/*      $NetBSD: dotlock.c,v 1.14 2021/02/17 21:09:39 christos Exp $    */

/*
 * Copyright (c) 1996 Christos Zoulas.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/cdefs.h>
#ifndef lint
__RCSID("$NetBSD: dotlock.c,v 1.14 2021/02/17 21:09:39 christos Exp $");
#endif

#include "rcv.h"
#include "extern.h"
#include "sig.h"

#ifndef O_SYNC
#define O_SYNC  0
#endif

static int create_exclusive(const char *);
/*
 * Create a unique file. O_EXCL does not really work over NFS so we follow
 * the following trick: [Inspired by  S.R. van den Berg]
 *
 * - make a mostly unique filename and try to create it.
 * - link the unique filename to our target
 * - get the link count of the target
 * - unlink the mostly unique filename
 * - if the link count was 2, then we are ok; else we've failed.
 */
static int
create_exclusive(const char *fname)
{
        char path[MAXPATHLEN], hostname[MAXHOSTNAMELEN + 1];
        const char *ptr;
        struct timeval tv;
        pid_t pid;
        size_t ntries, cookie;
        int fd, serrno;
        struct stat st;

        (void)gettimeofday(&tv, NULL);
        (void)gethostname(hostname, sizeof(hostname));
        hostname[sizeof(hostname) - 1] = '\0';
        pid = getpid();

        cookie = pid ^ tv.tv_usec;

        /*
         * We generate a semi-unique filename, from hostname.(pid ^ usec)
         */
        if ((ptr = strrchr(fname, '/')) == NULL)
                ptr = fname;
        else
                ptr++;

        (void)snprintf(path, sizeof(path), "%.*s.%s.%lx",
            (int)(ptr - fname), fname, hostname, (u_long)cookie);

        /*
         * We try to create the unique filename.
         */
        for (ntries = 0; ; ntries++) {
                fd = open(path,
                    O_WRONLY|O_CREAT|O_TRUNC|O_EXCL|O_SYNC|O_CLOEXEC, 0);
                if (fd != -1) {
                        (void)close(fd);
                        break;
                }
                else if (errno == EEXIST && ntries < 5)
                        continue;
                else
                        return -1;
        }

        /*
         * We link the path to the name
         */
        if (link(path, fname) == -1)
                goto bad;

        /*
         * Note that we stat our own exclusively created name, not the
         * destination, since the destination can be affected by others.
         */
        if (stat(path, &st) == -1)
                goto bad;

        (void)unlink(path);

        /*
         * If the number of links was two (one for the unique file and one
         * for the lock), we've won the race
         */
        if (st.st_nlink != 2) {
                errno = EEXIST;
                return -1;
        }
        return 0;

bad:
        serrno = errno;
        (void)unlink(path);
        errno = serrno;
        return -1;
}

/*
 * fname -- Pathname to lock
 * pollinterval -- Interval to check for lock, -1 return
 * fp -- File to print message
 * msg -- Message to print
 */
PUBLIC int
dot_lock(const char *fname, int pollinterval, FILE *fp, const char *msg)
{
        char path[MAXPATHLEN];
        sigset_t nset, oset;
        int retval;

        (void)sigemptyset(&nset);
        (void)sigaddset(&nset, SIGHUP);
        (void)sigaddset(&nset, SIGINT);
        (void)sigaddset(&nset, SIGQUIT);
        (void)sigaddset(&nset, SIGTERM);
        (void)sigaddset(&nset, SIGTTIN);
        (void)sigaddset(&nset, SIGTTOU);
        (void)sigaddset(&nset, SIGTSTP);
        (void)sigaddset(&nset, SIGCHLD);

        (void)snprintf(path, sizeof(path), "%s.lock", fname);

        retval = -1;
        for (;;) {
                sig_check();
                (void)sigprocmask(SIG_BLOCK, &nset, &oset);
                if (create_exclusive(path) != -1) {
                        (void)sigprocmask(SIG_SETMASK, &oset, NULL);
                        retval = 0;
                        break;
                }
                else
                        (void)sigprocmask(SIG_SETMASK, &oset, NULL);

                if (errno != EEXIST)
                        break;

                if (fp && msg)
                    (void)fputs(msg, fp);

                if (pollinterval) {
                        if (pollinterval == -1) {
                                errno = EEXIST;
                                break;
                        }
                        (void)sleep((unsigned int)pollinterval);
                }
        }
        sig_check();
        return retval;
}

PUBLIC void
dot_unlock(const char *fname)
{
        char path[MAXPATHLEN];

        (void)snprintf(path, sizeof(path), "%s.lock", fname);
        (void)unlink(path);
}