1 /* $NetBSD: dotlock.c,v 1.14 2021/02/17 21:09:39 christos 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
27 #include <sys/cdefs.h>
28 #ifndef lint
29 __RCSID("$NetBSD: dotlock.c,v 1.14 2021/02/17 21:09:39 christos Exp $");
30 #endif
31
32 #include "rcv.h"
33 #include "extern.h"
34 #include "sig.h"
35
36 #ifndef O_SYNC
37 #define O_SYNC 0
38 #endif
39
40 static int create_exclusive(const char *);
41 /*
42 * Create a unique file. O_EXCL does not really work over NFS so we follow
43 * the following trick: [Inspired by S.R. van den Berg]
44 *
45 * - make a mostly unique filename and try to create it.
46 * - link the unique filename to our target
47 * - get the link count of the target
48 * - unlink the mostly unique filename
49 * - if the link count was 2, then we are ok; else we've failed.
50 */
51 static int
create_exclusive(const char * fname)52 create_exclusive(const char *fname)
53 {
54 char path[MAXPATHLEN], hostname[MAXHOSTNAMELEN + 1];
55 const char *ptr;
56 struct timeval tv;
57 pid_t pid;
58 size_t ntries, cookie;
59 int fd, serrno;
60 struct stat st;
61
62 (void)gettimeofday(&tv, NULL);
63 (void)gethostname(hostname, sizeof(hostname));
64 hostname[sizeof(hostname) - 1] = '\0';
65 pid = getpid();
66
67 cookie = pid ^ tv.tv_usec;
68
69 /*
70 * We generate a semi-unique filename, from hostname.(pid ^ usec)
71 */
72 if ((ptr = strrchr(fname, '/')) == NULL)
73 ptr = fname;
74 else
75 ptr++;
76
77 (void)snprintf(path, sizeof(path), "%.*s.%s.%lx",
78 (int)(ptr - fname), fname, hostname, (u_long)cookie);
79
80 /*
81 * We try to create the unique filename.
82 */
83 for (ntries = 0; ; ntries++) {
84 fd = open(path,
85 O_WRONLY|O_CREAT|O_TRUNC|O_EXCL|O_SYNC|O_CLOEXEC, 0);
86 if (fd != -1) {
87 (void)close(fd);
88 break;
89 }
90 else if (errno == EEXIST && ntries < 5)
91 continue;
92 else
93 return -1;
94 }
95
96 /*
97 * We link the path to the name
98 */
99 if (link(path, fname) == -1)
100 goto bad;
101
102 /*
103 * Note that we stat our own exclusively created name, not the
104 * destination, since the destination can be affected by others.
105 */
106 if (stat(path, &st) == -1)
107 goto bad;
108
109 (void)unlink(path);
110
111 /*
112 * If the number of links was two (one for the unique file and one
113 * for the lock), we've won the race
114 */
115 if (st.st_nlink != 2) {
116 errno = EEXIST;
117 return -1;
118 }
119 return 0;
120
121 bad:
122 serrno = errno;
123 (void)unlink(path);
124 errno = serrno;
125 return -1;
126 }
127
128 /*
129 * fname -- Pathname to lock
130 * pollinterval -- Interval to check for lock, -1 return
131 * fp -- File to print message
132 * msg -- Message to print
133 */
134 PUBLIC int
dot_lock(const char * fname,int pollinterval,FILE * fp,const char * msg)135 dot_lock(const char *fname, int pollinterval, FILE *fp, const char *msg)
136 {
137 char path[MAXPATHLEN];
138 sigset_t nset, oset;
139 int retval;
140
141 (void)sigemptyset(&nset);
142 (void)sigaddset(&nset, SIGHUP);
143 (void)sigaddset(&nset, SIGINT);
144 (void)sigaddset(&nset, SIGQUIT);
145 (void)sigaddset(&nset, SIGTERM);
146 (void)sigaddset(&nset, SIGTTIN);
147 (void)sigaddset(&nset, SIGTTOU);
148 (void)sigaddset(&nset, SIGTSTP);
149 (void)sigaddset(&nset, SIGCHLD);
150
151 (void)snprintf(path, sizeof(path), "%s.lock", fname);
152
153 retval = -1;
154 for (;;) {
155 sig_check();
156 (void)sigprocmask(SIG_BLOCK, &nset, &oset);
157 if (create_exclusive(path) != -1) {
158 (void)sigprocmask(SIG_SETMASK, &oset, NULL);
159 retval = 0;
160 break;
161 }
162 else
163 (void)sigprocmask(SIG_SETMASK, &oset, NULL);
164
165 if (errno != EEXIST)
166 break;
167
168 if (fp && msg)
169 (void)fputs(msg, fp);
170
171 if (pollinterval) {
172 if (pollinterval == -1) {
173 errno = EEXIST;
174 break;
175 }
176 (void)sleep((unsigned int)pollinterval);
177 }
178 }
179 sig_check();
180 return retval;
181 }
182
183 PUBLIC void
dot_unlock(const char * fname)184 dot_unlock(const char *fname)
185 {
186 char path[MAXPATHLEN];
187
188 (void)snprintf(path, sizeof(path), "%s.lock", fname);
189 (void)unlink(path);
190 }
191