cloneSymlink will create a new symlink that points to the resolved path of sl.
// If sl is a relative symlink, dst will also be a relative symlink.
func cloneSymlink(sl, dst string) error {
	resolved, err := os.Readlink(sl)
	if err != nil {
		return err
	}

	return os.Symlink(resolved, dst)
}

// IsDir determines is the path given is a directory or not.
func IsDir(name string) (bool, error) {
	fi, err := os.Stat(name)
	if err != nil {
		return false, err
	}
	if !fi.IsDir() {
		return false, fmt.Errorf("%q is not a directory", name)
	}
	return true, nil
}

// IsSymlink determines if the given path is a symbolic link.
func IsSymlink(path string) (bool, error) {
	l, err := os.Lstat(path)
	if err != nil {
		return false, err
	}

	return l.Mode()&os.ModeSymlink == os.ModeSymlink, nil
}

// fixLongPath returns the extended-length (\\?\-prefixed) form of
// path when needed, in order to avoid the default 260 character file
// path limit imposed by Windows. If path is not easily converted to
// the extended-length form (for example, if path is a relative path
// or contains .. elements), or is short enough, fixLongPath returns
// path unmodified.
//
// See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath
func fixLongPath(path string) string {
	// Do nothing (and don't allocate) if the path is "short".
	// Empirically (at least on the Windows Server 2013 builder),
	// the kernel is arbitrarily okay with < 248 bytes. That
	// matches what the docs above say:
	// "When using an API to create a directory, the specified
	// path cannot be so long that you cannot append an 8.3 file
	// name (that is, the directory name cannot exceed MAX_PATH
	// minus 12)." Since MAX_PATH is 260, 260 - 12 = 248.
	//
	// The MSDN docs appear to say that a normal path that is 248 bytes long
	// will work; empirically the path must be less then 248 bytes long.
	if len(path) < 248 {
		// Don't fix. (This is how Go 1.7 and earlier worked,
		// not automatically generating the \\?\ form)
		return path
	}

	// The extended form begins with \\?\, as in
	// \\?\c:\windows\foo.txt or \\?\UNC\server\share\foo.txt.
	// The extended form disables evaluation of . and .. path
	// elements and disables the interpretation of / as equivalent
	// to \. The conversion here rewrites / to \ and elides
	// . elements as well as trailing or duplicate separators. For
	// simplicity it avoids the conversion entirely for relative
	// paths or paths containing .. elements. For now,
	// \\server\share paths are not converted to
	// \\?\UNC\server\share paths because the rules for doing so
	// are less well-specified.
	if len(path) >= 2 && path[:2] == `\\` {
		// Don't canonicalize UNC paths.
		return path
	}
	if !isAbs(path) {
		// Relative path
		return path
	}

	const prefix = `\\?`

	pathbuf := make([]byte, len(prefix)+len(path)+len(`\`))
	copy(pathbuf, prefix)
	n := len(path)
	r, w := 0, len(prefix)
	for r < n {
		switch {
		case os.IsPathSeparator(path[r]):
			// empty block
			r++
		case path[r] == '.' && (r+1 == n || os.IsPathSeparator(path[r+1])):
			// /./
			r++
		case r+1 < n && path[r] == '.' && path[r+1] == '.' && (r+2 == n || os.IsPathSeparator(path[r+2])):
			// /../ is currently unhandled
			return path
		default:
			pathbuf[w] = '\\'
			w++
			for ; r < n && !os.IsPathSeparator(path[r]); r++ {
				pathbuf[w] = path[r]
				w++
			}
		}
	}
	// A drive's root directory needs a trailing \
	if w == len(`\\?\c:`) {
		pathbuf[w] = '\\'
		w++
	}
	return string(pathbuf[:w])
}

func isAbs(path string) (b bool) {
	v := volumeName(path)
	if v == "" {
		return false
	}
	path = path[len(v):]
	if path == "" {
		return false
	}
	return os.IsPathSeparator(path[0])
}

func volumeName(path string) (v string) {
	if len(path) < 2 {
		return ""
	}
	// with drive letter
	c := path[0]
	if path[1] == ':' &&
		('0' <= c && c <= '9' || 'a' <= c && c <= 'z' ||
			'A' <= c && c <= 'Z') {
		return path[:2]
	}
	// is it UNC
	if l := len(path); l >= 5 && os.IsPathSeparator(path[0]) && os.IsPathSeparator(path[1]) &&
		!os.IsPathSeparator(path[2]) && path[2] != '.' {
		// first, leading `\\` and next shouldn't be `\`. its server name.
		for n := 3; n < l-1; n++ {
			// second, next '\' shouldn't be repeated.
			if os.IsPathSeparator(path[n]) {
				n++
				// third, following something characters. its share name.
				if !os.IsPathSeparator(path[n]) {
					if path[n] == '.' {
						break
					}
					for ; n < l; n++ {
						if os.IsPathSeparator(path[n]) {
							break
						}
					}
					return path[:n]
				}
				break
			}
		}
	}
	return ""
}