/* Traverse a file hierarchy.

   Copyright (C) 2004-2017 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

/*-
 * Copyright (c) 1990, 1993, 1994
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <config.h>

#if defined LIBC_SCCS && !defined GCC_LINT && !defined lint
static char sccsid[] = "@(#)fts.c       8.6 (Berkeley) 8/14/94";
#endif

#include "fts_.h"

#if !defined(ISSLASH) || !defined(IS_ABSOLUTE_FILE_NAME)
# include "dosname.h"
#endif

#if HAVE_SYS_PARAM_H || defined _LIBC
# include <sys/param.h>
#endif
#ifdef _LIBC
# include <include/sys/stat.h>
#else
# include <sys/stat.h>
#endif
#include <fcntl.h>
#include <errno.h>
#include <stdalign.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#if ! _LIBC
# include "fcntl--.h"
# include "dirent--.h"
# include "unistd--.h"
/* FIXME - use fcntl(F_DUPFD_CLOEXEC)/openat(O_CLOEXEC) once they are
   supported.  */
# include "cloexec.h"
# include "flexmember.h"
# include "openat.h"
# include "same-inode.h"
#endif

#include <dirent.h>
#ifndef _D_EXACT_NAMLEN
# define _D_EXACT_NAMLEN(dirent) strlen ((dirent)->d_name)
#endif

#if HAVE_STRUCT_DIRENT_D_TYPE
/* True if the type of the directory entry D is known.  */
# define DT_IS_KNOWN(d) ((d)->d_type != DT_UNKNOWN)
/* True if the type of the directory entry D must be T.  */
# define DT_MUST_BE(d, t) ((d)->d_type == (t))
# define D_TYPE(d) ((d)->d_type)
#else
# define DT_IS_KNOWN(d) false
# define DT_MUST_BE(d, t) false
# define D_TYPE(d) DT_UNKNOWN

# undef DT_UNKNOWN
# define DT_UNKNOWN 0

/* Any nonzero values will do here, so long as they're distinct.
   Undef any existing macros out of the way.  */
# undef DT_BLK
# undef DT_CHR
# undef DT_DIR
# undef DT_FIFO
# undef DT_LNK
# undef DT_REG
# undef DT_SOCK
# define DT_BLK 1
# define DT_CHR 2
# define DT_DIR 3
# define DT_FIFO 4
# define DT_LNK 5
# define DT_REG 6
# define DT_SOCK 7
#endif

#ifndef S_IFLNK
# define S_IFLNK 0
#endif
#ifndef S_IFSOCK
# define S_IFSOCK 0
#endif

enum
{
  NOT_AN_INODE_NUMBER = 0
};

#ifdef D_INO_IN_DIRENT
# define D_INO(dp) (dp)->d_ino
#else
/* Some systems don't have inodes, so fake them to avoid lots of ifdefs.  */
# define D_INO(dp) NOT_AN_INODE_NUMBER
#endif

/* If possible (see max_entries, below), read no more than this many directory
   entries at a time.  Without this limit (i.e., when using non-NULL
   fts_compar), processing a directory with 4,000,000 entries requires ~1GiB
   of memory, and handling 64M entries would require 16GiB of memory.  */
#ifndef FTS_MAX_READDIR_ENTRIES
# define FTS_MAX_READDIR_ENTRIES 100000
#endif

/* If there are more than this many entries in a directory,
   and the conditions mentioned below are satisfied, then sort
   the entries on inode number before any further processing.  */
#ifndef FTS_INODE_SORT_DIR_ENTRIES_THRESHOLD
# define FTS_INODE_SORT_DIR_ENTRIES_THRESHOLD 10000
#endif

enum
{
  _FTS_INODE_SORT_DIR_ENTRIES_THRESHOLD = FTS_INODE_SORT_DIR_ENTRIES_THRESHOLD
};

enum Fts_stat
{
  FTS_NO_STAT_REQUIRED = 1,
  FTS_STAT_REQUIRED = 2
};

#ifdef _LIBC
# undef close
# define close __close
# undef closedir
# define closedir __closedir
# undef fchdir
# define fchdir __fchdir
# undef open
# define open __open
# undef readdir
# define readdir __readdir
#else
# undef internal_function
# define internal_function /* empty */
#endif

#ifndef __set_errno
# define __set_errno(Val) errno = (Val)
#endif

/* If this host provides the openat function, then we can avoid
   attempting to open "." in some initialization code below.  */
#ifdef HAVE_OPENAT
# define HAVE_OPENAT_SUPPORT 1
#else
# define HAVE_OPENAT_SUPPORT 0
#endif

#ifdef NDEBUG
# define fts_assert(expr) ((void) (0 && (expr)))
#else
# define fts_assert(expr)       \
    do                          \
      {                         \
        if (!(expr))            \
          abort ();             \
      }                         \
    while (false)
#endif

static FTSENT   *fts_alloc (FTS *, const char *, size_t) internal_function;
static FTSENT   *fts_build (FTS *, int) internal_function;
static void      fts_lfree (FTSENT *) internal_function;
static void      fts_load (FTS *, FTSENT *) internal_function;
static size_t    fts_maxarglen (char * const *) internal_function;
static void      fts_padjust (FTS *, FTSENT *) internal_function;
static bool      fts_palloc (FTS *, size_t) internal_function;
static FTSENT   *fts_sort (FTS *, FTSENT *, size_t) internal_function;
static unsigned short int fts_stat (FTS *, FTSENT *, bool) internal_function;
static int      fts_safe_changedir (FTS *, FTSENT *, int, const char *)
     internal_function;

#include "fts-cycle.c"

#ifndef MAX
# define MAX(a,b) ((a) > (b) ? (a) : (b))
#endif

#ifndef SIZE_MAX
# define SIZE_MAX ((size_t) -1)
#endif

#define ISDOT(a)        (a[0] == '.' && (!a[1] || (a[1] == '.' && !a[2])))
#define STREQ(a, b)     (strcmp (a, b) == 0)

#define CLR(opt)        (sp->fts_options &= ~(opt))
#define ISSET(opt)      (sp->fts_options & (opt))
#define SET(opt)        (sp->fts_options |= (opt))

/* FIXME: FTS_NOCHDIR is now misnamed.
   Call it FTS_USE_FULL_RELATIVE_FILE_NAMES instead. */
#define FCHDIR(sp, fd)                                  \
  (!ISSET(FTS_NOCHDIR) && (ISSET(FTS_CWDFD)             \
                           ? (cwd_advance_fd ((sp), (fd), true), 0) \
                           : fchdir (fd)))


/* fts_build flags */
/* FIXME: make this an enum */
#define BCHILD          1               /* fts_children */
#define BNAMES          2               /* fts_children, names only */
#define BREAD           3               /* fts_read */

#if FTS_DEBUG
# include <inttypes.h>
# include <stdint.h>
# include <stdio.h>
# include "getcwdat.h"
bool fts_debug = false;
# define Dprintf(x) do { if (fts_debug) printf x; } while (false)
#else
# define Dprintf(x)
# define fd_ring_check(x)
# define fd_ring_print(a, b, c)
#endif

#define LEAVE_DIR(Fts, Ent, Tag)                                \
  do                                                            \
    {                                                           \
      Dprintf (("  %s-leaving: %s\n", Tag, (Ent)->fts_path));   \
      leave_dir (Fts, Ent);                                     \
      fd_ring_check (Fts);                                      \
    }                                                           \
  while (false)

static void
fd_ring_clear (I_ring *fd_ring)
{
  while ( ! i_ring_empty (fd_ring))
    {
      int fd = i_ring_pop (fd_ring);
      if (0 <= fd)
        close (fd);
    }
}

/* Overload the fts_statp->st_size member (otherwise unused, when
   fts_info is FTS_NSOK) to indicate whether fts_read should stat
   this entry or not.  */
static void
fts_set_stat_required (FTSENT *p, bool required)
{
  fts_assert (p->fts_info == FTS_NSOK);
  p->fts_statp->st_size = (required
                           ? FTS_STAT_REQUIRED
                           : FTS_NO_STAT_REQUIRED);
}

/* file-descriptor-relative opendir.  */
/* FIXME: if others need this function, move it into lib/openat.c */
static DIR *
internal_function
opendirat (int fd, char const *dir, int extra_flags, int *pdir_fd)
{
  int new_fd = openat (fd, dir,
                       (O_RDONLY | O_DIRECTORY | O_NOCTTY | O_NONBLOCK
                        | extra_flags));
  DIR *dirp;

  if (new_fd < 0)
    return NULL;
  set_cloexec_flag (new_fd, true);
  dirp = fdopendir (new_fd);
  if (dirp)
    *pdir_fd = new_fd;
  else
    {
      int saved_errno = errno;
      close (new_fd);
      errno = saved_errno;
    }
  return dirp;
}

/* Virtual fchdir.  Advance SP's working directory file descriptor,
   SP->fts_cwd_fd, to FD, and push the previous value onto the fd_ring.
   CHDIR_DOWN_ONE is true if FD corresponds to an entry in the directory
   open on sp->fts_cwd_fd; i.e., to move the working directory one level
   down.  */
static void
internal_function
cwd_advance_fd (FTS *sp, int fd, bool chdir_down_one)
{
  int old = sp->fts_cwd_fd;
  fts_assert (old != fd || old == AT_FDCWD);

  if (chdir_down_one)
    {
      /* Push "old" onto the ring.
         If the displaced file descriptor is non-negative, close it.  */
      int prev_fd_in_slot = i_ring_push (&sp->fts_fd_ring, old);
      fd_ring_print (sp, stderr, "post-push");
      if (0 <= prev_fd_in_slot)
        close (prev_fd_in_slot); /* ignore any close failure */
    }
  else if ( ! ISSET (FTS_NOCHDIR))
    {
      if (0 <= old)
        close (old); /* ignore any close failure */
    }

  sp->fts_cwd_fd = fd;
}

/* Restore the initial, pre-traversal, "working directory".
   In FTS_CWDFD mode, we merely call cwd_advance_fd, otherwise,
   we may actually change the working directory.
   Return 0 upon success. Upon failure, set errno and return nonzero.  */
static int
restore_initial_cwd (FTS *sp)
{
  int fail = FCHDIR (sp, ISSET (FTS_CWDFD) ? AT_FDCWD : sp->fts_rfd);
  fd_ring_clear (&(sp->fts_fd_ring));
  return fail;
}

/* Open the directory DIR if possible, and return a file
   descriptor.  Return -1 and set errno on failure.  It doesn't matter
   whether the file descriptor has read or write access.  */

static int
internal_function
diropen (FTS const *sp, char const *dir)
{
  int open_flags = (O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK
                    | (ISSET (FTS_PHYSICAL) ? O_NOFOLLOW : 0)
                    | (ISSET (FTS_NOATIME) ? O_NOATIME : 0));

  int fd = (ISSET (FTS_CWDFD)
            ? openat (sp->fts_cwd_fd, dir, open_flags)
            : open (dir, open_flags));
  if (0 <= fd)
    set_cloexec_flag (fd, true);
  return fd;
}

FTS *
fts_open (char * const *argv,
          register int options,
          int (*compar) (FTSENT const **, FTSENT const **))
{
        register FTS *sp;
        register FTSENT *p, *root;
        register size_t nitems;
        FTSENT *parent = NULL;
        FTSENT *tmp = NULL;     /* pacify gcc */
        bool defer_stat;

        /* Options check. */
        if (options & ~FTS_OPTIONMASK) {
                __set_errno (EINVAL);
                return (NULL);
        }
        if ((options & FTS_NOCHDIR) && (options & FTS_CWDFD)) {
                __set_errno (EINVAL);
                return (NULL);
        }
        if ( ! (options & (FTS_LOGICAL | FTS_PHYSICAL))) {
                __set_errno (EINVAL);
                return (NULL);
        }

        /* Allocate/initialize the stream */
        if ((sp = malloc(sizeof(FTS))) == NULL)
                return (NULL);
        memset(sp, 0, sizeof(FTS));
        sp->fts_compar = compar;
        sp->fts_options = options;

        /* Logical walks turn on NOCHDIR; symbolic links are too hard. */
        if (ISSET(FTS_LOGICAL)) {
                SET(FTS_NOCHDIR);
                CLR(FTS_CWDFD);
        }

        /* Initialize fts_cwd_fd.  */
        sp->fts_cwd_fd = AT_FDCWD;
        if ( ISSET(FTS_CWDFD) && ! HAVE_OPENAT_SUPPORT)
          {
            /* While it isn't technically necessary to open "." this
               early, doing it here saves us the trouble of ensuring
               later (where it'd be messier) that "." can in fact
               be opened.  If not, revert to FTS_NOCHDIR mode.  */
            int fd = open (".",
                           O_SEARCH | (ISSET (FTS_NOATIME) ? O_NOATIME : 0));
            if (fd < 0)
              {
                /* Even if "." is unreadable, don't revert to FTS_NOCHDIR mode
                   on systems like Linux+PROC_FS, where our openat emulation
                   is good enough.  Note: on a system that emulates
                   openat via /proc, this technique can still fail, but
                   only in extreme conditions, e.g., when the working
                   directory cannot be saved (i.e. save_cwd fails) --
                   and that happens on Linux only when "." is unreadable
                   and the CWD would be longer than PATH_MAX.
                   FIXME: once Linux kernel openat support is well established,
                   replace the above open call and this entire if/else block
                   with the body of the if-block below.  */
                if ( openat_needs_fchdir ())
                  {
                    SET(FTS_NOCHDIR);
                    CLR(FTS_CWDFD);
                  }
              }
            else
              {
                close (fd);
              }
          }

        /*
         * Start out with 1K of file name space, and enough, in any case,
         * to hold the user's file names.
         */
#ifndef MAXPATHLEN
# define MAXPATHLEN 1024
#endif
        {
          size_t maxarglen = fts_maxarglen(argv);
          if (! fts_palloc(sp, MAX(maxarglen, MAXPATHLEN)))
                  goto mem1;
        }

        /* Allocate/initialize root's parent. */
        if (*argv != NULL) {
                if ((parent = fts_alloc(sp, "", 0)) == NULL)
                        goto mem2;
                parent->fts_level = FTS_ROOTPARENTLEVEL;
          }

        /* The classic fts implementation would call fts_stat with
           a new entry for each iteration of the loop below.
           If the comparison function is not specified or if the
           FTS_DEFER_STAT option is in effect, don't stat any entry
           in this loop.  This is an attempt to minimize the interval
           between the initial stat/lstat/fstatat and the point at which
           a directory argument is first opened.  This matters for any
           directory command line argument that resides on a file system
           without genuine i-nodes.  If you specify FTS_DEFER_STAT along
           with a comparison function, that function must not access any
           data via the fts_statp pointer.  */
        defer_stat = (compar == NULL || ISSET(FTS_DEFER_STAT));

        /* Allocate/initialize root(s). */
        for (root = NULL, nitems = 0; *argv != NULL; ++argv, ++nitems) {
                /* *Do* allow zero-length file names. */
                size_t len = strlen(*argv);

                if ( ! (options & FTS_VERBATIM))
                  {
                    /* If there are two or more trailing slashes, trim all but one,
                       but don't change "//" to "/", and do map "///" to "/".  */
                    char const *v = *argv;
                    if (2 < len && ISSLASH(v[len - 1]))
                      while (1 < len && ISSLASH(v[len - 2]))
                        --len;
                  }

                if ((p = fts_alloc(sp, *argv, len)) == NULL)
                        goto mem3;
                p->fts_level = FTS_ROOTLEVEL;
                p->fts_parent = parent;
                p->fts_accpath = p->fts_name;
                /* Even when defer_stat is true, be sure to stat the first
                   command line argument, since fts_read (at least with
                   FTS_XDEV) requires that.  */
                if (defer_stat && root != NULL) {
                        p->fts_info = FTS_NSOK;
                        fts_set_stat_required(p, true);
                } else {
                        p->fts_info = fts_stat(sp, p, false);
                }

                /*
                 * If comparison routine supplied, traverse in sorted
                 * order; otherwise traverse in the order specified.
                 */
                if (compar) {
                        p->fts_link = root;
                        root = p;
                } else {
                        p->fts_link = NULL;
                        if (root == NULL)
                                tmp = root = p;
                        else {
                                tmp->fts_link = p;
                                tmp = p;
                        }
                }
        }
        if (compar && nitems > 1)
                root = fts_sort(sp, root, nitems);

        /*
         * Allocate a dummy pointer and make fts_read think that we've just
         * finished the node before the root(s); set p->fts_info to FTS_INIT
         * so that everything about the "current" node is ignored.
         */
        if ((sp->fts_cur = fts_alloc(sp, "", 0)) == NULL)
                goto mem3;
        sp->fts_cur->fts_link = root;
        sp->fts_cur->fts_info = FTS_INIT;
        if (! setup_dir (sp))
                goto mem3;

        /*
         * If using chdir(2), grab a file descriptor pointing to dot to ensure
         * that we can get back here; this could be avoided for some file names,
         * but almost certainly not worth the effort.  Slashes, symbolic links,
         * and ".." are all fairly nasty problems.  Note, if we can't get the
         * descriptor we run anyway, just more slowly.
         */
        if (!ISSET(FTS_NOCHDIR) && !ISSET(FTS_CWDFD)
            && (sp->fts_rfd = diropen (sp, ".")) < 0)
                SET(FTS_NOCHDIR);

        i_ring_init (&sp->fts_fd_ring, -1);
        return (sp);

mem3:   fts_lfree(root);
        free(parent);
mem2:   free(sp->fts_path);
mem1:   free(sp);
        return (NULL);
}

static void
internal_function
fts_load (FTS *sp, register FTSENT *p)
{
        register size_t len;
        register char *cp;

        /*
         * Load the stream structure for the next traversal.  Since we don't
         * actually enter the directory until after the preorder visit, set
         * the fts_accpath field specially so the chdir gets done to the right
         * place and the user can access the first node.  From fts_open it's
         * known that the file name will fit.
         */
        len = p->fts_pathlen = p->fts_namelen;
        memmove(sp->fts_path, p->fts_name, len + 1);
        if ((cp = strrchr(p->fts_name, '/')) && (cp != p->fts_name || cp[1])) {
                len = strlen(++cp);
                memmove(p->fts_name, cp, len + 1);
                p->fts_namelen = len;
        }
        p->fts_accpath = p->fts_path = sp->fts_path;
}

int
fts_close (FTS *sp)
{
        register FTSENT *freep, *p;
        int saved_errno = 0;

        /*
         * This still works if we haven't read anything -- the dummy structure
         * points to the root list, so we step through to the end of the root
         * list which has a valid parent pointer.
         */
        if (sp->fts_cur) {
                for (p = sp->fts_cur; p->fts_level >= FTS_ROOTLEVEL;) {
                        freep = p;
                        p = p->fts_link != NULL ? p->fts_link : p->fts_parent;
                        free(freep);
                }
                free(p);
        }

        /* Free up child linked list, sort array, file name buffer. */
        if (sp->fts_child)
                fts_lfree(sp->fts_child);
        free(sp->fts_array);
        free(sp->fts_path);

        if (ISSET(FTS_CWDFD))
          {
            if (0 <= sp->fts_cwd_fd)
              if (close (sp->fts_cwd_fd))
                saved_errno = errno;
          }
        else if (!ISSET(FTS_NOCHDIR))
          {
            /* Return to original directory, save errno if necessary. */
            if (fchdir(sp->fts_rfd))
              saved_errno = errno;

            /* If close fails, record errno only if saved_errno is zero,
               so that we report the probably-more-meaningful fchdir errno.  */
            if (close (sp->fts_rfd))
              if (saved_errno == 0)
                saved_errno = errno;
          }

        fd_ring_clear (&sp->fts_fd_ring);

        if (sp->fts_leaf_optimization_works_ht)
          hash_free (sp->fts_leaf_optimization_works_ht);

        free_dir (sp);

        /* Free up the stream pointer. */
        free(sp);

        /* Set errno and return. */
        if (saved_errno) {
                __set_errno (saved_errno);
                return (-1);
        }

        return (0);
}

#if defined __linux__ \
  && HAVE_SYS_VFS_H && HAVE_FSTATFS && HAVE_STRUCT_STATFS_F_TYPE

# include <sys/vfs.h>

/* Linux-specific constants from coreutils' src/fs.h */
# define S_MAGIC_TMPFS 0x1021994
# define S_MAGIC_NFS 0x6969
# define S_MAGIC_REISERFS 0x52654973
# define S_MAGIC_XFS 0x58465342
# define S_MAGIC_PROC 0x9FA0

/* Return false if it is easy to determine the file system type of
   the directory on which DIR_FD is open, and sorting dirents on
   inode numbers is known not to improve traversal performance with
   that type of file system.  Otherwise, return true.  */
static bool
dirent_inode_sort_may_be_useful (int dir_fd)
{
  /* Skip the sort only if we can determine efficiently
     that skipping it is the right thing to do.
     The cost of performing an unnecessary sort is negligible,
     while the cost of *not* performing it can be O(N^2) with
     a very large constant.  */
  struct statfs fs_buf;

  /* If fstatfs fails, assume sorting would be useful.  */
  if (fstatfs (dir_fd, &fs_buf) != 0)
    return true;

  /* FIXME: what about when f_type is not an integral type?
     deal with that if/when it's encountered.  */
  switch (fs_buf.f_type)
    {
    case S_MAGIC_TMPFS:
    case S_MAGIC_NFS:
      /* On a file system of any of these types, sorting
         is unnecessary, and hence wasteful.  */
      return false;

    default:
      return true;
    }
}

/* Given a file descriptor DIR_FD open on a directory D,
   return true if it is valid to apply the leaf-optimization
   technique of counting directories in D via stat.st_nlink.  */
static bool
leaf_optimization_applies (int dir_fd)
{
  struct statfs fs_buf;

  /* If fstatfs fails, assume we can't use the optimization.  */
  if (fstatfs (dir_fd, &fs_buf) != 0)
    return false;

  /* FIXME: do we need to detect AFS mount points?  I doubt it,
     unless fstatfs can report S_MAGIC_REISERFS for such a directory.  */

  switch (fs_buf.f_type)
    {
      /* List here the file system types that lack usable dirent.d_type
         info, yet for which the optimization does apply.  */
    case S_MAGIC_REISERFS:
    case S_MAGIC_XFS:
      return true;

      /* Explicitly list here any other file system type for which the
         optimization is not applicable, but need documentation.  */
    case S_MAGIC_NFS:
      /* NFS provides usable dirent.d_type but not necessarily for all entries
         of large directories, so as per <https://bugzilla.redhat.com/1252549>
         NFS should return true.  However st_nlink values are not accurate on
         all implementations as per <https://bugzilla.redhat.com/1299169>.  */
      /* fall through */
    case S_MAGIC_PROC:
      /* Per <http://bugs.debian.org/143111> /proc may have
         bogus stat.st_nlink values.  */
      /* fall through */
    default:
      return false;
    }
}

#else
static bool
dirent_inode_sort_may_be_useful (int dir_fd _GL_UNUSED) { return true; }
static bool
leaf_optimization_applies (int dir_fd _GL_UNUSED) { return false; }
#endif

/* link-count-optimization entry:
   map a stat.st_dev number to a boolean: leaf_optimization_works */
struct LCO_ent
{
  dev_t st_dev;
  bool opt_ok;
};

/* Use a tiny initial size.  If a traversal encounters more than
   a few devices, the cost of growing/rehashing this table will be
   rendered negligible by the number of inodes processed.  */
enum { LCO_HT_INITIAL_SIZE = 13 };

static size_t
LCO_hash (void const *x, size_t table_size)
{
  struct LCO_ent const *ax = x;
  return (uintmax_t) ax->st_dev % table_size;
}

static bool
LCO_compare (void const *x, void const *y)
{
  struct LCO_ent const *ax = x;
  struct LCO_ent const *ay = y;
  return ax->st_dev == ay->st_dev;
}

/* Ask the same question as leaf_optimization_applies, but query
   the cache first (FTS.fts_leaf_optimization_works_ht), and if necessary,
   update that cache.  */
static bool
link_count_optimize_ok (FTSENT const *p)
{
  FTS *sp = p->fts_fts;
  Hash_table *h = sp->fts_leaf_optimization_works_ht;
  struct LCO_ent tmp;
  struct LCO_ent *ent;
  bool opt_ok;
  struct LCO_ent *t2;

  /* If we're not in CWDFD mode, don't bother with this optimization,
     since the caller is not serious about performance. */
  if (!ISSET(FTS_CWDFD))
    return false;

  /* map st_dev to the boolean, leaf_optimization_works */
  if (h == NULL)
    {
      h = sp->fts_leaf_optimization_works_ht
        = hash_initialize (LCO_HT_INITIAL_SIZE, NULL, LCO_hash,
                           LCO_compare, free);
      if (h == NULL)
        return false;
    }
  tmp.st_dev = p->fts_statp->st_dev;
  ent = hash_lookup (h, &tmp);
  if (ent)
    return ent->opt_ok;

  /* Look-up failed.  Query directly and cache the result.  */
  t2 = malloc (sizeof *t2);
  if (t2 == NULL)
    return false;

  /* Is it ok to perform the optimization in the dir, FTS_CWD_FD?  */
  opt_ok = leaf_optimization_applies (sp->fts_cwd_fd);
  t2->opt_ok = opt_ok;
  t2->st_dev = p->fts_statp->st_dev;

  ent = hash_insert (h, t2);
  if (ent == NULL)
    {
      /* insertion failed */
      free (t2);
      return false;
    }
  fts_assert (ent == t2);

  return opt_ok;
}

/*
 * Special case of "/" at the end of the file name so that slashes aren't
 * appended which would cause file names to be written as "....//foo".
 */
#define NAPPEND(p)                                                      \
        (ISSLASH(p->fts_path[p->fts_pathlen - 1])                       \
            ? p->fts_pathlen - 1 : p->fts_pathlen)

FTSENT *
fts_read (register FTS *sp)
{
        register FTSENT *p, *tmp;
        register unsigned short int instr;
        register char *t;

        /* If finished or unrecoverable error, return NULL. */
        if (sp->fts_cur == NULL || ISSET(FTS_STOP))
                return (NULL);

        /* Set current node pointer. */
        p = sp->fts_cur;

        /* Save and zero out user instructions. */
        instr = p->fts_instr;
        p->fts_instr = FTS_NOINSTR;

        /* Any type of file may be re-visited; re-stat and re-turn. */
        if (instr == FTS_AGAIN) {
                p->fts_info = fts_stat(sp, p, false);
                return (p);
        }
        Dprintf (("fts_read: p=%s\n",
                  p->fts_info == FTS_INIT ? "" : p->fts_path));

        /*
         * Following a symlink -- SLNONE test allows application to see
         * SLNONE and recover.  If indirecting through a symlink, have
         * keep a pointer to current location.  If unable to get that
         * pointer, follow fails.
         */
        if (instr == FTS_FOLLOW &&
            (p->fts_info == FTS_SL || p->fts_info == FTS_SLNONE)) {
                p->fts_info = fts_stat(sp, p, true);
                if (p->fts_info == FTS_D && !ISSET(FTS_NOCHDIR)) {
                        if ((p->fts_symfd = diropen (sp, ".")) < 0) {
                                p->fts_errno = errno;
                                p->fts_info = FTS_ERR;
                        } else
                                p->fts_flags |= FTS_SYMFOLLOW;
                }
                goto check_for_dir;
        }

        /* Directory in pre-order. */
        if (p->fts_info == FTS_D) {
                /* If skipped or crossed mount point, do post-order visit. */
                if (instr == FTS_SKIP ||
                    (ISSET(FTS_XDEV) && p->fts_statp->st_dev != sp->fts_dev)) {
                        if (p->fts_flags & FTS_SYMFOLLOW)
                                (void)close(p->fts_symfd);
                        if (sp->fts_child) {
                                fts_lfree(sp->fts_child);
                                sp->fts_child = NULL;
                        }
                        p->fts_info = FTS_DP;
                        LEAVE_DIR (sp, p, "1");
                        return (p);
                }

                /* Rebuild if only read the names and now traversing. */
                if (sp->fts_child != NULL && ISSET(FTS_NAMEONLY)) {
                        CLR(FTS_NAMEONLY);
                        fts_lfree(sp->fts_child);
                        sp->fts_child = NULL;
                }

                /*
                 * Cd to the subdirectory.
                 *
                 * If have already read and now fail to chdir, whack the list
                 * to make the names come out right, and set the parent errno
                 * so the application will eventually get an error condition.
                 * Set the FTS_DONTCHDIR flag so that when we logically change
                 * directories back to the parent we don't do a chdir.
                 *
                 * If haven't read do so.  If the read fails, fts_build sets
                 * FTS_STOP or the fts_info field of the node.
                 */
                if (sp->fts_child != NULL) {
                        if (fts_safe_changedir(sp, p, -1, p->fts_accpath)) {
                                p->fts_errno = errno;
                                p->fts_flags |= FTS_DONTCHDIR;
                                for (p = sp->fts_child; p != NULL;
                                     p = p->fts_link)
                                        p->fts_accpath =
                                            p->fts_parent->fts_accpath;
                        }
                } else if ((sp->fts_child = fts_build(sp, BREAD)) == NULL) {
                        if (ISSET(FTS_STOP))
                                return (NULL);
                        /* If fts_build's call to fts_safe_changedir failed
                           because it was not able to fchdir into a
                           subdirectory, tell the caller.  */
                        if (p->fts_errno && p->fts_info != FTS_DNR)
                                p->fts_info = FTS_ERR;
                        LEAVE_DIR (sp, p, "2");
                        return (p);
                }
                p = sp->fts_child;
                sp->fts_child = NULL;
                goto name;
        }

        /* Move to the next node on this level. */
next:   tmp = p;

        /* If we have so many directory entries that we're reading them
           in batches, and we've reached the end of the current batch,
           read in a new batch.  */
        if (p->fts_link == NULL && p->fts_parent->fts_dirp)
          {
            p = tmp->fts_parent;
            sp->fts_cur = p;
            sp->fts_path[p->fts_pathlen] = '\0';

            if ((p = fts_build (sp, BREAD)) == NULL)
              {
                if (ISSET(FTS_STOP))
                  return NULL;
                goto cd_dot_dot;
              }

            free(tmp);
            goto name;
          }

        if ((p = p->fts_link) != NULL) {
                sp->fts_cur = p;
                free(tmp);

                /*
                 * If reached the top, return to the original directory (or
                 * the root of the tree), and load the file names for the next
                 * root.
                 */
                if (p->fts_level == FTS_ROOTLEVEL) {
                        if (restore_initial_cwd(sp)) {
                                SET(FTS_STOP);
                                return (NULL);
                        }
                        free_dir(sp);
                        fts_load(sp, p);
                        setup_dir(sp);
                        goto check_for_dir;
                }

                /*
                 * User may have called fts_set on the node.  If skipped,
                 * ignore.  If followed, get a file descriptor so we can
                 * get back if necessary.
                 */
                if (p->fts_instr == FTS_SKIP)
                        goto next;
                if (p->fts_instr == FTS_FOLLOW) {
                        p->fts_info = fts_stat(sp, p, true);
                        if (p->fts_info == FTS_D && !ISSET(FTS_NOCHDIR)) {
                                if ((p->fts_symfd = diropen (sp, ".")) < 0) {
                                        p->fts_errno = errno;
                                        p->fts_info = FTS_ERR;
                                } else
                                        p->fts_flags |= FTS_SYMFOLLOW;
                        }
                        p->fts_instr = FTS_NOINSTR;
                }

name:           t = sp->fts_path + NAPPEND(p->fts_parent);
                *t++ = '/';
                memmove(t, p->fts_name, p->fts_namelen + 1);
check_for_dir:
                sp->fts_cur = p;
                if (p->fts_info == FTS_NSOK)
                  {
                    if (p->fts_statp->st_size == FTS_STAT_REQUIRED)
                      {
                        FTSENT *parent = p->fts_parent;
                        if (FTS_ROOTLEVEL < p->fts_level
                            /* ->fts_n_dirs_remaining is not valid
                               for command-line-specified names.  */
                            && parent->fts_n_dirs_remaining == 0
                            && ISSET(FTS_NOSTAT)
                            && ISSET(FTS_PHYSICAL)
                            && link_count_optimize_ok (parent))
                          {
                            /* nothing more needed */
                          }
                        else
                          {
                            p->fts_info = fts_stat(sp, p, false);
                            if (S_ISDIR(p->fts_statp->st_mode)
                                && p->fts_level != FTS_ROOTLEVEL
                                && parent->fts_n_dirs_remaining)
                                  parent->fts_n_dirs_remaining--;
                          }
                      }
                    else
                      fts_assert (p->fts_statp->st_size == FTS_NO_STAT_REQUIRED);
                  }

                if (p->fts_info == FTS_D)
                  {
                    /* Now that P->fts_statp is guaranteed to be valid,
                       if this is a command-line directory, record its
                       device number, to be used for FTS_XDEV.  */
                    if (p->fts_level == FTS_ROOTLEVEL)
                      sp->fts_dev = p->fts_statp->st_dev;
                    Dprintf (("  entering: %s\n", p->fts_path));
                    if (! enter_dir (sp, p))
                      {
                        __set_errno (ENOMEM);
                        return NULL;
                      }
                  }
                return p;
        }
cd_dot_dot:

        /* Move up to the parent node. */
        p = tmp->fts_parent;
        sp->fts_cur = p;
        free(tmp);

        if (p->fts_level == FTS_ROOTPARENTLEVEL) {
                /*
                 * Done; free everything up and set errno to 0 so the user
                 * can distinguish between error and EOF.
                 */
                free(p);
                __set_errno (0);
                return (sp->fts_cur = NULL);
        }

        fts_assert (p->fts_info != FTS_NSOK);

        /* NUL terminate the file name.  */
        sp->fts_path[p->fts_pathlen] = '\0';

        /*
         * Return to the parent directory.  If at a root node, restore
         * the initial working directory.  If we came through a symlink,
         * go back through the file descriptor.  Otherwise, move up
         * one level, via "..".
         */
        if (p->fts_level == FTS_ROOTLEVEL) {
                if (restore_initial_cwd(sp)) {
                        p->fts_errno = errno;
                        SET(FTS_STOP);
                }
        } else if (p->fts_flags & FTS_SYMFOLLOW) {
                if (FCHDIR(sp, p->fts_symfd)) {
                        p->fts_errno = errno;
                        SET(FTS_STOP);
                }
                (void)close(p->fts_symfd);
        } else if (!(p->fts_flags & FTS_DONTCHDIR) &&
                   fts_safe_changedir(sp, p->fts_parent, -1, "..")) {
                p->fts_errno = errno;
                SET(FTS_STOP);
        }

        /* If the directory causes a cycle, preserve the FTS_DC flag and keep
           the corresponding dev/ino pair in the hash table.  It is going to be
           removed when leaving the original directory.  */
        if (p->fts_info != FTS_DC) {
                p->fts_info = p->fts_errno ? FTS_ERR : FTS_DP;
                if (p->fts_errno == 0)
                        LEAVE_DIR (sp, p, "3");
        }
#ifdef __DJGPP__
        /* Close also the second descriptor related to this directory.  */
        close(sp->dir_fd);
#endif
        return ISSET(FTS_STOP) ? NULL : p;
}

/*
 * Fts_set takes the stream as an argument although it's not used in this
 * implementation; it would be necessary if anyone wanted to add global
 * semantics to fts using fts_set.  An error return is allowed for similar
 * reasons.
 */
/* ARGSUSED */
int
fts_set(FTS *sp _GL_UNUSED, FTSENT *p, int instr)
{
        if (instr != 0 && instr != FTS_AGAIN && instr != FTS_FOLLOW &&
            instr != FTS_NOINSTR && instr != FTS_SKIP) {
                __set_errno (EINVAL);
                return (1);
        }
        p->fts_instr = instr;
        return (0);
}

FTSENT *
fts_children (register FTS *sp, int instr)
{
        register FTSENT *p;
        int fd;

        if (instr != 0 && instr != FTS_NAMEONLY) {
                __set_errno (EINVAL);
                return (NULL);
        }

        /* Set current node pointer. */
        p = sp->fts_cur;

        /*
         * Errno set to 0 so user can distinguish empty directory from
         * an error.
         */
        __set_errno (0);

        /* Fatal errors stop here. */
        if (ISSET(FTS_STOP))
                return (NULL);

        /* Return logical hierarchy of user's arguments. */
        if (p->fts_info == FTS_INIT)
                return (p->fts_link);

        /*
         * If not a directory being visited in pre-order, stop here.  Could
         * allow FTS_DNR, assuming the user has fixed the problem, but the
         * same effect is available with FTS_AGAIN.
         */
        if (p->fts_info != FTS_D /* && p->fts_info != FTS_DNR */)
                return (NULL);

        /* Free up any previous child list. */
        if (sp->fts_child != NULL)
                fts_lfree(sp->fts_child);

        if (instr == FTS_NAMEONLY) {
                SET(FTS_NAMEONLY);
                instr = BNAMES;
        } else
                instr = BCHILD;

        /*
         * If using chdir on a relative file name and called BEFORE fts_read
         * does its chdir to the root of a traversal, we can lose -- we need to
         * chdir into the subdirectory, and we don't know where the current
         * directory is, so we can't get back so that the upcoming chdir by
         * fts_read will work.
         */
        if (p->fts_level != FTS_ROOTLEVEL || IS_ABSOLUTE_FILE_NAME(p->fts_accpath) ||
            ISSET(FTS_NOCHDIR))
                return (sp->fts_child = fts_build(sp, instr));

        if ((fd = diropen (sp, ".")) < 0)
                return (sp->fts_child = NULL);
        sp->fts_child = fts_build(sp, instr);
        if (ISSET(FTS_CWDFD))
          {
            cwd_advance_fd (sp, fd, true);
          }
        else
          {
            if (fchdir(fd))
              {
                int saved_errno = errno;
                close (fd);
                __set_errno (saved_errno);
                return NULL;
              }
            close (fd);
          }
        return (sp->fts_child);
}

/* A comparison function to sort on increasing inode number.
   For some file system types, sorting either way makes a huge
   performance difference for a directory with very many entries,
   but sorting on increasing values is slightly better than sorting
   on decreasing values.  The difference is in the 5% range.  */
static int
fts_compare_ino (struct _ftsent const **a, struct _ftsent const **b)
{
  return (a[0]->fts_statp->st_ino < b[0]->fts_statp->st_ino ? -1
          : b[0]->fts_statp->st_ino < a[0]->fts_statp->st_ino ? 1 : 0);
}

/* Map the dirent.d_type value, DTYPE, to the corresponding stat.st_mode
   S_IF* bit and set ST.st_mode, thus clearing all other bits in that field.  */
static void
set_stat_type (struct stat *st, unsigned int dtype)
{
  mode_t type;
  switch (dtype)
    {
    case DT_BLK:
      type = S_IFBLK;
      break;
    case DT_CHR:
      type = S_IFCHR;
      break;
    case DT_DIR:
      type = S_IFDIR;
      break;
    case DT_FIFO:
      type = S_IFIFO;
      break;
    case DT_LNK:
      type = S_IFLNK;
      break;
    case DT_REG:
      type = S_IFREG;
      break;
    case DT_SOCK:
      type = S_IFSOCK;
      break;
    default:
      type = 0;
    }
  st->st_mode = type;
}

#define closedir_and_clear(dirp)                \
  do                                            \
    {                                           \
      closedir (dirp);                          \
      dirp = NULL;                              \
    }                                           \
  while (0)

#define fts_opendir(file, Pdir_fd)                              \
        opendirat((! ISSET(FTS_NOCHDIR) && ISSET(FTS_CWDFD)     \
                   ? sp->fts_cwd_fd : AT_FDCWD),                \
                  file,                                         \
                  (((ISSET(FTS_PHYSICAL)                        \
                     && ! (ISSET(FTS_COMFOLLOW)                 \
                           && cur->fts_level == FTS_ROOTLEVEL)) \
                    ? O_NOFOLLOW : 0)                           \
                   | (ISSET (FTS_NOATIME) ? O_NOATIME : 0)),    \
                  Pdir_fd)

/*
 * This is the tricky part -- do not casually change *anything* in here.  The
 * idea is to build the linked list of entries that are used by fts_children
 * and fts_read.  There are lots of special cases.
 *
 * The real slowdown in walking the tree is the stat calls.  If FTS_NOSTAT is
 * set and it's a physical walk (so that symbolic links can't be directories),
 * we can do things quickly.  First, if it's a 4.4BSD file system, the type
 * of the file is in the directory entry.  Otherwise, we assume that the number
 * of subdirectories in a node is equal to the number of links to the parent.
 * The former skips all stat calls.  The latter skips stat calls in any leaf
 * directories and for any files after the subdirectories in the directory have
 * been found, cutting the stat calls by about 2/3.
 */
static FTSENT *
internal_function
fts_build (register FTS *sp, int type)
{
        register FTSENT *p, *head;
        register size_t nitems;
        FTSENT *tail;
        void *oldaddr;
        int saved_errno;
        bool descend;
        bool doadjust;
        ptrdiff_t level;
        nlink_t nlinks;
        bool nostat;
        size_t len, maxlen, new_len;
        char *cp;
        int dir_fd;
        FTSENT *cur = sp->fts_cur;
        bool continue_readdir = !!cur->fts_dirp;
        size_t max_entries;

        /* When cur->fts_dirp is non-NULL, that means we should
           continue calling readdir on that existing DIR* pointer
           rather than opening a new one.  */
        if (continue_readdir)
          {
            DIR *dp = cur->fts_dirp;
            dir_fd = dirfd (dp);
            if (dir_fd < 0)
              {
                closedir_and_clear (cur->fts_dirp);
                if (type == BREAD)
                  {
                    cur->fts_info = FTS_DNR;
                    cur->fts_errno = errno;
                  }
                return NULL;
              }
          }
        else
          {
            /* Open the directory for reading.  If this fails, we're done.
               If being called from fts_read, set the fts_info field. */
            if ((cur->fts_dirp = fts_opendir(cur->fts_accpath, &dir_fd)) == NULL)
              {
                if (type == BREAD)
                  {
                    cur->fts_info = FTS_DNR;
                    cur->fts_errno = errno;
                  }
                return NULL;
              }
            /* Rather than calling fts_stat for each and every entry encountered
               in the readdir loop (below), stat each directory only right after
               opening it.  */
            if (cur->fts_info == FTS_NSOK)
              cur->fts_info = fts_stat(sp, cur, false);
            else if (sp->fts_options & FTS_TIGHT_CYCLE_CHECK)
              {
                /* Now read the stat info again after opening a directory to
                   reveal eventual changes caused by a submount triggered by
                   the traversal.  But do it only for utilities which use
                   FTS_TIGHT_CYCLE_CHECK.  Therefore, only find and du
                   benefit/suffer from this feature for now.  */
                LEAVE_DIR (sp, cur, "4");
                fts_stat (sp, cur, false);
                if (! enter_dir (sp, cur))
                  {
                    __set_errno (ENOMEM);
                    return NULL;
                  }
              }
#ifdef __DJGPP__
            /* Store the fd of the current directory that must be closed after leaving.
               This is necessary because this descriptor gets duplicated (see: if (ISSET(FTS_CWDFD)))
               and the new descriptor is used to close the directory and the old descriptor
               remains open and thus exhausting the number of available and free descriptors
               for further use.  This descriptor will be closed by the call of fts_read.  */
            sp->dir_fd = dir_fd;
#endif
          }

        /* Maximum number of readdir entries to read at one time.  This
           limitation is to avoid reading millions of entries into memory
           at once.  When an fts_compar function is specified, we have no
           choice: we must read all entries into memory before calling that
           function.  But when no such function is specified, we can read
           entries in batches that are large enough to help us with inode-
           sorting, yet not so large that we risk exhausting memory.  */
        max_entries = sp->fts_compar ? SIZE_MAX : FTS_MAX_READDIR_ENTRIES;

        /*
         * Nlinks is the number of possible entries of type directory in the
         * directory if we're cheating on stat calls, 0 if we're not doing
         * any stat calls at all, (nlink_t) -1 if we're statting everything.
         */
        if (type == BNAMES) {
                nlinks = 0;
                /* Be quiet about nostat, GCC. */
                nostat = false;
        } else if (ISSET(FTS_NOSTAT) && ISSET(FTS_PHYSICAL)) {
                nlinks = (cur->fts_statp->st_nlink
                          - (ISSET(FTS_SEEDOT) ? 0 : 2));
                nostat = true;
        } else {
                nlinks = -1;
                nostat = false;
        }

        /*
         * If we're going to need to stat anything or we want to descend
         * and stay in the directory, chdir.  If this fails we keep going,
         * but set a flag so we don't chdir after the post-order visit.
         * We won't be able to stat anything, but we can still return the
         * names themselves.  Note, that since fts_read won't be able to
         * chdir into the directory, it will have to return different file
         * names than before, i.e. "a/b" instead of "b".  Since the node
         * has already been visited in pre-order, have to wait until the
         * post-order visit to return the error.  There is a special case
         * here, if there was nothing to stat then it's not an error to
         * not be able to stat.  This is all fairly nasty.  If a program
         * needed sorted entries or stat information, they had better be
         * checking FTS_NS on the returned nodes.
         */
        if (continue_readdir)
          {
            /* When resuming a short readdir run, we already have
               the required dirp and dir_fd.  */
            descend = true;
          }
        else if (nlinks || type == BREAD) {
                if (ISSET(FTS_CWDFD))
                  {
                    dir_fd = dup (dir_fd);
                    if (0 <= dir_fd)
                      set_cloexec_flag (dir_fd, true);
                  }
                if (dir_fd < 0 || fts_safe_changedir(sp, cur, dir_fd, NULL)) {
                        if (nlinks && type == BREAD)
                                cur->fts_errno = errno;
                        cur->fts_flags |= FTS_DONTCHDIR;
                        descend = false;
                        closedir_and_clear(cur->fts_dirp);
                        if (ISSET(FTS_CWDFD) && 0 <= dir_fd)
                                close (dir_fd);
                        cur->fts_dirp = NULL;
                } else
                        descend = true;
        } else
                descend = false;

        /*
         * Figure out the max file name length that can be stored in the
         * current buffer -- the inner loop allocates more space as necessary.
         * We really wouldn't have to do the maxlen calculations here, we
         * could do them in fts_read before returning the name, but it's a
         * lot easier here since the length is part of the dirent structure.
         *
         * If not changing directories set a pointer so that can just append
         * each new component into the file name.
         */
        len = NAPPEND(cur);
        if (ISSET(FTS_NOCHDIR)) {
                cp = sp->fts_path + len;
                *cp++ = '/';
        } else {
                /* GCC, you're too verbose. */
                cp = NULL;
        }
        len++;
        maxlen = sp->fts_pathlen - len;

        level = cur->fts_level + 1;

        /* Read the directory, attaching each entry to the "link" pointer. */
        doadjust = false;
        head = NULL;
        tail = NULL;
        nitems = 0;
        while (cur->fts_dirp) {
                bool is_dir;
                size_t d_namelen;
                __set_errno (0);
                struct dirent *dp = readdir(cur->fts_dirp);
                if (dp == NULL) {
                        if (errno) {
                                cur->fts_errno = errno;
                                /* If we've not read any items yet, treat
                                   the error as if we can't access the dir.  */
                                cur->fts_info = (continue_readdir || nitems)
                                                ? FTS_ERR : FTS_DNR;
                        }
                        break;
                }
                if (!ISSET(FTS_SEEDOT) && ISDOT(dp->d_name))
                        continue;

                d_namelen = _D_EXACT_NAMLEN (dp);
                p = fts_alloc (sp, dp->d_name, d_namelen);
                if (!p)
                        goto mem1;
                if (d_namelen >= maxlen) {
                        /* include space for NUL */
                        oldaddr = sp->fts_path;
                        if (! fts_palloc(sp, d_namelen + len + 1)) {
                                /*
                                 * No more memory.  Save
                                 * errno, free up the current structure and the
                                 * structures already allocated.
                                 */
mem1:                           saved_errno = errno;
                                free(p);
                                fts_lfree(head);
                                closedir_and_clear(cur->fts_dirp);
                                cur->fts_info = FTS_ERR;
                                SET(FTS_STOP);
                                __set_errno (saved_errno);
                                return (NULL);
                        }
                        /* Did realloc() change the pointer? */
                        if (oldaddr != sp->fts_path) {
                                doadjust = true;
                                if (ISSET(FTS_NOCHDIR))
                                        cp = sp->fts_path + len;
                        }
                        maxlen = sp->fts_pathlen - len;
                }

                new_len = len + d_namelen;
                if (new_len < len) {
                        /*
                         * In the unlikely event that we would end up
                         * with a file name longer than SIZE_MAX, free up
                         * the current structure and the structures already
                         * allocated, then error out with ENAMETOOLONG.
                         */
                        free(p);
                        fts_lfree(head);
                        closedir_and_clear(cur->fts_dirp);
                        cur->fts_info = FTS_ERR;
                        SET(FTS_STOP);
                        __set_errno (ENAMETOOLONG);
                        return (NULL);
                }
                p->fts_level = level;
                p->fts_parent = sp->fts_cur;
                p->fts_pathlen = new_len;

                /* Store dirent.d_ino, in case we need to sort
                   entries before processing them.  */
                p->fts_statp->st_ino = D_INO (dp);

                /* Build a file name for fts_stat to stat. */
                if (ISSET(FTS_NOCHDIR)) {
                        p->fts_accpath = p->fts_path;
                        memmove(cp, p->fts_name, p->fts_namelen + 1);
                } else
                        p->fts_accpath = p->fts_name;

                if (sp->fts_compar == NULL || ISSET(FTS_DEFER_STAT)) {
                        /* Record what fts_read will have to do with this
                           entry. In many cases, it will simply fts_stat it,
                           but we can take advantage of any d_type information
                           to optimize away the unnecessary stat calls.  I.e.,
                           if FTS_NOSTAT is in effect and we're not following
                           symlinks (FTS_PHYSICAL) and d_type indicates this
                           is *not* a directory, then we won't have to stat it
                           at all.  If it *is* a directory, then (currently)
                           we stat it regardless, in order to get device and
                           inode numbers.  Some day we might optimize that
                           away, too, for directories where d_ino is known to
                           be valid.  */
                        bool skip_stat = (ISSET(FTS_PHYSICAL)
                                          && ISSET(FTS_NOSTAT)
                                          && DT_IS_KNOWN(dp)
                                          && ! DT_MUST_BE(dp, DT_DIR));
                        p->fts_info = FTS_NSOK;
                        /* Propagate dirent.d_type information back
                           to caller, when possible.  */
                        set_stat_type (p->fts_statp, D_TYPE (dp));
                        fts_set_stat_required(p, !skip_stat);
                        is_dir = (ISSET(FTS_PHYSICAL)
                                  && DT_MUST_BE(dp, DT_DIR));
                } else {
                        p->fts_info = fts_stat(sp, p, false);
                        is_dir = (p->fts_info == FTS_D
                                  || p->fts_info == FTS_DC
                                  || p->fts_info == FTS_DOT);
                }

                /* Decrement link count if applicable. */
                if (nlinks > 0 && is_dir)
                        nlinks -= nostat;

                /* We walk in directory order so "ls -f" doesn't get upset. */
                p->fts_link = NULL;
                if (head == NULL)
                        head = tail = p;
                else {
                        tail->fts_link = p;
                        tail = p;
                }
                ++nitems;
                if (max_entries <= nitems) {
                        /* When there are too many dir entries, leave
                           fts_dirp open, so that a subsequent fts_read
                           can take up where we leave off.  */
                        goto break_without_closedir;
                }
        }

        if (cur->fts_dirp)
                closedir_and_clear(cur->fts_dirp);

 break_without_closedir:

        /*
         * If realloc() changed the address of the file name, adjust the
         * addresses for the rest of the tree and the dir list.
         */
        if (doadjust)
                fts_padjust(sp, head);

        /*
         * If not changing directories, reset the file name back to original
         * state.
         */
        if (ISSET(FTS_NOCHDIR)) {
                if (len == sp->fts_pathlen || nitems == 0)
                        --cp;
                *cp = '\0';
        }

        /*
         * If descended after called from fts_children or after called from
         * fts_read and nothing found, get back.  At the root level we use
         * the saved fd; if one of fts_open()'s arguments is a relative name
         * to an empty directory, we wind up here with no other way back.  If
         * can't get back, we're done.
         */
        if (!continue_readdir && descend && (type == BCHILD || !nitems) &&
            (cur->fts_level == FTS_ROOTLEVEL
             ? restore_initial_cwd(sp)
             : fts_safe_changedir(sp, cur->fts_parent, -1, ".."))) {
                cur->fts_info = FTS_ERR;
                SET(FTS_STOP);
                fts_lfree(head);
                return (NULL);
        }

        /* If didn't find anything, return NULL. */
        if (!nitems) {
                if (type == BREAD
                    && cur->fts_info != FTS_DNR && cur->fts_info != FTS_ERR)
                        cur->fts_info = FTS_DP;
                fts_lfree(head);
                return (NULL);
        }

        /* If there are many entries, no sorting function has been specified,
           and this file system is of a type that may be slow with a large
           number of entries, then sort the directory entries on increasing
           inode numbers.  */
        if (nitems > _FTS_INODE_SORT_DIR_ENTRIES_THRESHOLD
            && !sp->fts_compar
            && ISSET (FTS_CWDFD)
            && dirent_inode_sort_may_be_useful (sp->fts_cwd_fd)) {
                sp->fts_compar = fts_compare_ino;
                head = fts_sort (sp, head, nitems);
                sp->fts_compar = NULL;
        }

        /* Sort the entries. */
        if (sp->fts_compar && nitems > 1)
                head = fts_sort(sp, head, nitems);
        return (head);
}

#if FTS_DEBUG

/* Walk ->fts_parent links starting at E_CURR, until the root of the
   current hierarchy.  There should be a directory with dev/inode
   matching those of AD.  If not, print a lot of diagnostics.  */
static void
find_matching_ancestor (FTSENT const *e_curr, struct Active_dir const *ad)
{
  FTSENT const *ent;
  for (ent = e_curr; ent->fts_level >= FTS_ROOTLEVEL; ent = ent->fts_parent)
    {
      if (ad->ino == ent->fts_statp->st_ino
          && ad->dev == ent->fts_statp->st_dev)
        return;
    }
  printf ("ERROR: tree dir, %s, not active\n", ad->fts_ent->fts_accpath);
  printf ("active dirs:\n");
  for (ent = e_curr;
       ent->fts_level >= FTS_ROOTLEVEL; ent = ent->fts_parent)
    printf ("  %s(%"PRIuMAX"/%"PRIuMAX") to %s(%"PRIuMAX"/%"PRIuMAX")...\n",
            ad->fts_ent->fts_accpath,
            (uintmax_t) ad->dev,
            (uintmax_t) ad->ino,
            ent->fts_accpath,
            (uintmax_t) ent->fts_statp->st_dev,
            (uintmax_t) ent->fts_statp->st_ino);
}

void
fts_cross_check (FTS const *sp)
{
  FTSENT const *ent = sp->fts_cur;
  FTSENT const *t;
  if ( ! ISSET (FTS_TIGHT_CYCLE_CHECK))
    return;

  Dprintf (("fts-cross-check cur=%s\n", ent->fts_path));
  /* Make sure every parent dir is in the tree.  */
  for (t = ent->fts_parent; t->fts_level >= FTS_ROOTLEVEL; t = t->fts_parent)
    {
      struct Active_dir ad;
      ad.ino = t->fts_statp->st_ino;
      ad.dev = t->fts_statp->st_dev;
      if ( ! hash_lookup (sp->fts_cycle.ht, &ad))
        printf ("ERROR: active dir, %s, not in tree\n", t->fts_path);
    }

  /* Make sure every dir in the tree is an active dir.
     But ENT is not necessarily a directory.  If so, just skip this part. */
  if (ent->fts_parent->fts_level >= FTS_ROOTLEVEL
      && (ent->fts_info == FTS_DP
          || ent->fts_info == FTS_D))
    {
      struct Active_dir *ad;
      for (ad = hash_get_first (sp->fts_cycle.ht); ad != NULL;
           ad = hash_get_next (sp->fts_cycle.ht, ad))
        {
          find_matching_ancestor (ent, ad);
        }
    }
}

static bool
same_fd (int fd1, int fd2)
{
  struct stat sb1, sb2;
  return (fstat (fd1, &sb1) == 0
          && fstat (fd2, &sb2) == 0
          && SAME_INODE (sb1, sb2));
}

static void
fd_ring_print (FTS const *sp, FILE *stream, char const *msg)
{
  I_ring const *fd_ring = &sp->fts_fd_ring;
  unsigned int i = fd_ring->fts_front;
  char *cwd = getcwdat (sp->fts_cwd_fd, NULL, 0);
  fprintf (stream, "=== %s ========== %s\n", msg, cwd);
  free (cwd);
  if (i_ring_empty (fd_ring))
    return;

  while (true)
    {
      int fd = fd_ring->fts_fd_ring[i];
      if (fd < 0)
        fprintf (stream, "%d: %d:\n", i, fd);
      else
        {
          char *wd = getcwdat (fd, NULL, 0);
          fprintf (stream, "%d: %d: %s\n", i, fd, wd);
          free (wd);
        }
      if (i == fd_ring->fts_back)
        break;
      i = (i + I_RING_SIZE - 1) % I_RING_SIZE;
    }
}

/* Ensure that each file descriptor on the fd_ring matches a
   parent, grandparent, etc. of the current working directory.  */
static void
fd_ring_check (FTS const *sp)
{
  if (!fts_debug)
    return;

  /* Make a writable copy.  */
  I_ring fd_w = sp->fts_fd_ring;

  int cwd_fd = sp->fts_cwd_fd;
  cwd_fd = dup (cwd_fd);
  char *dot = getcwdat (cwd_fd, NULL, 0);
  error (0, 0, "===== check ===== cwd: %s", dot);
  free (dot);
  while ( ! i_ring_empty (&fd_w))
    {
      int fd = i_ring_pop (&fd_w);
      if (0 <= fd)
        {
          int parent_fd = openat (cwd_fd, "..", O_SEARCH | O_NOATIME);
          if (parent_fd < 0)
            {
              // Warn?
              break;
            }
          if (!same_fd (fd, parent_fd))
            {
              char *cwd = getcwdat (fd, NULL, 0);
              error (0, errno, "ring  : %s", cwd);
              char *c2 = getcwdat (parent_fd, NULL, 0);
              error (0, errno, "parent: %s", c2);
              free (cwd);
              free (c2);
              fts_assert (0);
            }
          close (cwd_fd);
          cwd_fd = parent_fd;
        }
    }
  close (cwd_fd);
}
#endif

static unsigned short int
internal_function
fts_stat(FTS *sp, register FTSENT *p, bool follow)
{
        struct stat *sbp = p->fts_statp;
        int saved_errno;

        if (p->fts_level == FTS_ROOTLEVEL && ISSET(FTS_COMFOLLOW))
                follow = true;

        /*
         * If doing a logical walk, or application requested FTS_FOLLOW, do
         * a stat(2).  If that fails, check for a non-existent symlink.  If
         * fail, set the errno from the stat call.
         */
        if (ISSET(FTS_LOGICAL) || follow) {
                if (stat(p->fts_accpath, sbp)) {
                        saved_errno = errno;
                        if (errno == ENOENT
                            && lstat(p->fts_accpath, sbp) == 0) {
                                __set_errno (0);
                                return (FTS_SLNONE);
                        }
                        p->fts_errno = saved_errno;
                        goto err;
                }
        } else if (fstatat(sp->fts_cwd_fd, p->fts_accpath, sbp,
                           AT_SYMLINK_NOFOLLOW)) {
                p->fts_errno = errno;
err:            memset(sbp, 0, sizeof(struct stat));
                return (FTS_NS);
        }

        if (S_ISDIR(sbp->st_mode)) {
                p->fts_n_dirs_remaining = (sbp->st_nlink
                                           - (ISSET(FTS_SEEDOT) ? 0 : 2));
                if (ISDOT(p->fts_name)) {
                        /* Command-line "." and ".." are real directories. */
                        return (p->fts_level == FTS_ROOTLEVEL ? FTS_D : FTS_DOT);
                }

                return (FTS_D);
        }
        if (S_ISLNK(sbp->st_mode))
                return (FTS_SL);
        if (S_ISREG(sbp->st_mode))
                return (FTS_F);
        return (FTS_DEFAULT);
}

static int
fts_compar (void const *a, void const *b)
{
  /* Convert A and B to the correct types, to pacify the compiler, and
     for portability to bizarre hosts where "void const *" and "FTSENT
     const **" differ in runtime representation.  The comparison
     function cannot modify *a and *b, but there is no compile-time
     check for this.  */
  FTSENT const **pa = (FTSENT const **) a;
  FTSENT const **pb = (FTSENT const **) b;
  return pa[0]->fts_fts->fts_compar (pa, pb);
}

static FTSENT *
internal_function
fts_sort (FTS *sp, FTSENT *head, register size_t nitems)
{
        register FTSENT **ap, *p;

        /* On most modern hosts, void * and FTSENT ** have the same
           run-time representation, and one can convert sp->fts_compar to
           the type qsort expects without problem.  Use the heuristic that
           this is OK if the two pointer types are the same size, and if
           converting FTSENT ** to long int is the same as converting
           FTSENT ** to void * and then to long int.  This heuristic isn't
           valid in general but we don't know of any counterexamples.  */
        FTSENT *dummy;
        int (*compare) (void const *, void const *) =
          ((sizeof &dummy == sizeof (void *)
            && (long int) &dummy == (long int) (void *) &dummy)
           ? (int (*) (void const *, void const *)) sp->fts_compar
           : fts_compar);

        /*
         * Construct an array of pointers to the structures and call qsort(3).
         * Reassemble the array in the order returned by qsort.  If unable to
         * sort for memory reasons, return the directory entries in their
         * current order.  Allocate enough space for the current needs plus
         * 40 so don't realloc one entry at a time.
         */
        if (nitems > sp->fts_nitems) {
                FTSENT **a;

                sp->fts_nitems = nitems + 40;
                if (SIZE_MAX / sizeof *a < sp->fts_nitems
                    || ! (a = realloc (sp->fts_array,
                                       sp->fts_nitems * sizeof *a))) {
                        free(sp->fts_array);
                        sp->fts_array = NULL;
                        sp->fts_nitems = 0;
                        return (head);
                }
                sp->fts_array = a;
        }
        for (ap = sp->fts_array, p = head; p; p = p->fts_link)
                *ap++ = p;
        qsort((void *)sp->fts_array, nitems, sizeof(FTSENT *), compare);
        for (head = *(ap = sp->fts_array); --nitems; ++ap)
                ap[0]->fts_link = ap[1];
        ap[0]->fts_link = NULL;
        return (head);
}

static FTSENT *
internal_function
fts_alloc (FTS *sp, const char *name, register size_t namelen)
{
        register FTSENT *p;
        size_t len;

        /*
         * The file name is a variable length array.  Allocate the FTSENT
         * structure and the file name in one chunk.
         */
        len = FLEXSIZEOF(FTSENT, fts_name, namelen + 1);
        if ((p = malloc(len)) == NULL)
                return (NULL);

        /* Copy the name and guarantee NUL termination. */
        memcpy(p->fts_name, name, namelen);
        p->fts_name[namelen] = '\0';

        p->fts_namelen = namelen;
        p->fts_fts = sp;
        p->fts_path = sp->fts_path;
        p->fts_errno = 0;
        p->fts_dirp = NULL;
        p->fts_flags = 0;
        p->fts_instr = FTS_NOINSTR;
        p->fts_number = 0;
        p->fts_pointer = NULL;
        return (p);
}

static void
internal_function
fts_lfree (register FTSENT *head)
{
        register FTSENT *p;

        /* Free a linked list of structures. */
        while ((p = head)) {
                head = head->fts_link;
                if (p->fts_dirp)
                        closedir (p->fts_dirp);
                free(p);
        }
}

/*
 * Allow essentially unlimited file name lengths; find, rm, ls should
 * all work on any tree.  Most systems will allow creation of file
 * names much longer than MAXPATHLEN, even though the kernel won't
 * resolve them.  Add the size (not just what's needed) plus 256 bytes
 * so don't realloc the file name 2 bytes at a time.
 */
static bool
internal_function
fts_palloc (FTS *sp, size_t more)
{
        char *p;
        size_t new_len = sp->fts_pathlen + more + 256;

        /*
         * See if fts_pathlen would overflow.
         */
        if (new_len < sp->fts_pathlen) {
                free(sp->fts_path);
                sp->fts_path = NULL;
                __set_errno (ENAMETOOLONG);
                return false;
        }
        sp->fts_pathlen = new_len;
        p = realloc(sp->fts_path, sp->fts_pathlen);
        if (p == NULL) {
                free(sp->fts_path);
                sp->fts_path = NULL;
                return false;
        }
        sp->fts_path = p;
        return true;
}

/*
 * When the file name is realloc'd, have to fix all of the pointers in
 *  structures already returned.
 */
static void
internal_function
fts_padjust (FTS *sp, FTSENT *head)
{
        FTSENT *p;
        char *addr = sp->fts_path;

#define ADJUST(p) do {                                                  \
        if ((p)->fts_accpath != (p)->fts_name) {                        \
                (p)->fts_accpath =                                      \
                    (char *)addr + ((p)->fts_accpath - (p)->fts_path);  \
        }                                                               \
        (p)->fts_path = addr;                                           \
} while (0)
        /* Adjust the current set of children. */
        for (p = sp->fts_child; p; p = p->fts_link)
                ADJUST(p);

        /* Adjust the rest of the tree, including the current level. */
        for (p = head; p->fts_level >= FTS_ROOTLEVEL;) {
                ADJUST(p);
                p = p->fts_link ? p->fts_link : p->fts_parent;
        }
}

static size_t
internal_function _GL_ATTRIBUTE_PURE
fts_maxarglen (char * const *argv)
{
        size_t len, max;

        for (max = 0; *argv; ++argv)
                if ((len = strlen(*argv)) > max)
                        max = len;
        return (max + 1);
}

/*
 * Change to dir specified by fd or file name without getting
 * tricked by someone changing the world out from underneath us.
 * Assumes p->fts_statp->st_dev and p->fts_statp->st_ino are filled in.
 * If FD is non-negative, expect it to be used after this function returns,
 * and to be closed eventually.  So don't pass e.g., 'dirfd(dirp)' and then
 * do closedir(dirp), because that would invalidate the saved FD.
 * Upon failure, close FD immediately and return nonzero.
 */
static int
internal_function
fts_safe_changedir (FTS *sp, FTSENT *p, int fd, char const *dir)
{
        int ret;
        bool is_dotdot = dir && STREQ (dir, "..");
        int newfd;

        /* This clause handles the unusual case in which FTS_NOCHDIR
           is specified, along with FTS_CWDFD.  In that case, there is
           no need to change even the virtual cwd file descriptor.
           However, if FD is non-negative, we do close it here.  */
        if (ISSET (FTS_NOCHDIR))
          {
            if (ISSET (FTS_CWDFD) && 0 <= fd)
              close (fd);
            return 0;
          }

        if (fd < 0 && is_dotdot && ISSET (FTS_CWDFD))
          {
            /* When possible, skip the diropen and subsequent fstat+dev/ino
               comparison.  I.e., when changing to parent directory
               (chdir ("..")), use a file descriptor from the ring and
               save the overhead of diropen+fstat, as well as avoiding
               failure when we lack "x" access to the virtual cwd.  */
            if ( ! i_ring_empty (&sp->fts_fd_ring))
              {
                int parent_fd;
                fd_ring_print (sp, stderr, "pre-pop");
                parent_fd = i_ring_pop (&sp->fts_fd_ring);
                is_dotdot = true;
                if (0 <= parent_fd)
                  {
                    fd = parent_fd;
                    dir = NULL;
                  }
              }
          }

        newfd = fd;
        if (fd < 0 && (newfd = diropen (sp, dir)) < 0)
          return -1;

        /* The following dev/inode check is necessary if we're doing a
           "logical" traversal (through symlinks, a la chown -L), if the
           system lacks O_NOFOLLOW support, or if we're changing to ".."
           (but not via a popped file descriptor).  When changing to the
           name "..", O_NOFOLLOW can't help.  In general, when the target is
           not "..", diropen's use of O_NOFOLLOW ensures we don't mistakenly
           follow a symlink, so we can avoid the expense of this fstat.  */
        if (ISSET(FTS_LOGICAL) || ! HAVE_WORKING_O_NOFOLLOW
            || (dir && STREQ (dir, "..")))
          {
            struct stat sb;
            if (fstat(newfd, &sb))
              {
                ret = -1;
                goto bail;
              }
            if (p->fts_statp->st_dev != sb.st_dev
                || p->fts_statp->st_ino != sb.st_ino)
              {
                __set_errno (ENOENT);           /* disinformation */
                ret = -1;
                goto bail;
              }
          }

        if (ISSET(FTS_CWDFD))
          {
            cwd_advance_fd (sp, newfd, ! is_dotdot);
            return 0;
          }

        ret = fchdir(newfd);
bail:
        if (fd < 0)
          {
            int oerrno = errno;
            (void)close(newfd);
            __set_errno (oerrno);
          }
        return ret;
}
