src/shared/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 "missing_fcntl.h"
  16 #include "path-util.h"
  17
  18 int make_lock_file(const char *p, int operation, LockFile *ret) {
  19         _cleanup_close_ int fd = -1;
  20         _cleanup_free_ char *t = NULL;
  21         int r;
  22
  23         /*
  24          * We use UNPOSIX locks if they are available. They have nice
  25          * semantics, and are mostly compatible with NFS. However,
  26          * they are only available on new kernels. When we detect we
  27          * are running on an older kernel, then we fall back to good
  28          * old BSD locks. They also have nice semantics, but are
  29          * slightly problematic on NFS, where they are upgraded to
  30          * POSIX locks, even though locally they are orthogonal to
  31          * POSIX locks.
  32          */
  33
  34         t = strdup(p);
  35         if (!t)
  36                 return -ENOMEM;
  37
  38         for (;;) {
  39                 struct flock fl = {
  40                         .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
  41                         .l_whence = SEEK_SET,
  42                 };
  43                 struct stat st;
  44
  45                 fd = open(p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
  46                 if (fd < 0)
  47                         return -errno;
  48
  49                 r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
  50                 if (r < 0) {
  51
  52                         /* If the kernel is too old, use good old BSD locks */
  53                         if (errno == EINVAL)
  54                                 r = flock(fd, operation);
  55
  56                         if (r < 0)
  57                                 return errno == EAGAIN ? -EBUSY : -errno;
  58                 }
  59
  60                 /* If we acquired the lock, let's check if the file
  61                  * still exists in the file system. If not, then the
  62                  * previous exclusive owner removed it and then closed
  63                  * it. In such a case our acquired lock is worthless,
  64                  * hence try again. */
  65
  66                 r = fstat(fd, &st);
  67                 if (r < 0)
  68                         return -errno;
  69                 if (st.st_nlink > 0)
  70                         break;
  71
  72                 fd = safe_close(fd);
  73         }
  74
  75         ret->path = t;
  76         ret->fd = fd;
  77         ret->operation = operation;
  78
  79         fd = -1;
  80         t = NULL;
  81
  82         return r;
  83 }
  84
  85 int make_lock_file_for(const char *p, int operation, LockFile *ret) {
  86         const char *fn;
  87         char *t;
  88
  89         assert(p);
  90         assert(ret);
  91
  92         fn = basename(p);
  93         if (!filename_is_valid(fn))
  94                 return -EINVAL;
  95
  96         t = newa(char, strlen(p) + 2 + 4 + 1);
  97         stpcpy(stpcpy(stpcpy(mempcpy(t, p, fn - p), ".#"), fn), ".lck");
  98
  99         return make_lock_file(t, operation, ret);
 100 }
 101
 102 void release_lock_file(LockFile *f) {
 103         int r;
 104
 105         if (!f)
 106                 return;
 107
 108         if (f->path) {
 109
 110                 /* If we are the exclusive owner we can safely delete
 111                  * the lock file itself. If we are not the exclusive
 112                  * owner, we can try becoming it. */
 113
 114                 if (f->fd >= 0 &&
 115                     (f->operation & ~LOCK_NB) == LOCK_SH) {
 116                         static const struct flock fl = {
 117                                 .l_type = F_WRLCK,
 118                                 .l_whence = SEEK_SET,
 119                         };
 120
 121                         r = fcntl(f->fd, F_OFD_SETLK, &fl);
 122                         if (r < 0 && errno == EINVAL)
 123                                 r = flock(f->fd, LOCK_EX|LOCK_NB);
 124
 125                         if (r >= 0)
 126                                 f->operation = LOCK_EX|LOCK_NB;
 127                 }
 128
 129                 if ((f->operation & ~LOCK_NB) == LOCK_EX)
 130                         unlink_noerrno(f->path);
 131
 132                 f->path = mfree(f->path);
 133         }
 134
 135         f->fd = safe_close(f->fd);
 136         f->operation = 0;
 137 }