Subject: Re: [PATCH 3/9] maintenance: add loose-objects task

From: Emily Shaffer <[email-masked]>
Date: Wed, 12 Aug 2020 16:10:53 -0700

On Thu, Aug 06, 2020 at 04:30:18PM +0000, Derrick Stolee via GitGitGadget wrote:

> diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt
> index bb0d5eded4..898aff4726 100644
> --- a/Documentation/git-maintenance.txt
> +++ b/Documentation/git-maintenance.txt
> @@ -80,6 +80,17 @@ gc::
>  	It can also be disruptive in some situations, as it deletes stale
>  	data.
>  
> +loose-objects::
> +	The `loose-objects` job cleans up loose objects and places them into
> +	pack-files. In order to prevent race conditions with concurrent Git
> +	commands, it follows a two-step process. First, it deletes any loose
> +	objects that already exist in a pack-file; concurrent Git processes
> +	will examine the pack-file for the object data instead of the loose
> +	object. Second, it creates a new pack-file (starting with "loose-")
> +	containing a batch of loose objects. The batch size is limited to 50
> +	thousand objects to prevent the job from taking too long on a
> +	repository with many loose objects.

[emily] What's not said is what happens to loose-* packfile. Does this
get repacked and disappear, is it treated differently, etc?
[jonathantan] This is treated the same as any other pack. Is there a
reason to call it loose-*?
[jrnieder] It seems like unreachable objects will get stuck going in and
out of this pack, right? unreachable loose obj -> loose-*.pack ->
repack, unreachable becomes loose -> repeat

> +static int prune_packed(struct maintenance_opts *opts)
> +{
> +	struct child_process child = CHILD_PROCESS_INIT;
> +
> +	child.git_cmd = 1;
> +	strvec_push(&child.args, "prune-packed");
> +
> +	if (opts->quiet)
> +		strvec_push(&child.args, "--quiet");
> +
> +	return !!run_command(&child);
[emily] Why not report the error code here to the caller? Is there a
path to notify user of errors that require user intervention?
[jrnieder] Some errors might be expected and some might not, so should
we handle them differently?
[emily] Probably makes more sense to revisit this in the far future when
we teach 'git maintenance' to send us emails and flashing lights when
our jobs fail, instead of worrying about it now :)
[jrnieder] Imagine if we got exit 127 because the usage we are using is
wrong - in that case, for example, we would want to BUG()

> +static int write_loose_object_to_stdin(const struct object_id *oid,
> +				       const char *path,
> +				       void *data)
> +{
> +	struct write_loose_object_data *d = (struct write_loose_object_data *)data;
[jrnieder] Since we are enlightened C developers, not C++ developers,
you can skip the explicit cast.

> +++ b/t/t7900-maintenance.sh
> @@ -83,4 +83,43 @@ test_expect_success 'prefetch multiple remotes' '
>  	git log prefetch/remote2/two
>  '
>  
> +test_expect_success 'loose-objects task' '
[jrnieder] Unrelated to this change, this test makes me sad that we
don't have better low-level test helpers to enable this kind of testing.
Makes me wish for something better :)
> +	# Repack everything so we know the state of the object dir
> +	git repack -adk &&
> +
> +	# Hack to stop maintenance from running during "git commit"
> +	echo in use >.git/objects/maintenance.lock &&
> +
> +	# Assuming that "git commit" creates at least one loose object
> +	test_commit create-loose-object &&
> +	rm .git/objects/maintenance.lock &&
> +
> +	ls .git/objects >obj-dir-before &&
> +	test_file_not_empty obj-dir-before &&
> +	ls .git/objects/pack/*.pack >packs-before &&
> +	test_line_count = 1 packs-before &&
> +
> +	# The first run creates a pack-file
> +	# but does not delete loose objects.
> +	git maintenance run --task=loose-objects &&
> +	ls .git/objects >obj-dir-between &&
> +	test_cmp obj-dir-before obj-dir-between &&
> +	ls .git/objects/pack/*.pack >packs-between &&
> +	test_line_count = 2 packs-between &&
> +	ls .git/objects/pack/loose-*.pack >loose-packs &&
> +	test_line_count = 1 loose-packs &&
> +
> +	# The second run deletes loose objects
> +	# but does not create a pack-file.
> +	git maintenance run --task=loose-objects &&
> +	ls .git/objects >obj-dir-after &&
> +	cat >expect <<-\EOF &&
> +	info
> +	pack
> +	EOF
> +	test_cmp expect obj-dir-after &&
> +	ls .git/objects/pack/*.pack >packs-after &&
> +	test_cmp packs-between packs-after
> +'
> +