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From: Jonathan Nieder <jrnieder@gmail•com>
To: Michael Haggerty <mhagger@alum•mit.edu>
Cc: Junio C Hamano <gitster@pobox•com>, git@vger•kernel.org
Subject: Re: [PATCH 4/5] safe_create_leading_directories(): fix a mkdir/rmdir race
Date: Thu, 26 Dec 2013 15:02:04 -0800	[thread overview]
Message-ID: <20131226230204.GZ20443@google.com> (raw)
In-Reply-To: <1387696451-32224-5-git-send-email-mhagger@alum.mit.edu>

Hi,

Michael Haggerty wrote:

> It could be that some other process is trying to clean up empty
> directories at the same time that safe_create_leading_directories() is
> attempting to create them.  In this case, it could happen that
> directory "a/b" was present at the end of one iteration of the loop
> (either it was already present or we just created it ourselves), but
> by the time we try to create directory "a/b/c", directory "a/b" has
> been deleted.  In fact, directory "a" might also have been deleted.

When does this happen in practice?  Is this about git racing with
itself or with some other program?

I fear that the aggressive directory creator fighting the aggressive
directory remover might be waging a losing battle.

Is this about a push that creates a ref racing against a push that
deletes a ref from the same hierarchy?

> So, if a call to mkdir() fails with ENOENT, then try checking/making
> all directories again from the beginning.  Attempt up to three times
> before giving up.

If we are racing against a ref deletion, then we can retry while our
rival keeps walking up the directory tree and deleting parent
directories.  As soon as we successfully create a directory, we have
won the race.

But what happens if the entire safe_create_leading_directories
operation succeeds and *then* our racing partner deletes the
directory?  No one is putting in a file to reserve the directory for
the directory creator.

If we care enough to retry more than once, I fear this is retrying at
the wrong level.

> Signed-off-by: Michael Haggerty <mhagger@alum•mit.edu>
> ---
>  sha1_file.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)

Tests?

The code is clear and implements the design correctly.

Thanks for some food for thought,
Jonathan

next prev parent reply	other threads:[~2013-12-26 23:02 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-12-22  7:14 [PATCH 0/5] Fix two mkdir/rmdir races Michael Haggerty
2013-12-22  7:14 ` [PATCH 1/5] safe_create_leading_directories(): modernize format of "if" chaining Michael Haggerty
2013-12-26 21:52   ` Jonathan Nieder
2013-12-22  7:14 ` [PATCH 2/5] safe_create_leading_directories(): reduce scope of local variable Michael Haggerty
2013-12-26 21:55   ` Jonathan Nieder
2014-01-01 12:39     ` Michael Haggerty
2013-12-22  7:14 ` [PATCH 3/5] safe_create_leading_directories(): add "slash" pointer Michael Haggerty
2013-12-26 22:34   ` Jonathan Nieder
2014-01-01 21:10     ` Michael Haggerty
2013-12-22  7:14 ` [PATCH 4/5] safe_create_leading_directories(): fix a mkdir/rmdir race Michael Haggerty
2013-12-22 22:42   ` Ramsay Jones
2013-12-26 23:02   ` Jonathan Nieder [this message]
2014-01-02  0:53     ` Michael Haggerty
2013-12-22  7:14 ` [PATCH 5/5] rename_ref(): fix a mkdir()/rmdir() race Michael Haggerty
2013-12-26 23:20   ` Jonathan Nieder

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