git_open(): untangle possible NOATIME and CLOEXEC interactions

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>

cache.h

@@ -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);

read-cache.c

@@ -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];

sha1_file.c

@@ -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)