dotlock.c revision 316958 
1/*	$NetBSD: dotlock.c,v 1.11 2009/10/21 01:07:46 snj Exp $	*/
2
3/*
4 * Copyright (c) 1996 Christos Zoulas.  All rights reserved.
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 *    notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 *    notice, this list of conditions and the following disclaimer in the
13 *    documentation and/or other materials provided with the distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
16 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
17 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
18 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
19 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
20 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
21 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
22 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
23 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
24 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
25 */
26#include "sh.h"
27RCSID("$tcsh: dotlock.c,v 3.4 2015/11/03 21:04:13 christos Exp $")
28
29#include <stdio.h>
30#ifndef O_SYNC
31#define O_SYNC	0
32#endif
33
34#include "dotlock.h"
35
36static int create_exclusive(const char *);
37/*
38 * Create a unique file. O_EXCL does not really work over NFS so we follow
39 * the following trick: [Inspired by  S.R. van den Berg]
40 *
41 * - make a mostly unique filename and try to create it.
42 * - link the unique filename to our target
43 * - get the link count of the target
44 * - unlink the mostly unique filename
45 * - if the link count was 2, then we are ok; else we've failed.
46 */
47static int
48create_exclusive(const char *fname)
49{
50	char path[MAXPATHLEN], hostname[MAXHOSTNAMELEN + 1];
51	const char *ptr;
52	struct timeval tv;
53	pid_t pid;
54	size_t ntries, cookie;
55	int fd, serrno;
56	struct stat st;
57
58	(void)gettimeofday(&tv, NULL);
59	(void)gethostname(hostname, sizeof(hostname));
60	hostname[sizeof(hostname) - 1] = '\0';
61	pid = getpid();
62
63	cookie = pid ^ tv.tv_usec;
64
65	/*
66	 * We generate a semi-unique filename, from hostname.(pid ^ usec)
67	 */
68	if ((ptr = strrchr(fname, '/')) == NULL)
69		ptr = fname;
70	else
71		ptr++;
72
73	(void)snprintf(path, sizeof(path), "%.*s.%s.%lx",
74	    (int)(ptr - fname), fname, hostname, (u_long)cookie);
75
76	/*
77	 * We try to create the unique filename.
78	 */
79	for (ntries = 0; ntries < 5; ntries++) {
80		fd = open(path, O_WRONLY|O_CREAT|O_TRUNC|O_EXCL|O_SYNC, 0);
81		if (fd != -1) {
82			(void)close(fd);
83			break;
84		}
85		else if (errno == EEXIST)
86			continue;
87		else
88			return -1;
89	}
90
91	/*
92	 * We link the path to the name
93	 */
94	if (link(path, fname) == -1)
95		goto bad;
96
97	/*
98	 * Note that we stat our own exclusively created name, not the
99	 * destination, since the destination can be affected by others.
100	 */
101	if (stat(path, &st) == -1)
102		goto bad;
103
104	(void)unlink(path);
105
106	/*
107	 * If the number of links was two (one for the unique file and one
108	 * for the lock), we've won the race
109	 */
110	if (st.st_nlink != 2) {
111		errno = EEXIST;
112		return -1;
113	}
114	return 0;
115
116bad:
117	serrno = errno;
118	(void)unlink(path);
119	errno = serrno;
120	return -1;
121}
122
123/*
124 * fname -- Pathname to lock
125 * pollinterval -- Interval (miliseconds) to check for lock, -1 return
126 */
127int
128dot_lock(const char *fname, int pollinterval)
129{
130	char path[MAXPATHLEN];
131	sigset_t nset, oset;
132	int retval;
133
134	(void)sigemptyset(&nset);
135	(void)sigaddset(&nset, SIGHUP);
136	(void)sigaddset(&nset, SIGINT);
137	(void)sigaddset(&nset, SIGQUIT);
138	(void)sigaddset(&nset, SIGTERM);
139	(void)sigaddset(&nset, SIGTTIN);
140	(void)sigaddset(&nset, SIGTTOU);
141	(void)sigaddset(&nset, SIGTSTP);
142	(void)sigaddset(&nset, SIGCHLD);
143
144	(void)snprintf(path, sizeof(path), "%s.lock", fname);
145
146	retval = -1;
147	for (;;) {
148		handle_pending_signals();
149		(void)sigprocmask(SIG_BLOCK, &nset, &oset);
150		if (create_exclusive(path) != -1) {
151			(void)sigprocmask(SIG_SETMASK, &oset, NULL);
152			retval = 0;
153			break;
154		}
155		else
156			(void)sigprocmask(SIG_SETMASK, &oset, NULL);
157
158		if (errno != EEXIST)
159			break;
160
161		if (pollinterval) {
162			if (pollinterval == -1) {
163				errno = EEXIST;
164				break;
165			}
166			(void)usleep((unsigned int)pollinterval * 1000);
167		}
168	}
169	handle_pending_signals();
170	return retval;
171}
172
173void
174dot_unlock(const char *fname)
175{
176	char path[MAXPATHLEN];
177
178	(void)snprintf(path, sizeof(path), "%s.lock", fname);
179	(void)unlink(path);
180}
181