git repack leaks disk space on ENOSPC

git repack leaks disk space on ENOSPC

[Andreas Krey]

Hi all,

I observed (again) an annoying behavior of 'git repack':
When the new pack file cannot be fully written because
the disk gets full beforehand, the tmp_pack file isn't
deleted, meaning the disk stays full:

  $ df -h .; git repack -ad; df -h .; ls -lart .git/objects/pack/tmp*; rm .git/objects/pack/tmp*; df -h .
  Filesystem                        Size  Used Avail Use% Mounted on
  /dev/mapper/vg02-localworkspaces  250G  245G  5.1G  98% /workspaces/calvin
  Counting objects: 4715349, done.
  Delta compression using up to 8 threads.
  Compressing objects: 100% (978051/978051), done.
  fatal: sha1 file '.git/objects/pack/tmp_pack_xB7DMt' write error: No space left on device
  Filesystem                        Size  Used Avail Use% Mounted on
  /dev/mapper/vg02-localworkspaces  250G  250G   20K 100% /workspaces/calvin
  -r--r--r-- 1 andrkrey users 5438435328 Oct 11 17:03 .git/objects/pack/tmp_pack_xB7DMt
  rm: remove write-protected regular file '.git/objects/pack/tmp_pack_xB7DMt'? y
  Filesystem                        Size  Used Avail Use% Mounted on
  /dev/mapper/vg02-localworkspaces  250G  245G  5.1G  98% /workspaces/calvin

- Andreas

git version 2.15.0.rc0

-- 
"Totally trivial. Famous last words."
From: Linus Torvalds <torvalds@*.org>
Date: Fri, 22 Jan 2010 07:29:21 -0800

Re: git repack leaks disk space on ENOSPC

[Jonathan Nieder]

Hi Andreas,

Andreas Krey wrote:

> I observed (again) an annoying behavior of 'git repack':

Do you have context for this 'again'?  E.g. was this discussed
previously on-list?

> When the new pack file cannot be fully written because
> the disk gets full beforehand, the tmp_pack file isn't
> deleted, meaning the disk stays full:
>
>   $ df -h .; git repack -ad; df -h .; ls -lart .git/objects/pack/tmp*; rm .git/objects/pack/tmp*; df -h .
>   Filesystem                        Size  Used Avail Use% Mounted on
>   /dev/mapper/vg02-localworkspaces  250G  245G  5.1G  98% /workspaces/calvin
>   Counting objects: 4715349, done.
>   Delta compression using up to 8 threads.
>   Compressing objects: 100% (978051/978051), done.
>   fatal: sha1 file '.git/objects/pack/tmp_pack_xB7DMt' write error: No space left on device
>   Filesystem                        Size  Used Avail Use% Mounted on
>   /dev/mapper/vg02-localworkspaces  250G  250G   20K 100% /workspaces/calvin
>   -r--r--r-- 1 andrkrey users 5438435328 Oct 11 17:03 .git/objects/pack/tmp_pack_xB7DMt
>   rm: remove write-protected regular file '.git/objects/pack/tmp_pack_xB7DMt'? y
>   Filesystem                        Size  Used Avail Use% Mounted on
>   /dev/mapper/vg02-localworkspaces  250G  245G  5.1G  98% /workspaces/calvin
>
> git version 2.15.0.rc0

I can imagine this behavior of retaining tmp_pack being useful for
debugging in some circumstances, but I agree with you that it is
certainly not a good default.

Chasing this down, I find:

  pack-write.c::create_tmp_packfile chooses the filename
  builtin/pack-objects.c::write_pack_file writes to it and the .bitmap, calling
  pack-write.c::finish_tmp_packfile to rename it into place

Nothing tries to install an atexit handler to do anything special to it
on exit.

The natural thing, I'd expect, would be for pack-write to use the
tempfile API (see tempfile.h) to create and finish the file.  That way,
we'd get such atexit handlers for free.  If we want a way to keep temp
files for debugging on abnormal exit, we could set that up separately as
a generic feature of the tempfile API (e.g. an envvar
GIT_KEEP_TEMPFILES_ON_FAILURE), making that an orthogonal topic.

Does using create_tempfile there seem like a good path forward to you?
Would you be interested in working on it (either writing a patch with
such a fix or a test in t/ to make sure it keeps working)?

Thanks,
Jonathan

Re: git repack leaks disk space on ENOSPC

[Andreas Krey]

On Wed, 11 Oct 2017 20:17:03 +0000, Jonathan Nieder wrote:
> Hi Andreas,
> 
> Andreas Krey wrote:
> 
> > I observed (again) an annoying behavior of 'git repack':
> 
> Do you have context for this 'again'?  E.g. was this discussed
> previously on-list?

I think I posted about it, but no discussion. I poked a bit
at the code, with not much luck back then.

...
> Does using create_tempfile there seem like a good path forward to you?
> Would you be interested in working on it (either writing a patch with
> such a fix or a test in t/ to make sure it keeps working)?

I will look into creating a patch (thanks for the pointers),
but I don't see how to make a testcase for this - pre-filling the
disk doesn't sound like a good idea. Most people probably won't run in
this situation, and then won't have tmp_packs with a dozen GBytes each
lying around.

Andreas

-- 
"Totally trivial. Famous last words."
From: Linus Torvalds <torvalds@*.org>
Date: Fri, 22 Jan 2010 07:29:21 -0800

Re: git repack leaks disk space on ENOSPC

[brian m. carlson]

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On Thu, Oct 12, 2017 at 11:34:39AM +0200, Andreas Krey wrote:
> On Wed, 11 Oct 2017 20:17:03 +0000, Jonathan Nieder wrote:
> > Does using create_tempfile there seem like a good path forward to you?
> > Would you be interested in working on it (either writing a patch with
> > such a fix or a test in t/ to make sure it keeps working)?
> 
> I will look into creating a patch (thanks for the pointers),
> but I don't see how to make a testcase for this - pre-filling the
> disk doesn't sound like a good idea. Most people probably won't run in
> this situation, and then won't have tmp_packs with a dozen GBytes each
> lying around.

A patch would be very welcome.  We have this problem not infrequently at
work with development and test VMs, which tend to have a relatively
small amount of disk.

If you decide that you don't want to create a patch, I'll probably pick
it up eventually.
-- 
brian m. carlson / brian with sandals: Houston, Texas, US
https://www.crustytoothpaste.net/~bmc | My opinion only
OpenPGP: https://keybase.io/bk2204

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Re: git repack leaks disk space on ENOSPC

[Jeff King]

On Thu, Oct 12, 2017 at 11:34:39AM +0200, Andreas Krey wrote:

> > Does using create_tempfile there seem like a good path forward to you?
> > Would you be interested in working on it (either writing a patch with
> > such a fix or a test in t/ to make sure it keeps working)?
> 
> I will look into creating a patch (thanks for the pointers),
> but I don't see how to make a testcase for this - pre-filling the
> disk doesn't sound like a good idea. Most people probably won't run in
> this situation, and then won't have tmp_packs with a dozen GBytes each
> lying around.

It may be easier to trigger a case which rejects the pack for other
reasons. For an incoming index-pack, turning on transfer.fsckObjects is
an easy one. For a repack, perhaps corrupting a loose object
to-be-packed would work.

E.g.:

  git init
  echo content >file
  git add file
  git commit -m foo

  # corrupt the blob in a subtle way
  obj=.git/objects/$(git rev-parse HEAD:file | sed 's,..,&/,')
  chmod +w $obj
  echo cruft >>$obj

After that, I get:

  $ git repack -ad
  Counting objects: 3, done.
  error: garbage at end of loose object 'd95f3ad14dee633a758d2e331151e950dd13e4ed'
  fatal: loose object d95f3ad14dee633a758d2e331151e950dd13e4ed (stored in .git/objects/d9/5f3ad14dee633a758d2e331151e950dd13e4ed) is corrupt

  $ find -type f .git/objects/pack
  .git/objects/pack/tmp_pack_0GaXwk

-Peff

Re: git repack leaks disk space on ENOSPC

[Jeff King]

On Wed, Oct 11, 2017 at 08:17:03PM -0700, Jonathan Nieder wrote:

> I can imagine this behavior of retaining tmp_pack being useful for
> debugging in some circumstances, but I agree with you that it is
> certainly not a good default.
> 
> Chasing this down, I find:
> 
>   pack-write.c::create_tmp_packfile chooses the filename
>   builtin/pack-objects.c::write_pack_file writes to it and the .bitmap, calling
>   pack-write.c::finish_tmp_packfile to rename it into place
> 
> Nothing tries to install an atexit handler to do anything special to it
> on exit.
> 
> The natural thing, I'd expect, would be for pack-write to use the
> tempfile API (see tempfile.h) to create and finish the file.  That way,
> we'd get such atexit handlers for free.  If we want a way to keep temp
> files for debugging on abnormal exit, we could set that up separately as
> a generic feature of the tempfile API (e.g. an envvar
> GIT_KEEP_TEMPFILES_ON_FAILURE), making that an orthogonal topic.

Yes, I think this is the right direction. I've had a patch in GitHub's
fork for years that does so (since otherwise failures can fill up your
disk and need manual intervention).

The main reason that I hadn't submitted it upstream was because of the
"you can never free a struct tempfile" requirement. So my patch just
leaks the tempfile structs. That's OK for packs, of which we tend to
create only a few in a given process, but doesn't scale for loose
objects.

Now that 89563ec379 (Merge branch 'jk/incore-lockfile-removal',
2017-09-19) has landed, I think it makes sense to pursue that direction.

My patch roughly looks like:

  diff --git a/builtin/index-pack.c b/builtin/index-pack.c
  index 4ff567db47..7f261e56c4 100644
  --- a/builtin/index-pack.c
  +++ b/builtin/index-pack.c

  @@ -308,9 +348,11 @@ static const char *open_pack_file(const char *pack_name)
                  input_fd = 0;
                  if (!pack_name) {
                          struct strbuf tmp_file = STRBUF_INIT;
  +                       struct tempfile *t = xcalloc(1, sizeof(*t));
                          output_fd = odb_mkstemp(&tmp_file,
                                                  "pack/tmp_pack_XXXXXX");
                          pack_name = strbuf_detach(&tmp_file, NULL);
  +                       register_tempfile(t, pack_name);
                  } else {
                          output_fd = open(pack_name, O_CREAT|O_EXCL|O_RDWR, 0600);
                          if (output_fd < 0)

but note that's not quite what we'd want. It never closes the tempfile,
so:

  1. Under the new regime, we'd still leak the struct!

  2. Git will still try to unlink the tempfile on exit, even if we
     successfully moved it into place.

So I think all the code around open_pack_file() needs to learn to pass
around the tempfile struct, and eventually use rename_tempfile() to
cement it in place. I also suspect that odb_mkstemp should just take a
"struct tempfile".

-Peff