Subject: Re: [PATCH 4/5] safe_create_leading_directories(): fix a mkdir/rmdir race

From: Jonathan Nieder <[email-masked]>
Date: Thu, 26 Dec 2013 15:02:04 -0800

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 <[email-masked]>
> ---
>  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