src/basic/lockfile-util.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <errno.h>
   4 #include <fcntl.h>
   5 #include <stdio.h>
   6 #include <string.h>
   7 #include <sys/file.h>
   8 #include <sys/stat.h>
   9
  10 #include "alloc-util.h"
  11 #include "fd-util.h"
  12 #include "fs-util.h"
  13 #include "lockfile-util.h"
  14 #include "macro.h"
  15 #include "path-util.h"
  16
  17 int make_lock_file(const char *p, int operation, LockFile *ret) {
  18         _cleanup_close_ int fd = -1;
  19         _cleanup_free_ char *t = NULL;
  20         int r;
  21
  22         /*
  23          * We use UNPOSIX locks if they are available. They have nice
  24          * semantics, and are mostly compatible with NFS. However,
  25          * they are only available on new kernels. When we detect we
  26          * are running on an older kernel, then we fall back to good
  27          * old BSD locks. They also have nice semantics, but are
  28          * slightly problematic on NFS, where they are upgraded to
  29          * POSIX locks, even though locally they are orthogonal to
  30          * POSIX locks.
  31          */
  32
  33         t = strdup(p);
  34         if (!t)
  35                 return -ENOMEM;
  36
  37         for (;;) {
  38                 struct flock fl = {
  39                         .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
  40                         .l_whence = SEEK_SET,
  41                 };
  42                 struct stat st;
  43
  44                 fd = open(p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
  45                 if (fd < 0)
  46                         return -errno;
  47
  48                 r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
  49                 if (r < 0) {
  50
  51                         /* If the kernel is too old, use good old BSD locks */
  52                         if (errno == EINVAL)
  53                                 r = flock(fd, operation);
  54
  55                         if (r < 0)
  56                                 return errno == EAGAIN ? -EBUSY : -errno;
  57                 }
  58
  59                 /* If we acquired the lock, let's check if the file
  60                  * still exists in the file system. If not, then the
  61                  * previous exclusive owner removed it and then closed
  62                  * it. In such a case our acquired lock is worthless,
  63                  * hence try again. */
  64
  65                 r = fstat(fd, &st);
  66                 if (r < 0)
  67                         return -errno;
  68                 if (st.st_nlink > 0)
  69                         break;
  70
  71                 fd = safe_close(fd);
  72         }
  73
  74         ret->path = t;
  75         ret->fd = fd;
  76         ret->operation = operation;
  77
  78         fd = -1;
  79         t = NULL;
  80
  81         return r;
  82 }
  83
  84 int make_lock_file_for(const char *p, int operation, LockFile *ret) {
  85         const char *fn;
  86         char *t;
  87
  88         assert(p);
  89         assert(ret);
  90
  91         fn = basename(p);
  92         if (!filename_is_valid(fn))
  93                 return -EINVAL;
  94
  95         t = newa(char, strlen(p) + 2 + 4 + 1);
  96         stpcpy(stpcpy(stpcpy(mempcpy(t, p, fn - p), ".#"), fn), ".lck");
  97
  98         return make_lock_file(t, operation, ret);
  99 }
 100
 101 void release_lock_file(LockFile *f) {
 102         int r;
 103
 104         if (!f)
 105                 return;
 106
 107         if (f->path) {
 108
 109                 /* If we are the exclusive owner we can safely delete
 110                  * the lock file itself. If we are not the exclusive
 111                  * owner, we can try becoming it. */
 112
 113                 if (f->fd >= 0 &&
 114                     (f->operation & ~LOCK_NB) == LOCK_SH) {
 115                         static const struct flock fl = {
 116                                 .l_type = F_WRLCK,
 117                                 .l_whence = SEEK_SET,
 118                         };
 119
 120                         r = fcntl(f->fd, F_OFD_SETLK, &fl);
 121                         if (r < 0 && errno == EINVAL)
 122                                 r = flock(f->fd, LOCK_EX|LOCK_NB);
 123
 124                         if (r >= 0)
 125                                 f->operation = LOCK_EX|LOCK_NB;
 126                 }
 127
 128                 if ((f->operation & ~LOCK_NB) == LOCK_EX)
 129                         unlink_noerrno(f->path);
 130
 131                 f->path = mfree(f->path);
 132         }
 133
 134         f->fd = safe_close(f->fd);
 135         f->operation = 0;
 136 }