path/filepath: WalkDir does not treat "." as equivalent to os.Getwd()

[bcmills]

What version of Go are you using (go version)?

go1.18-16d6a5233a

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

linux/amd64 via play.golang.org.

What did you do?

(https://go.dev/play/p/OLEED5lI02l?v=gotip)

With PWD set to a path ending in a symlink to the actual working directory, compare the behavior of filepath.WalkDir with "." and with the directory reported by os.Getwd().

What did you expect to see?

Equivalent behavior for "." and os.Getwd(). Specifically, I expected filepath.WalkDir to report the current working directory as a symlink in both cases.

What did you see instead?

os.Chdir accepts the symlink as valid and changes to the underlying directory. With a matching PWD environment variable, os.Getwd() reports the symlink as the current working directory (as intended since #8400).

filepath.WalkDir(cwd, …) treats the current directory as the symlink itself, and does not list its contents.

However, filepath.WalkDir(".", …) treats "." as the underlying directory and walks its contents.

[bcmills]

This may be as close to “working as intended” as is feasible given #8400. The equivalent C program reports "." as a non-link, although on Linux getcwd also reports the current working directory as a non-link.

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
  size_t n = PATH_MAX;
  char *buf = malloc(n);
  char *cwd = NULL;
  while (!(cwd = getcwd(buf, n))) {
    n *= 2;
    buf = realloc(buf, n);
  }
  printf("cwd is %s\n", cwd);
  free(buf);

  struct stat statbuf;
  memset(&statbuf, 0, sizeof(statbuf));
	if (lstat(".", &statbuf) != 0) {
    perror(NULL);
    return 1;
  }

  if (S_ISLNK(statbuf.st_mode)) {
    printf(". is a link\n");
  } else {
    printf(". is not a link\n");
  }

  if (fflush(stdout) != 0) {
    perror(NULL);
    return 1;
  }

  return 0;
}

~/tmp/link$ gcc main.c && ./a.out
cwd is /usr/local/google/home/bcmills/tmp/dir
. is not a link

[bcmills]

I don't think this can be fixed without breaking compatibility, and — given that it can be worked around with a wrapper that reimplements the first layer of the walk — I don't think it's worth breaking compatibility to fix.

[bcmills]

This is closely related to #26033.