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annotate lib/cycle-check.c @ 17476:6057744acd2c default tip master

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author Karl Berry <karl@freefriends.org>
date Fri, 16 Aug 2013 06:32:22 -0700
parents e542fd46ad6f
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1 /* help detect directory cycles efficiently
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2
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3 Copyright (C) 2003-2006, 2009-2013 Free Software Foundation, Inc.
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4
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5 This program is free software: you can redistribute it and/or modify
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6 it under the terms of the GNU General Public License as published by
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7 the Free Software Foundation; either version 3 of the License, or
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8 (at your option) any later version.
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9
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10 This program is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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13 GNU General Public License for more details.
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14
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15 You should have received a copy of the GNU General Public License
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16 along with this program. If not, see <http://www.gnu.org/licenses/>. */
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17
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18 /* Written by Jim Meyering */
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19
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20 #include <config.h>
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21
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22 #include <sys/types.h>
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23 #include <sys/stat.h>
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24 #include <stdio.h>
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25 #include <assert.h>
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26 #include <stdlib.h>
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27
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28 #include <stdbool.h>
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29
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30 #include "cycle-check.h"
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31
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32 #define CC_MAGIC 9827862
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33
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34 /* Return true if I is a power of 2, or is zero. */
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35
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36 static bool
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37 is_zero_or_power_of_two (uintmax_t i)
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38 {
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39 return (i & (i - 1)) == 0;
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40 }
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41
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42 void
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43 cycle_check_init (struct cycle_check_state *state)
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44 {
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45 state->chdir_counter = 0;
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46 state->magic = CC_MAGIC;
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47 }
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48
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49 /* In traversing a directory hierarchy, call this function once for each
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50 descending chdir call, with SB corresponding to the chdir operand.
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51 If SB corresponds to a directory that has already been seen,
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52 return true to indicate that there is a directory cycle.
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53 Note that this is done "lazily", which means that some of
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54 the directories in the cycle may be processed twice before
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55 the cycle is detected. */
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56
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57 bool
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58 cycle_check (struct cycle_check_state *state, struct stat const *sb)
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59 {
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60 assert (state->magic == CC_MAGIC);
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61
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62 /* If the current directory ever happens to be the same
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63 as the one we last recorded for the cycle detection,
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64 then it's obviously part of a cycle. */
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65 if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
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66 return true;
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67
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68 /* If the number of "descending" chdir calls is a power of two,
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69 record the dev/ino of the current directory. */
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70 if (is_zero_or_power_of_two (++(state->chdir_counter)))
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71 {
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72 /* On all architectures that we know about, if the counter
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73 overflows then there is a directory cycle here somewhere,
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74 even if we haven't detected it yet. Typically this happens
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75 only after the counter is incremented 2**64 times, so it's a
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76 fairly theoretical point. */
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77 if (state->chdir_counter == 0)
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78 return true;
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79
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80 state->dev_ino.st_dev = sb->st_dev;
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81 state->dev_ino.st_ino = sb->st_ino;
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82 }
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84 return false;
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85 }