Re: [PATCH] bundle: dup() output descriptor closer to point-of-use

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Peff,

On Fri, 16 Nov 2018, Jeff King wrote:

> On Thu, Nov 15, 2018 at 09:01:07PM +0100, Johannes Schindelin wrote:
> 
> > > It seems like we should be checking that the stale lockfile isn't left,
> > > which is the real problem (the warning is annoying, but is a symptom of
> > > the same thing). I.e., something like:
> > > 
> > >   test_must_fail git bundle create foobar.bundle master..master &&
> > >   test_path_is_missing foobar.bundle.lock
> > > 
> > > That would already pass on non-Windows platforms, but that's OK. It will
> > > never give a false failure.
> > > 
> > > If you don't mind, can you confirm that the test above fails without
> > > either of the two patches under discussion?
> > 
> > This test succeeds with your patch as well as with Gaël's, and fails when
> > neither patch is applied. So you're right, it is the much better test.
> 
> Thanks for checking!
> 
> > > > Do you want to integrate this test into your patch and run with it, or
> > > > do you want me to shepherd your patch?
> > > 
> > > I'll wrap it up with a commit message and a test.
> 
> Actually, I realized there's an even simpler way to do this. Here it is.
> 
> -- >8 --
> Subject: [PATCH] bundle: dup() output descriptor closer to point-of-use
> 
> When writing a bundle to a file, the bundle code actually creates
> "your.bundle.lock" using our lockfile interface. We feed that output
> descriptor to a child git-pack-objects via run-command, which has the
> quirk that it closes the output descriptor in the parent.
> 
> To avoid confusing the lockfile code (which still thinks the descriptor
> is valid), we dup() it, and operate on the duplicate.
> 
> However, this has a confusing side effect: after the dup() but before we
> call pack-objects, we have _two_ descriptors open to the lockfile. If we
> call die() during that time, the lockfile code will try to clean up the
> partially-written file. It knows to close() the file before unlinking,
> since on some platforms (i.e., Windows) the open file would block the
> deletion. But it doesn't know about the duplicate descriptor. On
> Windows, triggering an error at the right part of the code will result
> in the cleanup failing and the lockfile being left in the filesystem.
> 
> We can solve this by moving the dup() much closer to start_command(),
> shrinking the window in which we have the second descriptor open. It's
> easy to place this in such a way that no die() is possible. We could
> still die due to a signal in the exact wrong moment, but we already
> tolerate races there (e.g., a signal could come before we manage to put
> the file on the cleanup list in the first place).
> 
> As a bonus, this shields create_bundle() itself from the duplicate-fd
> trick, and we can simplify its error handling (note that the lock
> rollback now happens unconditionally, but that's OK; it's a noop if we
> didn't open the lock in the first place).
> 
> The included test uses an empty bundle to cause a failure at the right
> spot in the code, because that's easy to trigger (the other likely
> errors are write() problems like ENOSPC).  Note that it would already
> pass on non-Windows systems (because they are happy to unlink an
> already-open file).

Thanks, this is a very nice explanation (and now that I do not feel so
stressed as I did yesterday, I can easily wrap my head around it).

I can confirm that the test fails without the changes to bundle.c, and
succeeds with the changes.

Thank you so much!
Dscho

> Based-on-a-patch-by: Gaël Lhez <gael.lhez@gmail.com>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  bundle.c                | 39 ++++++++++++++++++---------------------
>  t/t5607-clone-bundle.sh |  6 ++++++
>  2 files changed, 24 insertions(+), 21 deletions(-)
> 
> diff --git a/bundle.c b/bundle.c
> index 1ef584b93b..6b0f6d8f10 100644
> --- a/bundle.c
> +++ b/bundle.c
> @@ -243,7 +243,7 @@ static int is_tag_in_date_range(struct object *tag, struct rev_info *revs)
>  }
>  
>  
> -/* Write the pack data to bundle_fd, then close it if it is > 1. */
> +/* Write the pack data to bundle_fd */
>  static int write_pack_data(int bundle_fd, struct rev_info *revs)
>  {
>  	struct child_process pack_objects = CHILD_PROCESS_INIT;
> @@ -256,6 +256,20 @@ static int write_pack_data(int bundle_fd, struct rev_info *revs)
>  	pack_objects.in = -1;
>  	pack_objects.out = bundle_fd;
>  	pack_objects.git_cmd = 1;
> +
> +	/*
> +	 * start_command() will close our descriptor if it's >1. Duplicate it
> +	 * to avoid surprising the caller.
> +	 */
> +	if (pack_objects.out > 1) {
> +		pack_objects.out = dup(pack_objects.out);
> +		if (pack_objects.out < 0) {
> +			error_errno(_("unable to dup bundle descriptor"));
> +			child_process_clear(&pack_objects);
> +			return -1;
> +		}
> +	}
> +
>  	if (start_command(&pack_objects))
>  		return error(_("Could not spawn pack-objects"));
>  
> @@ -421,21 +435,10 @@ int create_bundle(struct bundle_header *header, const char *path,
>  	bundle_to_stdout = !strcmp(path, "-");
>  	if (bundle_to_stdout)
>  		bundle_fd = 1;
> -	else {
> +	else
>  		bundle_fd = hold_lock_file_for_update(&lock, path,
>  						      LOCK_DIE_ON_ERROR);
>  
> -		/*
> -		 * write_pack_data() will close the fd passed to it,
> -		 * but commit_lock_file() will also try to close the
> -		 * lockfile's fd. So make a copy of the file
> -		 * descriptor to avoid trying to close it twice.
> -		 */
> -		bundle_fd = dup(bundle_fd);
> -		if (bundle_fd < 0)
> -			die_errno("unable to dup file descriptor");
> -	}
> -
>  	/* write signature */
>  	write_or_die(bundle_fd, bundle_signature, strlen(bundle_signature));
>  
> @@ -463,10 +466,8 @@ int create_bundle(struct bundle_header *header, const char *path,
>  		goto err;
>  
>  	/* write pack */
> -	if (write_pack_data(bundle_fd, &revs)) {
> -		bundle_fd = -1; /* already closed by the above call */
> +	if (write_pack_data(bundle_fd, &revs))
>  		goto err;
> -	}
>  
>  	if (!bundle_to_stdout) {
>  		if (commit_lock_file(&lock))
> @@ -474,11 +475,7 @@ int create_bundle(struct bundle_header *header, const char *path,
>  	}
>  	return 0;
>  err:
> -	if (!bundle_to_stdout) {
> -		if (0 <= bundle_fd)
> -			close(bundle_fd);
> -		rollback_lock_file(&lock);
> -	}
> +	rollback_lock_file(&lock);
>  	return -1;
>  }
>  
> diff --git a/t/t5607-clone-bundle.sh b/t/t5607-clone-bundle.sh
> index 348d9b3bc7..cf39e9e243 100755
> --- a/t/t5607-clone-bundle.sh
> +++ b/t/t5607-clone-bundle.sh
> @@ -71,4 +71,10 @@ test_expect_success 'prerequisites with an empty commit message' '
>  	git bundle verify bundle
>  '
>  
> +test_expect_success 'failed bundle creation does not leave cruft' '
> +	# This fails because the bundle would be empty.
> +	test_must_fail git bundle create fail.bundle master..master &&
> +	test_path_is_missing fail.bundle.lock
> +'
> +
>  test_done
> -- 
> 2.19.1.1636.gc7a073d580
> 
>