Re: [PATCH v3 2/3] sha1_file: open window into packfiles with O_CLOEXEC

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 28, 2016, 13:32 UTC
Message-ID: <xmqqh97w38gj.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <xmqq60od42s0.fsf@gitster.mtv.corp.google.com>

Junio C Hamano <gitster@pobox.com> writes:

> Hmph.  This may not fly well in practice, though.

We can flip the order around, and then it becomes simpler to later drop atime support. The first step goes like this.

-- >8 --
From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 28 Oct 2016 06:23:07 -0700
Subject: [PATCH] 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      |  1 +
 read-cache.c |  9 +--------
 sha1_file.c  | 39 +++++++++++++++++++--------------------
 3 files changed, 21 insertions(+), 28 deletions(-)

diff --git a/cache.h b/cache.h
index a902ca1f8e..6f1c21a352 100644
--- a/cache.h
+++ b/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);
diff --git a/read-cache.c b/read-cache.c
index db5d910642..c27d3e240b 100644
--- a/read-cache.c
+++ b/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];
diff --git a/sha1_file.c b/sha1_file.c
index 09045df1dc..dfbf398183 100644
--- a/sha1_file.c
+++ b/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(const char *name)
+int git_open_cloexec(const char *name, int flags)
 {
-	static int sha1_file_open_flag = O_NOATIME | O_CLOEXEC;
-
-	for (;;) {
-		int fd;
-
-		errno = 0;
-		fd = open(name, O_RDONLY | sha1_file_open_flag);
-		if (fd >= 0)
-			return fd;
+	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 */
-		if ((sha1_file_open_flag & O_CLOEXEC) && errno == EINVAL) {
-			sha1_file_open_flag &= ~O_CLOEXEC;
-			continue;
-		}
+		cloexec &= ~O_CLOEXEC;
+		fd = open(name, flags | cloexec);
+	}
+	return fd;
+}
 
-		/* 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;
+int git_open(const char *name)
+{
+	static int noatime = O_NOATIME;
+	int fd = git_open_cloexec(name, O_RDONLY);
+
+	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)

-- 
2.10.1-791-g404733b9cf