/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License, Version 1.0 only
 * (the "License").  You may not use this file except in compliance
 * with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2004 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/param.h>
#include <sys/stat.h>
#include <errno.h>


/*
 * Format a message telling why the lock could not be created.
 */
/* VARARGS5 */
static void
file_lock_error(char *msg, char *file, int err, const char *str,
    char *arg1, char *arg2, size_t mlen)
{
        size_t  len;
        char    *errstr;

        (void) snprintf(msg, mlen, "Could not lock file `%s'; ", file);
        len = strlen(msg);
        (void) snprintf(&msg[len], (mlen - len), str, arg1, arg2);
        (void) strcat(msg, " failed - ");
        if ((errstr = strerror(err)) != NULL) {
                (void) strlcat(msg, errstr, mlen);
        } else {
                len = strlen(msg);
                (void) sprintf(&msg[len], "errno %d", err);
        }
}

/*
 * This code stolen from the NSE library and changed to not depend
 * upon any NSE routines or header files.
 *
 * Simple file locking.
 * Create a symlink to a file.  The "test and set" will be
 * atomic as creating the symlink provides both functions.
 *
 * The timeout value specifies how long to wait for stale locks
 * to disappear.  If the lock is more than 'timeout' seconds old
 * then it is ok to blow it away.  This part has a small window
 * of vunerability as the operations of testing the time,
 * removing the lock and creating a new one are not atomic.
 * It would be possible for two processes to both decide to blow
 * away the lock and then have process A remove the lock and establish
 * its own, and then then have process B remove the lock which accidentily
 * removes A's lock rather than the stale one.
 *
 * A further complication is with the NFS.  If the file in question is
 * being served by an NFS server, then its time is set by that server.
 * We can not use the time on the client machine to check for a stale
 * lock.  Therefore, a temp file on the server is created to get
 * the servers current time.
 *
 * Returns an error message.  NULL return means the lock was obtained.
 *
 */
char *
file_lock(char *name, char *lockname, int timeout)
{
        int             r;
        int             fd;
        struct  stat    statb;
        struct  stat    fs_statb;
        char            tmpname[MAXPATHLEN];
        static  char    msg[MAXPATHLEN];

        if (timeout <= 0) {
                timeout = 15;
        }
        for (;;) {
                r = symlink(name, lockname);
                if (r == 0) {
                        return (NULL);
                }
                if (errno != EEXIST) {
                        file_lock_error(msg, name, errno,
                            (const char *)"symlink(%s, %s)", name, lockname,
                            sizeof (msg));
                        return (msg);
                }
                for (;;) {
                        (void) sleep(1);
                        r = lstat(lockname, &statb);
                        if (r == -1) {
                                /*
                                 * The lock must have just gone away - try
                                 * again.
                                 */
                                break;
                        }

                        /*
                         * With the NFS the time given a file is the time on
                         * the file server.  This time may vary from the
                         * client's time.  Therefore, we create a tmpfile in
                         * the same directory to establish the time on the
                         * server and use this time to see if the lock has
                         * expired.
                         */
                        (void) snprintf(tmpname, MAXPATHLEN, "%s.XXXXXX",
                            lockname);
                        (void) mktemp(tmpname);
                        fd = creat(tmpname, 0666);
                        if (fd != -1) {
                                (void) close(fd);
                        } else {
                                file_lock_error(msg, name, errno,
                                    (const char *)"creat(%s)", tmpname, 0,
                                    sizeof (msg));
                                return (msg);
                        }
                        if (stat(tmpname, &fs_statb) == -1) {
                                file_lock_error(msg, name, errno,
                                    (const char *)"stat(%s)", tmpname, 0,
                                    sizeof (msg));
                                return (msg);
                        }
                        (void) unlink(tmpname);
                        if (statb.st_mtime + timeout < fs_statb.st_mtime) {
                                /*
                                 * The lock has expired - blow it away.
                                 */
                                (void) unlink(lockname);
                                break;
                        }
                }
        }
        /* NOTREACHED */
}