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
35 static 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 */
46 static int
create_exclusive(const char * fname)47 create_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
115 bad:
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 */
126 int
dot_lock(const char * fname,int pollinterval)127 dot_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
172 void
dot_unlock(const char * fname)173 dot_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