1 /* help detect directory cycles efficiently
2
3 Copyright (C) 2003-2006, 2009-2014 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 <sys/types.h>
23 #include <sys/stat.h>
24 #include <stdio.h>
25 #include <assert.h>
26 #include <stdlib.h>
27
28 #include <stdbool.h>
29
30 #include "cycle-check.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
is_zero_or_power_of_two(uintmax_t i)37 is_zero_or_power_of_two (uintmax_t i)
38 {
39 return (i & (i - 1)) == 0;
40 }
41
42 void
cycle_check_init(struct cycle_check_state * state)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
cycle_check(struct cycle_check_state * state,struct stat const * sb)58 cycle_check (struct cycle_check_state *state, struct stat const *sb)
59 {
60 assert (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 }
86