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 <errno.h>
  23 #include <limits.h>
  24 #include <stdbool.h>
  25 #include <stdio.h>
  26 #include <stdlib.h>
  27 #include <string.h>
  28 #include <sys/file.h>
  29
  30 #include "alloc-util.h"
  31 #include "fd-util.h"
  32 #include "fileio.h"
  33 #include "fs-util.h"
  34 #include "lockfile-util.h"
  35 #include "path-util.h"
  36 #include "util.h"
  37
  38 int make_lock_file(const char *p, int operation, LockFile *ret) {
  39         _cleanup_close_ int fd = -1;
  40         _cleanup_free_ char *t = NULL;
  41         int r;
  42
  43         /*
  44          * We use UNPOSIX locks if they are available. They have nice
  45          * semantics, and are mostly compatible with NFS. However,
  46          * they are only available on new kernels. When we detect we
  47          * are running on an older kernel, then we fall back to good
  48          * old BSD locks. They also have nice semantics, but are
  49          * slightly problematic on NFS, where they are upgraded to
  50          * POSIX locks, even though locally they are orthogonal to
  51          * POSIX locks.
  52          */
  53
  54         t = strdup(p);
  55         if (!t)
  56                 return -ENOMEM;
  57
  58         for (;;) {
  59                 struct flock fl = {
  60                         .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
  61                         .l_whence = SEEK_SET,
  62                 };
  63                 struct stat st;
  64
  65                 fd = open(p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
  66                 if (fd < 0)
  67                         return -errno;
  68
  69                 r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
  70                 if (r < 0) {
  71
  72                         /* If the kernel is too old, use good old BSD locks */
  73                         if (errno == EINVAL)
  74                                 r = flock(fd, operation);
  75
  76                         if (r < 0)
  77                                 return errno == EAGAIN ? -EBUSY : -errno;
  78                 }
  79
  80                 /* If we acquired the lock, let's check if the file
  81                  * still exists in the file system. If not, then the
  82                  * previous exclusive owner removed it and then closed
  83                  * it. In such a case our acquired lock is worthless,
  84                  * hence try again. */
  85
  86                 r = fstat(fd, &st);
  87                 if (r < 0)
  88                         return -errno;
  89                 if (st.st_nlink > 0)
  90                         break;
  91
  92                 fd = safe_close(fd);
  93         }
  94
  95         ret->path = t;
  96         ret->fd = fd;
  97         ret->operation = operation;
  98
  99         fd = -1;
 100         t = NULL;
 101
 102         return r;
 103 }
 104
 105 int make_lock_file_for(const char *p, int operation, LockFile *ret) {
 106         const char *fn;
 107         char *t;
 108
 109         assert(p);
 110         assert(ret);
 111
 112         fn = basename(p);
 113         if (!filename_is_valid(fn))
 114                 return -EINVAL;
 115
 116         t = newa(char, strlen(p) + 2 + 4 + 1);
 117         stpcpy(stpcpy(stpcpy(mempcpy(t, p, fn - p), ".#"), fn), ".lck");
 118
 119         return make_lock_file(t, operation, ret);
 120 }
 121
 122 void release_lock_file(LockFile *f) {
 123         int r;
 124
 125         if (!f)
 126                 return;
 127
 128         if (f->path) {
 129
 130                 /* If we are the exclusive owner we can safely delete
 131                  * the lock file itself. If we are not the exclusive
 132                  * owner, we can try becoming it. */
 133
 134                 if (f->fd >= 0 &&
 135                     (f->operation & ~LOCK_NB) == LOCK_SH) {
 136                         static const struct flock fl = {
 137                                 .l_type = F_WRLCK,
 138                                 .l_whence = SEEK_SET,
 139                         };
 140
 141                         r = fcntl(f->fd, F_OFD_SETLK, &fl);
 142                         if (r < 0 && errno == EINVAL)
 143                                 r = flock(f->fd, LOCK_EX|LOCK_NB);
 144
 145                         if (r >= 0)
 146                                 f->operation = LOCK_EX|LOCK_NB;
 147                 }
 148
 149                 if ((f->operation & ~LOCK_NB) == LOCK_EX)
 150                         unlink_noerrno(f->path);
 151
 152                 f->path = mfree(f->path);
 153         }
 154
 155         f->fd = safe_close(f->fd);
 156         f->operation = 0;
 157 }