86 lines
2.6 KiB
C
86 lines
2.6 KiB
C
/* help detect directory cycles efficiently
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Copyright (C) 2003-2006, 2009-2020 Free Software Foundation, Inc.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Written by Jim Meyering */
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#include <config.h>
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#include "cycle-check.h"
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <stdbool.h>
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#include "assure.h"
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#define CC_MAGIC 9827862
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/* Return true if I is a power of 2, or is zero. */
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static bool
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is_zero_or_power_of_two (uintmax_t i)
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{
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return (i & (i - 1)) == 0;
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}
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void
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cycle_check_init (struct cycle_check_state *state)
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{
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state->chdir_counter = 0;
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state->magic = CC_MAGIC;
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}
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/* In traversing a directory hierarchy, call this function once for each
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descending chdir call, with SB corresponding to the chdir operand.
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If SB corresponds to a directory that has already been seen,
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return true to indicate that there is a directory cycle.
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Note that this is done "lazily", which means that some of
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the directories in the cycle may be processed twice before
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the cycle is detected. */
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bool
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cycle_check (struct cycle_check_state *state, struct stat const *sb)
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{
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assure (state->magic == CC_MAGIC);
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/* If the current directory ever happens to be the same
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as the one we last recorded for the cycle detection,
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then it's obviously part of a cycle. */
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if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
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return true;
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/* If the number of "descending" chdir calls is a power of two,
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record the dev/ino of the current directory. */
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if (is_zero_or_power_of_two (++(state->chdir_counter)))
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{
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/* On all architectures that we know about, if the counter
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overflows then there is a directory cycle here somewhere,
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even if we haven't detected it yet. Typically this happens
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only after the counter is incremented 2**64 times, so it's a
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fairly theoretical point. */
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if (state->chdir_counter == 0)
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return true;
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state->dev_ino.st_dev = sb->st_dev;
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state->dev_ino.st_ino = sb->st_ino;
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}
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return false;
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}
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