Subject: Re: [PATCH 2/3] index-pack: no blobs during outgoing link check

From: Patrick Steinhardt <[email-masked]>
Date: Tue, 3 Dec 2024 07:00:44 +0100

On Mon, Dec 02, 2024 at 12:18:39PM -0800, Jonathan Tan wrote:
> As a follow-up to the parent of this commit, it was found that not
> checking for the existence of blobs linked from trees sped up the fetch
> from 24m47.815s to 2m2.127s. Teach Git to do that.
> 
> The benefit of doing this is as above (fetch speedup), but the drawback
> is that if the packfile to be indexed references a local blob directly
> (that is, not through a local tree), that local blob is in danger of
> being garbage collected. Such a situation may arise if we push local
> commits, including one with a change to a blob in the root tree,
> and then the server incorporates them into its main branch through a
> "rebase" or "squash" merge strategy, and then we fetch the new main
> branch from the server.

Okay, so we know that we are basically doing the wrong thing with the
optimization, but by skipping blobs we can get a significant speedup and
the failure mode is that we will re-fetch the object in a later step.
And because we think the situation is rare it shouldn't be a huge issue
in practice.

> This situation has not been observed yet - we have only noticed missing
> commits, not missing trees or blobs. (In fact, if it were believed that
> only missing commits are problematic, one could argue that we should
> also exclude trees during the outgoing link check; but it is safer to
> include them.)
> 
> Due to the rarity of the situation (it has not been observed to happen
> in real life), and because the "penalty" in such a situation is merely
> to refetch the missing blob when it's needed, the tradeoff seems
> worth it.

So is this a one-off event that may happen once per blob, or would we
eventually evict the refetched blob and run into the same situation
repeatedly?

> diff --git a/builtin/index-pack.c b/builtin/index-pack.c
> index 8e7d14c17e..58d24540dc 100644
> --- a/builtin/index-pack.c
> +++ b/builtin/index-pack.c
> @@ -830,8 +830,10 @@ static void do_record_outgoing_links(struct object *obj)
>  			 * verified, so do not print any here.
>  			 */
>  			return;
> -		while (tree_entry_gently(&desc, &entry))
> -			record_outgoing_link(&entry.oid);
> +		while (tree_entry_gently(&desc, &entry)) {
> +			if (S_ISDIR(entry.mode))
> +				record_outgoing_link(&entry.oid);
> +		}

Without the context of the commit message this code snippet likely would
not make any sense to a reader. The "correct" logic would be to record
all objects, regardless of whether they are an object ID or not. But we
explicitly choose not to as a tradeoff between performance and
correctness.

All to say that we should have a comment here that explains what is
going on.

Patrick