1 /* 2 * CDDL HEADER START 3 * 4 * The contents of this file are subject to the terms of the 5 * Common Development and Distribution License, Version 1.0 only 6 * (the "License"). You may not use this file except in compliance 7 * with the License. 8 * 9 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE 10 * or http://www.opensolaris.org/os/licensing. 11 * See the License for the specific language governing permissions 12 * and limitations under the License. 13 * 14 * When distributing Covered Code, include this CDDL HEADER in each 15 * file and include the License file at usr/src/OPENSOLARIS.LICENSE. 16 * If applicable, add the following below this CDDL HEADER, with the 17 * fields enclosed by brackets "[]" replaced with your own identifying 18 * information: Portions Copyright [yyyy] [name of copyright owner] 19 * 20 * CDDL HEADER END 21 */ 22 /* 23 * Copyright 2004 Sun Microsystems, Inc. All rights reserved. 24 * Use is subject to license terms. 25 */ 26 27 #include <stdio.h> 28 #include <stdlib.h> 29 #include <unistd.h> 30 #include <string.h> 31 #include <fcntl.h> 32 #include <sys/types.h> 33 #include <sys/param.h> 34 #include <sys/stat.h> 35 #include <errno.h> 36 37 38 /* 39 * Format a message telling why the lock could not be created. 40 */ 41 /* VARARGS5 */ 42 static void 43 file_lock_error(char *msg, char *file, int err, const char *str, 44 char *arg1, char *arg2, size_t mlen) 45 { 46 size_t len; 47 char *errstr; 48 49 (void) snprintf(msg, mlen, "Could not lock file `%s'; ", file); 50 len = strlen(msg); 51 (void) snprintf(&msg[len], (mlen - len), str, arg1, arg2); 52 (void) strcat(msg, " failed - "); 53 if ((errstr = strerror(err)) != NULL) { 54 (void) strlcat(msg, errstr, mlen); 55 } else { 56 len = strlen(msg); 57 (void) sprintf(&msg[len], "errno %d", err); 58 } 59 } 60 61 /* 62 * This code stolen from the NSE library and changed to not depend 63 * upon any NSE routines or header files. 64 * 65 * Simple file locking. 66 * Create a symlink to a file. The "test and set" will be 67 * atomic as creating the symlink provides both functions. 68 * 69 * The timeout value specifies how long to wait for stale locks 70 * to disappear. If the lock is more than 'timeout' seconds old 71 * then it is ok to blow it away. This part has a small window 72 * of vunerability as the operations of testing the time, 73 * removing the lock and creating a new one are not atomic. 74 * It would be possible for two processes to both decide to blow 75 * away the lock and then have process A remove the lock and establish 76 * its own, and then then have process B remove the lock which accidentily 77 * removes A's lock rather than the stale one. 78 * 79 * A further complication is with the NFS. If the file in question is 80 * being served by an NFS server, then its time is set by that server. 81 * We can not use the time on the client machine to check for a stale 82 * lock. Therefore, a temp file on the server is created to get 83 * the servers current time. 84 * 85 * Returns an error message. NULL return means the lock was obtained. 86 * 87 */ 88 char * 89 file_lock(char *name, char *lockname, int timeout) 90 { 91 int r; 92 int fd; 93 struct stat statb; 94 struct stat fs_statb; 95 char tmpname[MAXPATHLEN]; 96 static char msg[MAXPATHLEN]; 97 98 if (timeout <= 0) { 99 timeout = 15; 100 } 101 for (;;) { 102 r = symlink(name, lockname); 103 if (r == 0) { 104 return (NULL); 105 } 106 if (errno != EEXIST) { 107 file_lock_error(msg, name, errno, 108 (const char *)"symlink(%s, %s)", name, lockname, 109 sizeof (msg)); 110 return (msg); 111 } 112 for (;;) { 113 (void) sleep(1); 114 r = lstat(lockname, &statb); 115 if (r == -1) { 116 /* 117 * The lock must have just gone away - try 118 * again. 119 */ 120 break; 121 } 122 123 /* 124 * With the NFS the time given a file is the time on 125 * the file server. This time may vary from the 126 * client's time. Therefore, we create a tmpfile in 127 * the same directory to establish the time on the 128 * server and use this time to see if the lock has 129 * expired. 130 */ 131 (void) snprintf(tmpname, MAXPATHLEN, "%s.XXXXXX", 132 lockname); 133 (void) mktemp(tmpname); 134 fd = creat(tmpname, 0666); 135 if (fd != -1) { 136 (void) close(fd); 137 } else { 138 file_lock_error(msg, name, errno, 139 (const char *)"creat(%s)", tmpname, 0, 140 sizeof (msg)); 141 return (msg); 142 } 143 if (stat(tmpname, &fs_statb) == -1) { 144 file_lock_error(msg, name, errno, 145 (const char *)"stat(%s)", tmpname, 0, 146 sizeof (msg)); 147 return (msg); 148 } 149 (void) unlink(tmpname); 150 if (statb.st_mtime + timeout < fs_statb.st_mtime) { 151 /* 152 * The lock has expired - blow it away. 153 */ 154 (void) unlink(lockname); 155 break; 156 } 157 } 158 } 159 /* NOTREACHED */ 160 } 161