src/basic/lockfile-util.c

   1 /*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
   2
   3 /***
   4   This file is part of systemd.
   5
   6   Copyright 2010 Lennart Poettering
   7
   8   systemd is free software; you can redistribute it and/or modify it
   9   under the terms of the GNU Lesser General Public License as published by
  10   the Free Software Foundation; either version 2.1 of the License, or
  11   (at your option) any later version.
  12
  13   systemd is distributed in the hope that it will be useful, but
  14   WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  16   Lesser General Public License for more details.
  17
  18   You should have received a copy of the GNU Lesser General Public License
  19   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  20 ***/
  21
  22 #include <stdlib.h>
  23 #include <stdbool.h>
  24 #include <errno.h>
  25 #include <string.h>
  26 #include <stdio.h>
  27 #include <limits.h>
  28 #include <sys/file.h>
  29
  30 #include "util.h"
  31 #include "lockfile-util.h"
  32 #include "fileio.h"
  33
  34 int make_lock_file(const char *p, int operation, LockFile *ret) {
  35         _cleanup_close_ int fd = -1;
  36         _cleanup_free_ char *t = NULL;
  37         int r;
  38
  39         /*
  40          * We use UNPOSIX locks if they are available. They have nice
  41          * semantics, and are mostly compatible with NFS. However,
  42          * they are only available on new kernels. When we detect we
  43          * are running on an older kernel, then we fall back to good
  44          * old BSD locks. They also have nice semantics, but are
  45          * slightly problematic on NFS, where they are upgraded to
  46          * POSIX locks, even though locally they are orthogonal to
  47          * POSIX locks.
  48          */
  49
  50         t = strdup(p);
  51         if (!t)
  52                 return -ENOMEM;
  53
  54         for (;;) {
  55                 struct flock fl = {
  56                         .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
  57                         .l_whence = SEEK_SET,
  58                 };
  59                 struct stat st;
  60
  61                 fd = open(p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
  62                 if (fd < 0)
  63                         return -errno;
  64
  65                 r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
  66                 if (r < 0) {
  67
  68                         /* If the kernel is too old, use good old BSD locks */
  69                         if (errno == EINVAL)
  70                                 r = flock(fd, operation);
  71
  72                         if (r < 0)
  73                                 return errno == EAGAIN ? -EBUSY : -errno;
  74                 }
  75
  76                 /* If we acquired the lock, let's check if the file
  77                  * still exists in the file system. If not, then the
  78                  * previous exclusive owner removed it and then closed
  79                  * it. In such a case our acquired lock is worthless,
  80                  * hence try again. */
  81
  82                 r = fstat(fd, &st);
  83                 if (r < 0)
  84                         return -errno;
  85                 if (st.st_nlink > 0)
  86                         break;
  87
  88                 fd = safe_close(fd);
  89         }
  90
  91         ret->path = t;
  92         ret->fd = fd;
  93         ret->operation = operation;
  94
  95         fd = -1;
  96         t = NULL;
  97
  98         return r;
  99 }
 100
 101 int make_lock_file_for(const char *p, int operation, LockFile *ret) {
 102         const char *fn;
 103         char *t;
 104
 105         assert(p);
 106         assert(ret);
 107
 108         fn = basename(p);
 109         if (!filename_is_valid(fn))
 110                 return -EINVAL;
 111
 112         t = newa(char, strlen(p) + 2 + 4 + 1);
 113         stpcpy(stpcpy(stpcpy(mempcpy(t, p, fn - p), ".#"), fn), ".lck");
 114
 115         return make_lock_file(t, operation, ret);
 116 }
 117
 118 void release_lock_file(LockFile *f) {
 119         int r;
 120
 121         if (!f)
 122                 return;
 123
 124         if (f->path) {
 125
 126                 /* If we are the exclusive owner we can safely delete
 127                  * the lock file itself. If we are not the exclusive
 128                  * owner, we can try becoming it. */
 129
 130                 if (f->fd >= 0 &&
 131                     (f->operation & ~LOCK_NB) == LOCK_SH) {
 132                         static const struct flock fl = {
 133                                 .l_type = F_WRLCK,
 134                                 .l_whence = SEEK_SET,
 135                         };
 136
 137                         r = fcntl(f->fd, F_OFD_SETLK, &fl);
 138                         if (r < 0 && errno == EINVAL)
 139                                 r = flock(f->fd, LOCK_EX|LOCK_NB);
 140
 141                         if (r >= 0)
 142                                 f->operation = LOCK_EX|LOCK_NB;
 143                 }
 144
 145                 if ((f->operation & ~LOCK_NB) == LOCK_EX)
 146                         unlink_noerrno(f->path);
 147
 148                 f->path = mfree(f->path);
 149         }
 150
 151         f->fd = safe_close(f->fd);
 152         f->operation = 0;
 153 }