Subject: Re: [PATCH v4 2/7] fetch-pack: add refetch

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Thu, 31 Mar 2022 17:09:19 +0200

On Mon, Mar 28 2022, Robert Coup via GitGitGadget wrote:

> From: Robert Coup <[email-masked]>
>
> Allow a "refetch" where the contents of the local object store are
> ignored and a full fetch is performed, not attempting to find or
> negotiate common commits with the remote.
>
> A key use case is to apply a new partial clone blob/tree filter and
> refetch all the associated matching content, which would otherwise not
> be transferred when the commit objects are already present locally.

FWIW it's not clear to me re earlier comments on earlier iterations
whether the "noop fetch" is per-se wanted for this feature for some
reason, or if it's just much easier to implement than doing what I
suggested in:
https://lore.kernel.org/git/[email-masked]/

I don't think such a thing should hold this series up, but as it would
be a bit kinder to servers I think it's worth at least noting in the
commit message what's desired per-se here, v.s. what's just needed for
the convenience of implementation.

I.e. when this series was in an earlier iteration the scope was to
repair repository corruption, which I pointed out to you it really
couldn't do without more wider changes to the object store management,
and at that point having it be NOOP definitely makes sense. The object
lookups etc. take shortcuts that "fsck" wouldn't do, so we could be
negotiating on the basis of corrupt content.

But now that it's a "fetch what's missing" wouldn't it make more sense
to descend from our otherwise-negotiated tips, and find the OIDs that
are "complete", if any, and negotiate with those?

Which again, I think it's fine to say "yeah, that would be ideal, but
this is easier". I'm just checking if I'm missing some subtlety here...

> Signed-off-by: Robert Coup <[email-masked]>
> ---
>  fetch-pack.c | 46 +++++++++++++++++++++++++++++-----------------
>  fetch-pack.h |  1 +
>  2 files changed, 30 insertions(+), 17 deletions(-)
>
> diff --git a/fetch-pack.c b/fetch-pack.c
> index 87657907e78..4e1e88eea09 100644
> --- a/fetch-pack.c
> +++ b/fetch-pack.c
> @@ -312,19 +312,21 @@ static int find_common(struct fetch_negotiator *negotiator,
>  		const char *remote_hex;
>  		struct object *o;
>  
> -		/*
> -		 * If that object is complete (i.e. it is an ancestor of a
> -		 * local ref), we tell them we have it but do not have to
> -		 * tell them about its ancestors, which they already know
> -		 * about.
> -		 *
> -		 * We use lookup_object here because we are only
> -		 * interested in the case we *know* the object is
> -		 * reachable and we have already scanned it.
> -		 */
> -		if (((o = lookup_object(the_repository, remote)) != NULL) &&
> -				(o->flags & COMPLETE)) {
> -			continue;
> +		if (!args->refetch) {
> +			/*
> +			* If that object is complete (i.e. it is an ancestor of a
> +			* local ref), we tell them we have it but do not have to
> +			* tell them about its ancestors, which they already know
> +			* about.
> +			*
> +			* We use lookup_object here because we are only
> +			* interested in the case we *know* the object is
> +			* reachable and we have already scanned it.
> +			*/
> +			if (((o = lookup_object(the_repository, remote)) != NULL) &&
> +					(o->flags & COMPLETE)) {
> +				continue;
> +			}

nit: remove the {} here
nit: this double-indented if can just be one if-statement
nit: don't compare against NULL, use ! instead.
>  		}
>  
>  		remote_hex = oid_to_hex(remote);
> @@ -692,6 +694,9 @@ static void mark_complete_and_common_ref(struct fetch_negotiator *negotiator,
>  	int old_save_commit_buffer = save_commit_buffer;
>  	timestamp_t cutoff = 0;
>  
> +	if (args->refetch)
> +		return;
> +
>  	save_commit_buffer = 0;

nit: This function has only two callers, perhaps it's clearer to do do
this "early abort" in those calls?

>  	trace2_region_enter("fetch-pack", "parse_remote_refs_and_find_cutoff", NULL);
> @@ -1028,7 +1033,11 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
>  	struct fetch_negotiator *negotiator;
>  
>  	negotiator = &negotiator_alloc;
> -	fetch_negotiator_init(r, negotiator);
> +	if (args->refetch) {
> +		fetch_negotiator_init_noop(negotiator);
> +	} else {
> +		fetch_negotiator_init(r, negotiator);
> +	}

More needless {}

>  	sort_ref_list(&ref, ref_compare_name);
>  	QSORT(sought, nr_sought, cmp_ref_by_name);
> @@ -1121,7 +1130,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
>  
>  	mark_complete_and_common_ref(negotiator, args, &ref);
>  	filter_refs(args, &ref, sought, nr_sought);
> -	if (everything_local(args, &ref)) {
> +	if (!args->refetch && everything_local(args, &ref)) {
>  		packet_flush(fd[1]);
>  		goto all_done;
>  	}

Here everything_local() is doing what I suggested for
mark_complete_and_common_ref() above, i.e. we check args->refetch first.

> @@ -1587,7 +1596,10 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
>  	struct strvec index_pack_args = STRVEC_INIT;
>  
>  	negotiator = &negotiator_alloc;
> -	fetch_negotiator_init(r, negotiator);
> +	if (args->refetch)
> +		fetch_negotiator_init_noop(negotiator);
> +	else
> +		fetch_negotiator_init(r, negotiator);

This one doesn't have braces (good), unlike do_fetch_pack()