Subject: Re: [PATCH 1/4] fsck: create scaffolding for rev-index checks

From: Taylor Blau <[email-masked]>
Date: Mon, 17 Apr 2023 18:20:00 -0400

On Mon, Apr 17, 2023 at 04:21:38PM +0000, Derrick Stolee via GitGitGadget wrote:
> From: Derrick Stolee <[email-masked]>
>
> The 'fsck' builtin checks many of Git's on-disk data structures, but
> does not currently validate the pack rev-index files (a .rev file to
> pair with a .pack and .idx file).
>
> Before doing a more-involved check process, create the scaffolding
> within builtin/fsck.c to have a new error type and add that error type
> when the API method verify_pack_revindex() returns an error. That method
> does nothing currently, but we will add checks to it in later changes.
>
> For now, check that 'git fsck' succeeds without any errors in the normal
> case. Future checks will be paired with tests that corrupt the .rev file
> appropriately.
>
> Signed-off-by: Derrick Stolee <[email-masked]>
> ---
>  builtin/fsck.c           | 30 ++++++++++++++++++++++++++++++
>  pack-revindex.c          | 11 +++++++++++
>  pack-revindex.h          |  8 ++++++++
>  t/t5325-reverse-index.sh | 14 ++++++++++++++
>  4 files changed, 63 insertions(+)
>
> diff --git a/builtin/fsck.c b/builtin/fsck.c
> index 095b39d3980..2ab78129bde 100644
> --- a/builtin/fsck.c
> +++ b/builtin/fsck.c
> @@ -24,6 +24,7 @@
>  #include "resolve-undo.h"
>  #include "run-command.h"
>  #include "worktree.h"
> +#include "pack-revindex.h"
>
>  #define REACHABLE 0x0001
>  #define SEEN      0x0002
> @@ -53,6 +54,7 @@ static int name_objects;
>  #define ERROR_REFS 010
>  #define ERROR_COMMIT_GRAPH 020
>  #define ERROR_MULTI_PACK_INDEX 040
> +#define ERROR_PACK_REV_INDEX 0100
>
>  static const char *describe_object(const struct object_id *oid)
>  {
> @@ -856,6 +858,32 @@ static int mark_packed_for_connectivity(const struct object_id *oid,
>  	return 0;
>  }
>
> +static int check_pack_rev_indexes(struct repository *r, int show_progress)
> +{
> +	struct progress *progress = NULL;
> +	uint32_t pack_count = 0;
> +	int res = 0;
> +
> +	if (show_progress) {
> +		for (struct packed_git *p = get_all_packs(the_repository); p; p = p->next)

It's going to take me a while to get used to these declarations inside
of for-loops!

> +			pack_count++;
> +		progress = start_delayed_progress("Verifying reverse pack-indexes", pack_count);

I wonder if we want to count over the sum of objects in packs rather
than the number of packs themselves. My worry would be that a rather
large pack would make it appear as if nothing is happening when in
reality we're just churning through a lot of objects.

> +		pack_count = 0;
> +	}
> +
> +	for (struct packed_git *p = get_all_packs(the_repository); p; p = p->next) {
> +		if (!load_pack_revindex(the_repository, p) &&

I was going to comment that I wasn't sure if `load_pack_revindex()` was
the right thing here, since we don't care about validating the
on-the-fly reverse indexes that we generate.

But I see in your 3/4 that you are comparing the values on disk to those
in memory, which is very nice.

> +		    verify_pack_revindex(p)) {

Inside of verify_pack_revindex(), it says that a negative number is
returned on error. Do we care about disambiguating >= 0 here? IOW,
should this be:

    if (!load_pack_revindex(the_repository, p) || verify_pack_revindex(p) < 0)

?

All looking good otherwise.

Thanks,
Taylor