/*      $NetBSD: fts.c,v 1.52 2022/04/19 20:32:15 rillig Exp $  */

/*-
 * 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.
 * 3. 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.
 */

#if HAVE_NBTOOL_CONFIG_H
#include "nbtool_config.h"
#endif

#include <sys/cdefs.h>
#if defined(LIBC_SCCS) && !defined(lint)
#if 0
static char sccsid[] = "@(#)fts.c       8.6 (Berkeley) 8/14/94";
#else
__RCSID("$NetBSD: fts.c,v 1.52 2022/04/19 20:32:15 rillig Exp $");
#endif
#endif /* LIBC_SCCS and not lint */

#include "namespace.h"
#include <sys/param.h>
#include <sys/stat.h>

#include <assert.h>
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <fts.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#if ! HAVE_NBTOOL_CONFIG_H
#define HAVE_STRUCT_DIRENT_D_NAMLEN
#endif

static FTSENT   *fts_alloc(FTS *, const char *, size_t);
static FTSENT   *fts_build(FTS *, int);
static void      fts_free(FTSENT *);
static void      fts_lfree(FTSENT *);
static void      fts_load(FTS *, FTSENT *);
static size_t    fts_maxarglen(char * const *);
static size_t    fts_pow2(size_t);
static int       fts_palloc(FTS *, size_t);
static void      fts_padjust(FTS *, FTSENT *);
static FTSENT   *fts_sort(FTS *, FTSENT *, size_t);
static unsigned short fts_stat(FTS *, FTSENT *, int);
static int       fts_safe_changedir(const FTS *, const FTSENT *, int,
    const char *);

#if defined(ALIGNBYTES) && defined(ALIGN)
#define FTS_ALLOC_ALIGNED       1
#else
#undef  FTS_ALLOC_ALIGNED
#endif

#ifndef ftsent_namelen_truncate
#define ftsent_namelen_truncate(a)      \
    ((a) > UINT_MAX ? UINT_MAX : (unsigned int)(a))
#endif
#ifndef ftsent_pathlen_truncate
#define ftsent_pathlen_truncate(a) \
    ((a) > UINT_MAX ? UINT_MAX : (unsigned int)(a))
#endif
#ifndef fts_pathlen_truncate
#define fts_pathlen_truncate(a) \
    ((a) > UINT_MAX ? UINT_MAX : (unsigned int)(a))
#endif
#ifndef fts_nitems_truncate
#define fts_nitems_truncate(a) \
    ((a) > UINT_MAX ? UINT_MAX : (unsigned int)(a))
#endif

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

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

#define CHDIR(sp, path) (!ISSET(FTS_NOCHDIR) && chdir(path))
#define FCHDIR(sp, fd)  (!ISSET(FTS_NOCHDIR) && fchdir(fd))

/* fts_build flags */
#define BCHILD          1               /* fts_children */
#define BNAMES          2               /* fts_children, names only */
#define BREAD           3               /* fts_read */

#ifndef DTF_HIDEW
#undef FTS_WHITEOUT
#endif

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

        _DIAGASSERT(argv != NULL);

        /* Options check. */
        if (options & ~FTS_OPTIONMASK) {
                errno = EINVAL;
                return (NULL);
        }

        /* Allocate/initialize the stream */
        if ((sp = calloc(1, sizeof(FTS))) == NULL)
                return (NULL);
        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);

        /*
         * Start out with 1K of path space, and enough, in any case,
         * to hold the user's paths.
         */
        if (fts_palloc(sp, MAX(fts_maxarglen(argv), MAXPATHLEN)))
                goto mem1;

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

        /* Allocate/initialize root(s). */
        for (root = NULL, nitems = 0; *argv; ++argv, ++nitems) {
                /* Don't allow zero-length paths. */
                if ((len = strlen(*argv)) == 0) {
                        errno = ENOENT;
                        goto mem3;
                }

                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;
                p->fts_info = fts_stat(sp, p, ISSET(FTS_COMFOLLOW));

                /* Command-line "." and ".." are real directories. */
                if (p->fts_info == FTS_DOT)
                        p->fts_info = FTS_D;

                /*
                 * 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 using chdir(2), grab a file descriptor pointing to dot to ensure
         * that we can get back here; this could be avoided for some paths,
         * 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.
         */
#ifndef O_CLOEXEC
#define O_CLOEXEC 0
#endif
        if (!ISSET(FTS_NOCHDIR)) {
                if ((sp->fts_rfd = open(".", O_RDONLY | O_CLOEXEC, 0)) == -1)
                        SET(FTS_NOCHDIR);
        }

        if (nitems == 0)
                fts_free(parent);

        return (sp);

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

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

        _DIAGASSERT(sp != NULL);
        _DIAGASSERT(p != NULL);

        /*
         * 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 path 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 = ftsent_namelen_truncate(len);
        }
        p->fts_accpath = p->fts_path = sp->fts_path;
        sp->fts_dev = p->fts_dev;
}

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

        _DIAGASSERT(sp != NULL);

        /*
         * 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) {
                if (sp->fts_cur->fts_flags & FTS_SYMFOLLOW)
                        (void)close(sp->fts_cur->fts_symfd);
                for (p = sp->fts_cur; p->fts_level >= FTS_ROOTLEVEL;) {
                        freep = p;
                        p = p->fts_link ? p->fts_link : p->fts_parent;
                        fts_free(freep);
                }
                fts_free(p);
        }

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

        /* Return to original directory, save errno if necessary. */
        if (!ISSET(FTS_NOCHDIR)) {
                if (fchdir(sp->fts_rfd) == -1)
                        saved_errno = errno;
                (void)close(sp->fts_rfd);
        }

        /* Free up the stream pointer. */
        free(sp);
        if (saved_errno) {
                errno = saved_errno;
                return -1;
        }

        return 0;
}

#if !defined(__FTS_COMPAT_TAILINGSLASH)

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

#else /* !defined(__FTS_COMPAT_TAILINGSLASH) */

/*
 * compatibility with the old behaviour.
 *
 * Special case a root of "/" so that slashes aren't appended which would
 * cause paths to be written as "//foo".
 */

#define NAPPEND(p)                                                      \
        (p->fts_level == FTS_ROOTLEVEL && p->fts_pathlen == 1 &&        \
            p->fts_path[0] == '/' ? 0 : p->fts_pathlen)

#endif /* !defined(__FTS_COMPAT_TAILINGSLASH) */

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

        _DIAGASSERT(sp != NULL);

        /* 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, 0);
                return (p);
        }

        /*
         * 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, 1);
                if (p->fts_info == FTS_D && !ISSET(FTS_NOCHDIR)) {
                        if ((p->fts_symfd = open(".", O_RDONLY | O_CLOEXEC, 0))
                            == -1) {
                                p->fts_errno = errno;
                                p->fts_info = FTS_ERR;
                        } else
                                p->fts_flags |= FTS_SYMFOLLOW;
                }
                return (p);
        }

        /* 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_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;
                        return (p);
                }

                /* Rebuild if only read the names and now traversing. */
                if (sp->fts_child && 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) {
                        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; 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);
                        return (p);
                }
                p = sp->fts_child;
                sp->fts_child = NULL;
                goto name;
        }

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

        /* 
         * We are going to free sp->fts_cur, set it to NULL so 
         * that fts_close() does not attempt to free it again 
         * if we exit without setting it to a new value because
         * FCHDIR() failed below.
         */
        assert(tmp == sp->fts_cur);
        sp->fts_cur = NULL;
        
        if ((p = p->fts_link) != NULL) {
                fts_free(tmp);

                /*
                 * If reached the top, return to the original directory, and
                 * load the paths for the next root.
                 */
                if (p->fts_level == FTS_ROOTLEVEL) {
                        if (FCHDIR(sp, sp->fts_rfd)) {
                                SET(FTS_STOP);
                                return (NULL);
                        }
                        fts_load(sp, p);
                        return (sp->fts_cur = p);
                }

                /*
                 * 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, 1);
                        if (p->fts_info == FTS_D && !ISSET(FTS_NOCHDIR)) {
                                if ((p->fts_symfd =
                                    open(".", O_RDONLY | O_CLOEXEC, 0)) == -1) {
                                        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, (size_t)(p->fts_namelen + 1));
                return (sp->fts_cur = p);
        }

        /* Move up to the parent node. */
        p = tmp->fts_parent;
        fts_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.
                 */
                fts_free(p);
                errno = 0;
                return (sp->fts_cur = NULL);
        }

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

        /*
         * Return to the parent directory.  If at a root node or came through
         * a symlink, go back through the file descriptor.  Otherwise, cd up
         * one directory.
         */
        if (p->fts_level == FTS_ROOTLEVEL) {
                if (FCHDIR(sp, sp->fts_rfd)) {
                        SET(FTS_STOP);
                        return (NULL);
                }
        } else if (p->fts_flags & FTS_SYMFOLLOW) {
                if (FCHDIR(sp, p->fts_symfd)) {
                        saved_errno = errno;
                        (void)close(p->fts_symfd);
                        errno = saved_errno;
                        SET(FTS_STOP);
                        return (NULL);
                }
                (void)close(p->fts_symfd);
        } else if (!(p->fts_flags & FTS_DONTCHDIR) &&
            fts_safe_changedir(sp, p->fts_parent, -1, "..")) {
                SET(FTS_STOP);
                return (NULL);
        }
        p->fts_info = p->fts_errno ? FTS_ERR : FTS_DP;
        return (sp->fts_cur = 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, FTSENT *p, int instr)
{

        _DIAGASSERT(sp != NULL);
        _DIAGASSERT(p != NULL);

        if (instr && instr != FTS_AGAIN && instr != FTS_FOLLOW &&
            instr != FTS_NOINSTR && instr != FTS_SKIP) {
                errno = EINVAL;
                return (1);
        }
        p->fts_instr = instr;
        return (0);
}

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

        _DIAGASSERT(sp != NULL);

        if (instr && instr != FTS_NAMEONLY) {
                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.
         */
        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)
                fts_lfree(sp->fts_child);

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

        /*
         * If using chdir on a relative path 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 || p->fts_accpath[0] == '/' ||
            ISSET(FTS_NOCHDIR))
                return (sp->fts_child = fts_build(sp, instr));

        if ((fd = open(".", O_RDONLY | O_CLOEXEC, 0)) == -1)
                return (sp->fts_child = NULL);
        sp->fts_child = fts_build(sp, instr);
        if (fchdir(fd)) {
                (void)close(fd);
                return (NULL);
        }
        (void)close(fd);
        return (sp->fts_child);
}

/*
 * 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 *
fts_build(FTS *sp, int type)
{
        struct dirent *dp;
        FTSENT *p, *head;
        size_t nitems;
        FTSENT *cur, *tail;
        DIR *dirp;
        void *oldaddr;
        size_t dnamlen;
        int cderrno, descend, level, nlinks, saved_errno, nostat, doadjust;
        size_t len, maxlen;
#ifdef FTS_WHITEOUT
        int oflag;
#endif
        char *cp = NULL;        /* pacify gcc */

        _DIAGASSERT(sp != NULL);

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

        /*
         * Open the directory for reading.  If this fails, we're done.
         * If being called from fts_read, set the fts_info field.
         */
#ifdef FTS_WHITEOUT
        if (ISSET(FTS_WHITEOUT))
                oflag = DTF_NODUP|DTF_REWIND;
        else
                oflag = DTF_HIDEW|DTF_NODUP|DTF_REWIND;
#else
#define __opendir2(path, flag) opendir(path)
#endif
        if ((dirp = __opendir2(cur->fts_accpath, oflag)) == NULL) {
                if (type == BREAD) {
                        cur->fts_info = FTS_DNR;
                        cur->fts_errno = errno;
                }
                return (NULL);
        }

        /*
         * 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, -1 if we're doing stats on everything.
         */
        if (type == BNAMES) {
                nlinks = 0;
                nostat = 1;
        } else if (ISSET(FTS_NOSTAT) && ISSET(FTS_PHYSICAL)) {
                nlinks = cur->fts_nlink - (ISSET(FTS_SEEDOT) ? 0 : 2);
                nostat = 1;
        } else {
                nlinks = -1;
                nostat = 0;
        }

#ifdef notdef
        (void)printf("nlinks == %d (cur: %d)\n", nlinks, cur->fts_nlink);
        (void)printf("NOSTAT %d PHYSICAL %d SEEDOT %d\n",
            ISSET(FTS_NOSTAT), ISSET(FTS_PHYSICAL), ISSET(FTS_SEEDOT));
#endif
        /*
         * 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 path
         * 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.
         */
        cderrno = 0;
        if (nlinks || type == BREAD) {
                if (fts_safe_changedir(sp, cur, dirfd(dirp), NULL)) {
                        if (nlinks && type == BREAD)
                                cur->fts_errno = errno;
                        cur->fts_flags |= FTS_DONTCHDIR;
                        descend = 0;
                        cderrno = errno;
                } else
                        descend = 1;
        } else
                descend = 0;

        /*
         * Figure out the max file name length that can be stored in the
         * current path -- the inner loop allocates more path as necessary.
         * We really wouldn't have to do the maxlen calculations here, we
         * could do them in fts_read before returning the path, 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 name into the path.
         */
        len = NAPPEND(cur);
        if (ISSET(FTS_NOCHDIR)) {
                cp = sp->fts_path + len;
                *cp++ = '/';
        }
        len++;
        maxlen = sp->fts_pathlen - len;

#if defined(__FTS_COMPAT_LEVEL)
        if (cur->fts_level == SHRT_MAX) {
                (void)closedir(dirp);
                cur->fts_info = FTS_ERR;
                SET(FTS_STOP);
                errno = ENAMETOOLONG;
                return (NULL);
        }
#endif

        level = cur->fts_level + 1;

        /* Read the directory, attaching each entry to the `link' pointer. */
        doadjust = 0;
        for (head = tail = NULL, nitems = 0; (dp = readdir(dirp)) != NULL;) {

                if (!ISSET(FTS_SEEDOT) && ISDOT(dp->d_name))
                        continue;

#if defined(HAVE_STRUCT_DIRENT_D_NAMLEN)
                dnamlen = dp->d_namlen;
#else
                dnamlen = strlen(dp->d_name);
#endif
                if ((p = fts_alloc(sp, dp->d_name, dnamlen)) == NULL)
                        goto mem1;
                if (dnamlen >= maxlen) {        /* include space for NUL */
                        oldaddr = sp->fts_path;
                        if (fts_palloc(sp, dnamlen + len + 1)) {
                                /*
                                 * No more memory for path or structures.  Save
                                 * errno, free up the current structure and the
                                 * structures already allocated.
                                 */
mem1:                           saved_errno = errno;
                                if (p)
                                        fts_free(p);
                                fts_lfree(head);
                                (void)closedir(dirp);
                                errno = saved_errno;
                                cur->fts_info = FTS_ERR;
                                SET(FTS_STOP);
                                return (NULL);
                        }
                        /* Did realloc() change the pointer? */
                        if (oldaddr != sp->fts_path) {
                                doadjust = 1;
                                if (ISSET(FTS_NOCHDIR))
                                        cp = sp->fts_path + len;
                        }
                        maxlen = sp->fts_pathlen - len;
                }

#if defined(__FTS_COMPAT_LENGTH)
                if (len + dnamlen >= USHRT_MAX) {
                        /*
                         * In an FTSENT, fts_pathlen is an unsigned short
                         * so it is possible to wraparound here.
                         * If we do, free up the current structure and the
                         * structures already allocated, then error out
                         * with ENAMETOOLONG.
                         */
                        fts_free(p);
                        fts_lfree(head);
                        (void)closedir(dirp);
                        cur->fts_info = FTS_ERR;
                        SET(FTS_STOP);
                        errno = ENAMETOOLONG;
                        return (NULL);
                }
#endif
                p->fts_level = level;
                p->fts_pathlen = ftsent_pathlen_truncate(len + dnamlen);
                p->fts_parent = sp->fts_cur;

#ifdef FTS_WHITEOUT
                if (dp->d_type == DT_WHT)
                        p->fts_flags |= FTS_ISW;
#endif

                if (cderrno) {
                        if (nlinks) {
                                p->fts_info = FTS_NS;
                                p->fts_errno = cderrno;
                        } else
                                p->fts_info = FTS_NSOK;
                        p->fts_accpath = cur->fts_accpath;
                } else if (nlinks == 0
#ifdef DT_DIR
                    || (nostat &&
                    dp->d_type != DT_DIR && dp->d_type != DT_UNKNOWN)
#endif
                    ) {
                        p->fts_accpath =
                            ISSET(FTS_NOCHDIR) ? p->fts_path : p->fts_name;
                        p->fts_info = FTS_NSOK;
                } else {
                        /* Build a file name for fts_stat to stat. */
                        if (ISSET(FTS_NOCHDIR)) {
                                p->fts_accpath = p->fts_path;
                                memmove(cp, p->fts_name,
                                        (size_t)(p->fts_namelen + 1));
                        } else
                                p->fts_accpath = p->fts_name;
                        /* Stat it. */
                        p->fts_info = fts_stat(sp, p, 0);

                        /* Decrement link count if applicable. */
                        if (nlinks > 0 && (p->fts_info == FTS_D ||
                            p->fts_info == FTS_DC || p->fts_info == FTS_DOT))
                                --nlinks;
                }

                /* 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;
        }
        (void)closedir(dirp);

        /*
         * If had to realloc the path, adjust the addresses for the rest
         * of the tree.
         */
        if (doadjust)
                fts_padjust(sp, head);

        /*
         * If not changing directories, reset the path 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 path
         * to an empty directory, we wind up here with no other way back.  If
         * can't get back, we're done.
         */
        if (descend && (type == BCHILD || !nitems) &&
            (cur->fts_level == FTS_ROOTLEVEL ?
            FCHDIR(sp, sp->fts_rfd) :
            fts_safe_changedir(sp, cur->fts_parent, -1, ".."))) {
                cur->fts_info = FTS_ERR;
                SET(FTS_STOP);
                return (NULL);
        }

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

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

static unsigned short
fts_stat(FTS *sp, FTSENT *p, int follow)
{
        FTSENT *t;
        dev_t dev;
        __fts_ino_t ino;
        __fts_stat_t *sbp, sb;
        int saved_errno;

        _DIAGASSERT(sp != NULL);
        _DIAGASSERT(p != NULL);

        /* If user needs stat info, stat buffer already allocated. */
        sbp = ISSET(FTS_NOSTAT) ? &sb : p->fts_statp;

#ifdef FTS_WHITEOUT
        /* check for whiteout */
        if (p->fts_flags & FTS_ISW) {
                if (sbp != &sb) {
                        memset(sbp, '\0', sizeof (*sbp));
                        sbp->st_mode = S_IFWHT;
                }
                return (FTS_W);
        }
#endif

        /*
         * 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 (!lstat(p->fts_accpath, sbp)) {
                                errno = 0;
                                return (FTS_SLNONE);
                        }
                        p->fts_errno = saved_errno;
                        goto err;
                }
        } else if (lstat(p->fts_accpath, sbp)) {
                p->fts_errno = errno;
err:            memset(sbp, 0, sizeof(*sbp));
                return (FTS_NS);
        }

        if (S_ISDIR(sbp->st_mode)) {
                /*
                 * Set the device/inode.  Used to find cycles and check for
                 * crossing mount points.  Also remember the link count, used
                 * in fts_build to limit the number of stat calls.  It is
                 * understood that these fields are only referenced if fts_info
                 * is set to FTS_D.
                 */
                dev = p->fts_dev = sbp->st_dev;
                ino = p->fts_ino = sbp->st_ino;
                p->fts_nlink = sbp->st_nlink;

                if (ISDOT(p->fts_name))
                        return (FTS_DOT);

                /*
                 * Cycle detection is done by brute force when the directory
                 * is first encountered.  If the tree gets deep enough or the
                 * number of symbolic links to directories is high enough,
                 * something faster might be worthwhile.
                 */
                for (t = p->fts_parent;
                    t->fts_level >= FTS_ROOTLEVEL; t = t->fts_parent)
                        if (ino == t->fts_ino && dev == t->fts_dev) {
                                p->fts_cycle = t;
                                return (FTS_DC);
                        }
                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 FTSENT *
fts_sort(FTS *sp, FTSENT *head, size_t nitems)
{
        FTSENT **ap, *p;

        _DIAGASSERT(sp != NULL);
        _DIAGASSERT(head != NULL);

        /*
         * 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) {
                errno = reallocarr(&sp->fts_array,
                    nitems + 40, sizeof(*sp->fts_array));
                if (errno)
                        return head;
                sp->fts_nitems = fts_nitems_truncate(nitems + 40);
        }
        for (ap = sp->fts_array, p = head; p; p = p->fts_link)
                *ap++ = p;
        qsort((void *)sp->fts_array, nitems, sizeof(FTSENT *),
                (int (*)(const void *, const void *))sp->fts_compar);
        for (head = *(ap = sp->fts_array); --nitems; ++ap)
                ap[0]->fts_link = ap[1];
        ap[0]->fts_link = NULL;
        return (head);
}

static FTSENT *
fts_alloc(FTS *sp, const char *name, size_t namelen)
{
        FTSENT *p;
#if defined(FTS_ALLOC_ALIGNED)
        size_t len;
#endif

        _DIAGASSERT(sp != NULL);
        _DIAGASSERT(name != NULL);

#if defined(FTS_ALLOC_ALIGNED)
        /*
         * The file name is a variable length array and no stat structure is
         * necessary if the user has set the nostat bit.  Allocate the FTSENT
         * structure, the file name and the stat structure in one chunk, but
         * be careful that the stat structure is reasonably aligned.  Since the
         * fts_name field is declared to be of size 1, the fts_name pointer is
         * namelen + 2 before the first possible address of the stat structure.
         */
        len = sizeof(FTSENT) + namelen;
        if (!ISSET(FTS_NOSTAT))
                len += sizeof(*(p->fts_statp)) + ALIGNBYTES;
        if ((p = malloc(len)) == NULL)
                return (NULL);

        if (!ISSET(FTS_NOSTAT))
                p->fts_statp = (__fts_stat_t *)ALIGN(
                    (unsigned long)(p->fts_name + namelen + 2));
#else
        if ((p = malloc(sizeof(FTSENT) + namelen)) == NULL)
                return (NULL);

        if (!ISSET(FTS_NOSTAT))
                if ((p->fts_statp = malloc(sizeof(*(p->fts_statp)))) == NULL) {
                        free(p);
                        return (NULL);
                }
#endif

        if (ISSET(FTS_NOSTAT))
                p->fts_statp = NULL;

        /* Copy the name plus the trailing NULL. */
        memmove(p->fts_name, name, namelen + 1);

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

static void
fts_free(FTSENT *p)
{
#if !defined(FTS_ALLOC_ALIGNED)
        if (p->fts_statp)
                free(p->fts_statp);
#endif
        free(p);
}

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

        /* XXX: head may be NULL ? */

        /* Free a linked list of structures. */
        while ((p = head) != NULL) {
                head = head->fts_link;
                fts_free(p);
        }
}

static size_t
fts_pow2(size_t x)
{

        x--;
        x |= x>>1;
        x |= x>>2;
        x |= x>>4;
        x |= x>>8;
        x |= x>>16;
#if LONG_BIT > 32
        x |= x>>32;
#endif
#if LONG_BIT > 64
        x |= x>>64;
#endif
        x++;
        return (x);
}

/*
 * Allow essentially unlimited paths; find, rm, ls should all work on any tree.
 * Most systems will allow creation of paths much longer than MAXPATHLEN, even
 * though the kernel won't resolve them.  Round up the new size to a power of 2,
 * so we don't realloc the path 2 bytes at a time.
 */
static int
fts_palloc(FTS *sp, size_t size)
{
        char *new;

        _DIAGASSERT(sp != NULL);

#ifdef __FTS_COMPAT_LENGTH
        /* Protect against fts_pathlen overflow. */
        if (size > USHRT_MAX + 1) {
                errno = ENAMETOOLONG;
                return (1);
        }
#endif
        size = fts_pow2(size);
        new = realloc(sp->fts_path, size);
        if (new == 0)
                return (1);
        sp->fts_path = new;
        sp->fts_pathlen = fts_pathlen_truncate(size);
        return (0);
}

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

        _DIAGASSERT(sp != NULL);

#define ADJUST(p) do {                                                  \
        if ((p)->fts_accpath != (p)->fts_name)                          \
                (p)->fts_accpath =                                      \
                    addr + ((p)->fts_accpath - (p)->fts_path);          \
        (p)->fts_path = addr;                                           \
} while (0)

        addr = sp->fts_path;

        /* 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
fts_maxarglen(char * const *argv)
{
        size_t len, max;

        _DIAGASSERT(argv != NULL);

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

/*
 * Change to dir specified by fd or p->fts_accpath without getting
 * tricked by someone changing the world out from underneath us.
 * Assumes p->fts_dev and p->fts_ino are filled in.
 */
static int
fts_safe_changedir(const FTS *sp, const FTSENT *p, int fd, const char *path)
{
        int oldfd = fd, ret = -1;
        __fts_stat_t sb;

        if (ISSET(FTS_NOCHDIR))
                return 0;

        if (oldfd < 0 && (fd = open(path, O_RDONLY | O_CLOEXEC)) == -1)
                return -1;

        if (fstat(fd, &sb) == -1)
                goto bail;

        if (sb.st_ino != p->fts_ino || sb.st_dev != p->fts_dev) {
                errno = ENOENT;
                goto bail;
        }

        ret = fchdir(fd);

bail:
        if (oldfd < 0) {
                int save_errno = errno;
                (void)close(fd);
                errno = save_errno;
        }
        return ret;
}