Source: symlink.go in package path/filepath


package filepath

import (
	"errors"
	"io/fs"
	"os"
	"runtime"
	"syscall"
)

func walkSymlinks(path string) (string, error) {
	volLen := volumeNameLen(path)
	pathSeparator := string(os.PathSeparator)

	if volLen < len(path) && os.IsPathSeparator(path[volLen]) {
		volLen++
	}
	vol := path[:volLen]
	dest := vol
	linksWalked := 0
	for start, end := volLen, volLen; start < len(path); start = end {
		for start < len(path) && os.IsPathSeparator(path[start]) {
			start++
		}
		end = start
		for end < len(path) && !os.IsPathSeparator(path[end]) {
			end++
		}

On Windows, "." can be a symlink. We look it up, and use the value if it is absolute. If not, we just return ".".

		isWindowsDot := runtime.GOOS == "windows" && path[volumeNameLen(path):] == "."

The next path component is in path[start:end].

No more path components.

			break

Ignore path component ".".

			continue

Back up to previous component if possible. Note that volLen includes any leading slash.

Set r to the index of the last slash in dest, after the volume.

			var r int
			for r = len(dest) - 1; r >= volLen; r-- {
				if os.IsPathSeparator(dest[r]) {
					break
				}
			}

Either path has no slashes (it's empty or just "C:") or it ends in a ".." we had to keep. Either way, keep this "..".

				if len(dest) > volLen {
					dest += pathSeparator
				}
				dest += ".."

Discard everything since the last slash.

				dest = dest[:r]
			}
			continue
		}

Ordinary path component. Add it to result.


		if len(dest) > volumeNameLen(dest) && !os.IsPathSeparator(dest[len(dest)-1]) {
			dest += pathSeparator
		}

		dest += path[start:end]

Resolve symlink.


		fi, err := os.Lstat(dest)
		if err != nil {
			return "", err
		}

		if fi.Mode()&fs.ModeSymlink == 0 {
			if !fi.Mode().IsDir() && end < len(path) {
				return "", syscall.ENOTDIR
			}
			continue
		}

Found symlink.


		linksWalked++
		if linksWalked > 255 {
			return "", errors.New("EvalSymlinks: too many links")
		}

		link, err := os.Readlink(dest)
		if err != nil {
			return "", err
		}

On Windows, if "." is a relative symlink, just return ".".

			break
		}

		path = link + path[end:]

		v := volumeNameLen(link)

Symlink to drive name is an absolute path.

			if v < len(link) && os.IsPathSeparator(link[v]) {
				v++
			}
			vol = link[:v]
			dest = vol
			end = len(vol)

Symlink to absolute path.

			dest = link[:1]
			end = 1

Symlink to relative path; replace last path component in dest.

			var r int
			for r = len(dest) - 1; r >= volLen; r-- {
				if os.IsPathSeparator(dest[r]) {
					break
				}
			}
			if r < volLen {
				dest = vol
			} else {
				dest = dest[:r]
			}
			end = 0
		}
	}
	return Clean(dest), nil