summaryrefslogtreecommitdiff
path: root/src/basic/lockfile-util.c
blob: b94195cc5dd4d616d4194e9e05a42a27f4401742 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
/* SPDX-License-Identifier: LGPL-2.1+ */
/***
  This file is part of systemd.

  Copyright 2010 Lennart Poettering
***/

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/file.h>
#include <sys/stat.h>

#include "alloc-util.h"
#include "fd-util.h"
#include "fs-util.h"
#include "lockfile-util.h"
#include "macro.h"
#include "path-util.h"

int make_lock_file(const char *p, int operation, LockFile *ret) {
        _cleanup_close_ int fd = -1;
        _cleanup_free_ char *t = NULL;
        int r;

        /*
         * We use UNPOSIX locks if they are available. They have nice
         * semantics, and are mostly compatible with NFS. However,
         * they are only available on new kernels. When we detect we
         * are running on an older kernel, then we fall back to good
         * old BSD locks. They also have nice semantics, but are
         * slightly problematic on NFS, where they are upgraded to
         * POSIX locks, even though locally they are orthogonal to
         * POSIX locks.
         */

        t = strdup(p);
        if (!t)
                return -ENOMEM;

        for (;;) {
                struct flock fl = {
                        .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
                        .l_whence = SEEK_SET,
                };
                struct stat st;

                fd = open(p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
                if (fd < 0)
                        return -errno;

                r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
                if (r < 0) {

                        /* If the kernel is too old, use good old BSD locks */
                        if (errno == EINVAL)
                                r = flock(fd, operation);

                        if (r < 0)
                                return errno == EAGAIN ? -EBUSY : -errno;
                }

                /* If we acquired the lock, let's check if the file
                 * still exists in the file system. If not, then the
                 * previous exclusive owner removed it and then closed
                 * it. In such a case our acquired lock is worthless,
                 * hence try again. */

                r = fstat(fd, &st);
                if (r < 0)
                        return -errno;
                if (st.st_nlink > 0)
                        break;

                fd = safe_close(fd);
        }

        ret->path = t;
        ret->fd = fd;
        ret->operation = operation;

        fd = -1;
        t = NULL;

        return r;
}

int make_lock_file_for(const char *p, int operation, LockFile *ret) {
        const char *fn;
        char *t;

        assert(p);
        assert(ret);

        fn = basename(p);
        if (!filename_is_valid(fn))
                return -EINVAL;

        t = newa(char, strlen(p) + 2 + 4 + 1);
        stpcpy(stpcpy(stpcpy(mempcpy(t, p, fn - p), ".#"), fn), ".lck");

        return make_lock_file(t, operation, ret);
}

void release_lock_file(LockFile *f) {
        int r;

        if (!f)
                return;

        if (f->path) {

                /* If we are the exclusive owner we can safely delete
                 * the lock file itself. If we are not the exclusive
                 * owner, we can try becoming it. */

                if (f->fd >= 0 &&
                    (f->operation & ~LOCK_NB) == LOCK_SH) {
                        static const struct flock fl = {
                                .l_type = F_WRLCK,
                                .l_whence = SEEK_SET,
                        };

                        r = fcntl(f->fd, F_OFD_SETLK, &fl);
                        if (r < 0 && errno == EINVAL)
                                r = flock(f->fd, LOCK_EX|LOCK_NB);

                        if (r >= 0)
                                f->operation = LOCK_EX|LOCK_NB;
                }

                if ((f->operation & ~LOCK_NB) == LOCK_EX)
                        unlink_noerrno(f->path);

                f->path = mfree(f->path);
        }

        f->fd = safe_close(f->fd);
        f->operation = 0;
}