lib/cycle-check.c

   1 /* help detect directory cycles efficiently
   2
   3    Copyright (C) 2003-2006, 2009-2017 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Jim Meyering */
  19
  20 #include <config.h>
  21
  22 #include "cycle-check.h"
  23
  24 #include <sys/types.h>
  25 #include <sys/stat.h>
  26 #include <stdio.h>
  27 #include <stdlib.h>
  28 #include <stdbool.h>
  29
  30 #include "assure.h"
  31
  32 #define CC_MAGIC 9827862
  33
  34 /* Return true if I is a power of 2, or is zero.  */
  35
  36 static bool
  37 is_zero_or_power_of_two (uintmax_t i)
  38 {
  39   return (i & (i - 1)) == 0;
  40 }
  41
  42 void
  43 cycle_check_init (struct cycle_check_state *state)
  44 {
  45   state->chdir_counter = 0;
  46   state->magic = CC_MAGIC;
  47 }
  48
  49 /* In traversing a directory hierarchy, call this function once for each
  50    descending chdir call, with SB corresponding to the chdir operand.
  51    If SB corresponds to a directory that has already been seen,
  52    return true to indicate that there is a directory cycle.
  53    Note that this is done "lazily", which means that some of
  54    the directories in the cycle may be processed twice before
  55    the cycle is detected.  */
  56
  57 bool
  58 cycle_check (struct cycle_check_state *state, struct stat const *sb)
  59 {
  60   assure (state->magic == CC_MAGIC);
  61
  62   /* If the current directory ever happens to be the same
  63      as the one we last recorded for the cycle detection,
  64      then it's obviously part of a cycle.  */
  65   if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
  66     return true;
  67
  68   /* If the number of "descending" chdir calls is a power of two,
  69      record the dev/ino of the current directory.  */
  70   if (is_zero_or_power_of_two (++(state->chdir_counter)))
  71     {
  72       /* On all architectures that we know about, if the counter
  73          overflows then there is a directory cycle here somewhere,
  74          even if we haven't detected it yet.  Typically this happens
  75          only after the counter is incremented 2**64 times, so it's a
  76          fairly theoretical point.  */
  77       if (state->chdir_counter == 0)
  78         return true;
  79
  80       state->dev_ino.st_dev = sb->st_dev;
  81       state->dev_ino.st_ino = sb->st_ino;
  82     }
  83
  84   return false;
  85 }