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"
27
28#include <stdio.h>
29#ifndef O_SYNC
30#define O_SYNC	0
31#endif
32
33#include "dotlock.h"
34
35static int create_exclusive(const char *);
36/*
37 * Create a unique file. O_EXCL does not really work over NFS so we follow
38 * the following trick: [Inspired by  S.R. van den Berg]
39 *
40 * - make a mostly unique filename and try to create it.
41 * - link the unique filename to our target
42 * - get the link count of the target
43 * - unlink the mostly unique filename
44 * - if the link count was 2, then we are ok; else we've failed.
45 */
46static int
47create_exclusive(const char *fname)
48{
49	char path[MAXPATHLEN], hostname[MAXHOSTNAMELEN + 1];
50	const char *ptr;
51	struct timeval tv;
52	pid_t pid;
53	size_t ntries, cookie;
54	int fd, serrno;
55	struct stat st;
56
57	(void)gettimeofday(&tv, NULL);
58	(void)gethostname(hostname, sizeof(hostname));
59	hostname[sizeof(hostname) - 1] = '\0';
60	pid = getpid();
61
62	cookie = pid ^ tv.tv_usec;
63
64	/*
65	 * We generate a semi-unique filename, from hostname.(pid ^ usec)
66	 */
67	if ((ptr = strrchr(fname, '/')) == NULL)
68		ptr = fname;
69	else
70		ptr++;
71
72	(void)snprintf(path, sizeof(path), "%.*s.%s.%lx",
73	    (int)(ptr - fname), fname, hostname, (u_long)cookie);
74
75	/*
76	 * We try to create the unique filename.
77	 */
78	for (ntries = 0; ntries < 5; ntries++) {
79		fd = open(path, O_WRONLY|O_CREAT|O_TRUNC|O_EXCL|O_SYNC, 0);
80		if (fd != -1) {
81			(void)close(fd);
82			break;
83		}
84		else if (errno == EEXIST)
85			continue;
86		else
87			return -1;
88	}
89
90	/*
91	 * We link the path to the name
92	 */
93	if (link(path, fname) == -1)
94		goto bad;
95
96	/*
97	 * Note that we stat our own exclusively created name, not the
98	 * destination, since the destination can be affected by others.
99	 */
100	if (stat(path, &st) == -1)
101		goto bad;
102
103	(void)unlink(path);
104
105	/*
106	 * If the number of links was two (one for the unique file and one
107	 * for the lock), we've won the race
108	 */
109	if (st.st_nlink != 2) {
110		errno = EEXIST;
111		return -1;
112	}
113	return 0;
114
115bad:
116	serrno = errno;
117	(void)unlink(path);
118	errno = serrno;
119	return -1;
120}
121
122/*
123 * fname -- Pathname to lock
124 * pollinterval -- Interval (miliseconds) to check for lock, -1 return
125 */
126int
127dot_lock(const char *fname, int pollinterval)
128{
129	char path[MAXPATHLEN];
130	sigset_t nset, oset;
131	int retval;
132
133	(void)sigemptyset(&nset);
134	(void)sigaddset(&nset, SIGHUP);
135	(void)sigaddset(&nset, SIGINT);
136	(void)sigaddset(&nset, SIGQUIT);
137	(void)sigaddset(&nset, SIGTERM);
138	(void)sigaddset(&nset, SIGTTIN);
139	(void)sigaddset(&nset, SIGTTOU);
140	(void)sigaddset(&nset, SIGTSTP);
141	(void)sigaddset(&nset, SIGCHLD);
142
143	(void)snprintf(path, sizeof(path), "%s.lock", fname);
144
145	retval = -1;
146	for (;;) {
147		handle_pending_signals();
148		(void)sigprocmask(SIG_BLOCK, &nset, &oset);
149		if (create_exclusive(path) != -1) {
150			(void)sigprocmask(SIG_SETMASK, &oset, NULL);
151			retval = 0;
152			break;
153		}
154		else
155			(void)sigprocmask(SIG_SETMASK, &oset, NULL);
156
157		if (errno != EEXIST)
158			break;
159
160		if (pollinterval) {
161			if (pollinterval == -1) {
162				errno = EEXIST;
163				break;
164			}
165			(void)usleep((unsigned int)pollinterval * 1000);
166		}
167	}
168	handle_pending_signals();
169	return retval;
170}
171
172void
173dot_unlock(const char *fname)
174{
175	char path[MAXPATHLEN];
176
177	(void)snprintf(path, sizeof(path), "%s.lock", fname);
178	(void)unlink(path);
179}
180