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fs-util: add openat_report_new() wrapper around openat()

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This is a wrapper around openat(). It works mostly the same, except for
one thing: it race-freely reports whether we just created the indicated
file in case O_CREAT is passed without O_EXCL.
This commit is contained in:
Lennart Poettering 2022-03-21 18:11:26 +01:00
parent f5d0f21c37
commit ca8503f168
3 changed files with 91 additions and 0 deletions
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39
src/basic/fs-util.c
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@ -1083,3 +1083,42 @@ int open_mkdir_at(int dirfd, const char *path, int flags, mode_t mode) {
return TAKE_FD(fd);
}
int openat_report_new(int dirfd, const char *pathname, int flags, mode_t mode, bool *ret_newly_created) {
unsigned attempts = 7;
/* Just like openat(), but adds one thing: optionally returns whether we created the file anew or if
* it already existed before. This is only relevant of O_CREAT is set without O_EXCL, and thus will
* shortcut to openat() otherwise */
if (!FLAGS_SET(flags, O_CREAT) || FLAGS_SET(flags, O_EXCL) || !ret_newly_created)
return RET_NERRNO(openat(dirfd, pathname, flags, mode));
for (;;) {
int fd;
/* First, attempt to open without O_CREAT/O_EXCL, i.e. open existing file */
fd = openat(dirfd, pathname, flags & ~(O_CREAT | O_EXCL), mode);
if (fd >= 0) {
*ret_newly_created = false;
return fd;
}
if (errno != ENOENT)
return -errno;
/* So the file didn't exist yet, hence create it with O_CREAT/O_EXCL. */
fd = openat(dirfd, pathname, flags | O_CREAT | O_EXCL, mode);
if (fd >= 0) {
*ret_newly_created = true;
return fd;
}
if (errno != EEXIST)
return -errno;
/* Hmm, so now we got EEXIST? So it apparently exists now? If so, let's try to open again
* without the two flags. But let's not spin forever, hnce put a limit on things */
if (--attempts == 0) /* Give up eventually, somebody is playing with us */
return -EEXIST;
}
}

2
src/basic/fs-util.h
View File

@ -110,3 +110,5 @@ int posix_fallocate_loop(int fd, uint64_t offset, uint64_t size);
int parse_cifs_service(const char *s, char **ret_host, char **ret_service, char **ret_path);
int open_mkdir_at(int dirfd, const char *path, int flags, mode_t mode);
int openat_report_new(int dirfd, const char *pathname, int flags, mode_t mode, bool *ret_newly_created);

50
src/test/test-fs-util.c
View File

@ -970,6 +970,56 @@ TEST(open_mkdir_at) {
assert_se(subsubdir_fd >= 0);
}
TEST(openat_report_new) {
_cleanup_free_ char *j = NULL;
_cleanup_(rm_rf_physical_and_freep) char *d = NULL;
_cleanup_close_ int fd = -1;
bool b;
assert_se(mkdtemp_malloc(NULL, &d) >= 0);
j = path_join(d, "test");
assert_se(j);
fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
assert_se(fd >= 0);
fd = safe_close(fd);
assert_se(b);
fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
assert_se(fd >= 0);
fd = safe_close(fd);
assert_se(!b);
fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
assert_se(fd >= 0);
fd = safe_close(fd);
assert_se(!b);
assert_se(unlink(j) >= 0);
fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
assert_se(fd >= 0);
fd = safe_close(fd);
assert_se(b);
fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
assert_se(fd >= 0);
fd = safe_close(fd);
assert_se(!b);
assert_se(unlink(j) >= 0);
fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, NULL);
assert_se(fd >= 0);
fd = safe_close(fd);
fd = openat_report_new(AT_FDCWD, j, O_RDWR|O_CREAT, 0666, &b);
assert_se(fd >= 0);
fd = safe_close(fd);
assert_se(!b);
}
static int intro(void) {
arg_test_dir = saved_argv[1];
return EXIT_SUCCESS;