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113 lines
3.4 KiB
Go
113 lines
3.4 KiB
Go
package sysx
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import (
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"os"
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"path/filepath"
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"github.com/containerd/continuity/syscallx"
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)
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// Readlink returns the destination of the named symbolic link.
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// If there is an error, it will be of type *PathError.
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func Readlink(name string) (string, error) {
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for len := 128; ; len *= 2 {
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b := make([]byte, len)
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n, e := fixCount(syscallx.Readlink(fixLongPath(name), b))
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if e != nil {
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return "", &os.PathError{Op: "readlink", Path: name, Err: e}
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}
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if n < len {
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return string(b[0:n]), nil
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}
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}
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}
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// Many functions in package syscall return a count of -1 instead of 0.
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// Using fixCount(call()) instead of call() corrects the count.
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func fixCount(n int, err error) (int, error) {
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if n < 0 {
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n = 0
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}
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return n, err
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}
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// fixLongPath returns the extended-length (\\?\-prefixed) form of
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// path when needed, in order to avoid the default 260 character file
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// path limit imposed by Windows. If path is not easily converted to
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// the extended-length form (for example, if path is a relative path
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// or contains .. elements), or is short enough, fixLongPath returns
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// path unmodified.
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//
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// See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath
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func fixLongPath(path string) string {
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// Do nothing (and don't allocate) if the path is "short".
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// Empirically (at least on the Windows Server 2013 builder),
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// the kernel is arbitrarily okay with < 248 bytes. That
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// matches what the docs above say:
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// "When using an API to create a directory, the specified
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// path cannot be so long that you cannot append an 8.3 file
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// name (that is, the directory name cannot exceed MAX_PATH
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// minus 12)." Since MAX_PATH is 260, 260 - 12 = 248.
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//
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// The MSDN docs appear to say that a normal path that is 248 bytes long
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// will work; empirically the path must be less then 248 bytes long.
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if len(path) < 248 {
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// Don't fix. (This is how Go 1.7 and earlier worked,
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// not automatically generating the \\?\ form)
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return path
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}
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// The extended form begins with \\?\, as in
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// \\?\c:\windows\foo.txt or \\?\UNC\server\share\foo.txt.
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// The extended form disables evaluation of . and .. path
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// elements and disables the interpretation of / as equivalent
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// to \. The conversion here rewrites / to \ and elides
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// . elements as well as trailing or duplicate separators. For
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// simplicity it avoids the conversion entirely for relative
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// paths or paths containing .. elements. For now,
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// \\server\share paths are not converted to
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// \\?\UNC\server\share paths because the rules for doing so
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// are less well-specified.
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if len(path) >= 2 && path[:2] == `\\` {
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// Don't canonicalize UNC paths.
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return path
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}
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if !filepath.IsAbs(path) {
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// Relative path
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return path
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}
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const prefix = `\\?`
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pathbuf := make([]byte, len(prefix)+len(path)+len(`\`))
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copy(pathbuf, prefix)
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n := len(path)
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r, w := 0, len(prefix)
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for r < n {
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switch {
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case os.IsPathSeparator(path[r]):
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// empty block
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r++
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case path[r] == '.' && (r+1 == n || os.IsPathSeparator(path[r+1])):
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// /./
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r++
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case r+1 < n && path[r] == '.' && path[r+1] == '.' && (r+2 == n || os.IsPathSeparator(path[r+2])):
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// /../ is currently unhandled
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return path
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default:
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pathbuf[w] = '\\'
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w++
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for ; r < n && !os.IsPathSeparator(path[r]); r++ {
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pathbuf[w] = path[r]
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w++
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}
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}
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}
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// A drive's root directory needs a trailing \
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if w == len(`\\?\c:`) {
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pathbuf[w] = '\\'
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w++
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}
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return string(pathbuf[:w])
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}
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