syncthing/lib/fs/debug_symlink_windows.go

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// Copyright 2009 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 (
"os"
"path/filepath"
"syscall"
)
// DebugSymlinkForTestsOnly is os.Symlink taken from the 1.9.2 stdlib,
// hacked with the SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag to
// create symlinks when not elevated.
//
// This is not and should not be used in Syncthing code, hence the
// cumbersome name to make it obvious if this ever leaks. Nonetheless it's
// useful in tests.
func DebugSymlinkForTestsOnly(oldFs, newFS Filesystem, oldname, newname string) error {
oldname = filepath.Join(oldFs.URI(), oldname)
newname = filepath.Join(newFS.URI(), newname)
// CreateSymbolicLink is not supported before Windows Vista
if syscall.LoadCreateSymbolicLink() != nil {
return &os.LinkError{"symlink", oldname, newname, syscall.EWINDOWS}
}
// '/' does not work in link's content
oldname = filepath.FromSlash(oldname)
// need the exact location of the oldname when it's relative to determine if it's a directory
destpath := oldname
if !filepath.IsAbs(oldname) {
destpath = filepath.Dir(newname) + `\` + oldname
}
fi, err := os.Lstat(destpath)
isdir := err == nil && fi.IsDir()
n, err := syscall.UTF16PtrFromString(fixLongPath(newname))
if err != nil {
return &os.LinkError{"symlink", oldname, newname, err}
}
o, err := syscall.UTF16PtrFromString(fixLongPath(oldname))
if err != nil {
return &os.LinkError{"symlink", oldname, newname, err}
}
var flags uint32
if isdir {
flags |= syscall.SYMBOLIC_LINK_FLAG_DIRECTORY
}
flags |= 0x02 // SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
err = syscall.CreateSymbolicLink(n, o, flags)
if err != nil {
return &os.LinkError{"symlink", oldname, newname, err}
}
return 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 than 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 !filepath.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])
}