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git_open(): untangle possible NOATIME and CLOEXEC interactions

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The way we structured the fallback/retry mechanism for opening with
O_NOATIME and O_CLOEXEC meant that if we failed due to lack of
support to open the file with O_NOATIME option (i.e. EINVAL), we
would still try to drop O_CLOEXEC first and retry, and then drop
O_NOATIME.  A platform on which O_NOATIME is defined in the header
without support from the kernel wouldn't have a chance to open with
O_CLOEXEC option due to this code structure.

Arguably, O_CLOEXEC is more important than O_NOATIME, as the latter
is mostly about performance, while the former can affect correctness.

Instead use O_CLOEXEC to open the file, and then use fcntl(2) to set
O_NOATIME on the resulting file descriptor.  open(2) itself does not
cause atime to be updated according to Linus [*1*].

The helper to do the former can be usable in the codepath in
ce_compare_data() that was recently added to open a file descriptor
with O_CLOEXEC; use it while we are at it.

*1* <CA+55aFw83E+zOd+z5h-CA-3NhrLjVr-anL6pubrSWttYx3zu8g@mail.gmail.com>

Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Junio C Hamano 2016-10-28 06:23:07 -07:00
parent a0a6cb9662
commit 1b8ac5ead5
3 changed files with 22 additions and 29 deletions
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1
cache.h
View file

@ -1122,6 +1122,7 @@ extern int write_sha1_file(const void *buf, unsigned long len, const char *type,
extern int hash_sha1_file_literally(const void *buf, unsigned long len, const char *type, unsigned char *sha1, unsigned flags);
extern int pretend_sha1_file(void *, unsigned long, enum object_type, unsigned char *);
extern int force_object_loose(const unsigned char *sha1, time_t mtime);
extern int git_open_cloexec(const char *name, int flags);
extern int git_open(const char *name);
extern void *map_sha1_file(const unsigned char *sha1, unsigned long *size);
extern int unpack_sha1_header(git_zstream *stream, unsigned char *map, unsigned long mapsize, void *buffer, unsigned long bufsiz);

9
read-cache.c
View file

@ -156,14 +156,7 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
{
int match = -1;
static int cloexec = O_CLOEXEC;
int fd = open(ce->name, O_RDONLY | cloexec);
if ((cloexec & O_CLOEXEC) && fd < 0 && errno == EINVAL) {
/* Try again w/o O_CLOEXEC: the kernel might not support it */
cloexec &= ~O_CLOEXEC;
fd = open(ce->name, O_RDONLY | cloexec);
}
int fd = git_open_cloexec(ce->name, O_RDONLY);
if (fd >= 0) {
unsigned char sha1[20];

41
sha1_file.c
View file

@ -1559,31 +1559,30 @@ int check_sha1_signature(const unsigned char *sha1, void *map,
return hashcmp(sha1, real_sha1) ? -1 : 0;
}
int git_open_cloexec(const char *name, int flags)
{
static int cloexec = O_CLOEXEC;
int fd = open(name, flags | cloexec);
if ((cloexec & O_CLOEXEC) && fd < 0 && errno == EINVAL) {
/* Try again w/o O_CLOEXEC: the kernel might not support it */
cloexec &= ~O_CLOEXEC;
fd = open(name, flags | cloexec);
}
return fd;
}
int git_open(const char *name)
{
static int sha1_file_open_flag = O_NOATIME | O_CLOEXEC;
static int noatime = O_NOATIME;
int fd = git_open_cloexec(name, O_RDONLY);
for (;;) {
int fd;
errno = 0;
fd = open(name, O_RDONLY | sha1_file_open_flag);
if (fd >= 0)
return fd;
/* Try again w/o O_CLOEXEC: the kernel might not support it */
if ((sha1_file_open_flag & O_CLOEXEC) && errno == EINVAL) {
sha1_file_open_flag &= ~O_CLOEXEC;
continue;
}
/* Might the failure be due to O_NOATIME? */
if (errno != ENOENT && (sha1_file_open_flag & O_NOATIME)) {
sha1_file_open_flag &= ~O_NOATIME;
continue;
}
return -1;
if (0 <= fd && (noatime & O_NOATIME)) {
int flags = fcntl(fd, F_GETFL);
if (fcntl(fd, F_SETFL, flags | noatime))
noatime = 0;
}
return fd;
}
static int stat_sha1_file(const unsigned char *sha1, struct stat *st)