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From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: Johannes Sixt <j6t@kdbg.org>,
	git@vger.kernel.org, tytso@mit.edu,
	Junio C Hamano <gitster@pobox.com>,
	Christoph Hellwig <hch@lst.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [RFC PATCH 1/2] sha1-file: fsync() loose dir entry when core.fsyncObjectFiles
Date: Thu, 19 Nov 2020 12:38:28 +0100 (CET)
Message-ID: <nycvar.QRO.7.76.6.2011191232490.56@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <87a6xii5gs.fsf@evledraar.gmail.com>


[-- Attachment #1: Type: text/plain, Size: 3109 bytes --]

Hi Ævar,

On Tue, 22 Sep 2020, Ævar Arnfjörð Bjarmason wrote:

> On Thu, Sep 17 2020, Johannes Sixt wrote:
>
> > Am 17.09.20 um 13:28 schrieb Ævar Arnfjörð Bjarmason:
> >> Change the behavior of core.fsyncObjectFiles to also sync the
> >> directory entry. I don't have a case where this broke, just going by
> >> paranoia and the fsync(2) manual page's guarantees about its behavior.
> >>
> >> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> >> ---
> >>  sha1-file.c | 19 ++++++++++++++-----
> >>  1 file changed, 14 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/sha1-file.c b/sha1-file.c
> >> index dd65bd5c68..d286346921 100644
> >> --- a/sha1-file.c
> >> +++ b/sha1-file.c
> >> @@ -1784,10 +1784,14 @@ int hash_object_file(const struct git_hash_algo *algo, const void *buf,
> >>  }
> >>
> >>  /* Finalize a file on disk, and close it. */
> >> -static void close_loose_object(int fd)
> >> +static void close_loose_object(int fd, const struct strbuf *dirname)
> >>  {
> >> -	if (fsync_object_files)
> >> +	int dirfd;
> >> +	if (fsync_object_files) {
> >>  		fsync_or_die(fd, "loose object file");
> >> +		dirfd = xopen(dirname->buf, O_RDONLY);
> >> +		fsync_or_die(dirfd, "loose object directory");
> >
> > Did you have the opportunity to verify that this works on Windows?
> > Opening a directory with open(2), I mean: It's disallowed according to
> > the docs:
> > https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/open-wopen?view=vs-2019#return-value
>
> I did not, just did a quick hack for an RFC discussion (didn't even
> close() that fd), but if I pursue this I'll do it properly.
>
> Doing some research on it now reveals that we should probably have some
> Windows-specific code here, e.g. browsing GNUlib's source code reveals
> that it uses FlushFileBuffers(), and that code itself is taken from
> sqlite. SQLite also has special-case code for some Unix warts,
> e.g. OSX's and AIX's special fsync behaviors in its src/os_unix.c

If I understand correctly, the idea to `fsync()` directories is to ensure
that metadata updates (such as renames) are flushed, too?

If so (and please note that my understanding of NTFS is not super deep in
this regard), I think that we need not worry on Windows. I have come to
believe that the `rename()` operations are flushed pretty much
immediately, without any `FlushFileBuffers()` (or `_commit()`, as we
actually do in `compat/mingw.h`, to convince yourself see
https://github.com/git/git/blob/v2.29.2/compat/mingw.h#L135-L136).

Directories are not mentioned in `FlushFileBuffers()`'s documentation
(https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-flushfilebuffers)
nor in the documentation of `_commit()`:
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/commit?view=msvc-160

Therefore, I believe that there is not even a Win32 equivalent of
`fsync()`ing directories.

Ciao,
Dscho

>
> >> +	} if (close(fd) != 0) die_errno(_("error when closing loose object
> >> file")); }
> >
> > -- Hannes
>
>
>

  reply index

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-17 18:48 [PATCH] enable core.fsyncObjectFiles by default Christoph Hellwig
2018-01-17 19:04 ` Junio C Hamano
2018-01-17 19:35   ` Christoph Hellwig
2018-01-17 20:05     ` Andreas Schwab
2018-01-17 19:37   ` Matthew Wilcox
2018-01-17 19:42     ` Christoph Hellwig
2018-01-17 21:44   ` Ævar Arnfjörð Bjarmason
2018-01-17 22:07     ` Linus Torvalds
2018-01-17 22:25       ` Linus Torvalds
2018-01-17 23:16       ` Ævar Arnfjörð Bjarmason
2018-01-17 23:42         ` Linus Torvalds
2018-01-17 23:52       ` Theodore Ts'o
2018-01-17 23:57         ` Linus Torvalds
2018-01-18 16:27           ` Christoph Hellwig
2018-01-19 19:08             ` Junio C Hamano
2018-01-20 22:14               ` Theodore Ts'o
2018-01-20 22:27                 ` Junio C Hamano
2018-01-22 15:09                   ` Ævar Arnfjörð Bjarmason
2018-01-22 18:09                     ` Theodore Ts'o
2018-01-23  0:47                       ` Jeff King
2018-01-23  5:45                         ` Theodore Ts'o
2018-01-23 16:17                           ` Jeff King
2018-01-23  0:25                     ` Jeff King
2018-01-21 21:32             ` Chris Mason
2020-09-17 11:06         ` Ævar Arnfjörð Bjarmason
2020-09-17 11:28           ` [RFC PATCH 0/2] should core.fsyncObjectFiles fsync the dir entry + docs Ævar Arnfjörð Bjarmason
2020-09-17 11:28           ` [RFC PATCH 1/2] sha1-file: fsync() loose dir entry when core.fsyncObjectFiles Ævar Arnfjörð Bjarmason
2020-09-17 13:16             ` Jeff King
2020-09-17 15:09               ` Christoph Hellwig
2020-09-17 14:09             ` Christoph Hellwig
2020-09-17 14:55               ` Jeff King
2020-09-17 14:56                 ` Christoph Hellwig
2020-09-17 15:37                   ` Junio C Hamano
2020-09-17 17:12                     ` Jeff King
2020-09-17 20:37                       ` Taylor Blau
2020-09-22 10:42               ` Ævar Arnfjörð Bjarmason
2020-09-17 20:21             ` Johannes Sixt
2020-09-22  8:24               ` Ævar Arnfjörð Bjarmason
2020-11-19 11:38                 ` Johannes Schindelin [this message]
2020-09-17 11:28           ` [RFC PATCH 2/2] core.fsyncObjectFiles: make the docs less flippant Ævar Arnfjörð Bjarmason
2020-09-17 14:12             ` Christoph Hellwig
2020-09-17 15:43             ` Junio C Hamano
2020-09-17 20:15               ` Johannes Sixt
2020-10-08  8:13               ` Johannes Schindelin
2020-10-08 15:57                 ` Ævar Arnfjörð Bjarmason
2020-10-08 18:53                   ` Junio C Hamano
2020-10-09 10:44                   ` Johannes Schindelin
2020-09-17 19:21             ` Marc Branchaud
2020-09-17 14:14           ` [PATCH] enable core.fsyncObjectFiles by default Christoph Hellwig
2020-09-17 15:30           ` Junio C Hamano
2018-01-17 20:55 ` Jeff King
2018-01-17 21:10   ` Christoph Hellwig

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