Re: [PATCH v2] fetch: fix non-conflicting tags not being committed

[Patrick Steinhardt <ps@pks.im>]

On Thu, Nov 06, 2025 at 09:39:25AM +0100, Karthik Nayak wrote:
> The commit 0e358de64a (fetch: use batched reference updates, 2025-05-19)
> updated the 'git-fetch(1)' command to use batched updates. This batches
> updates to gain performance improvements. When fetching references, each
> update is added to the transaction. Finally, when committing, individual
> updates are allowed to fail with reason, while the transaction itself
> succeeds.
> 
> One scenario which was missed here, was fetching tags. When fetching
> conflicting tags, the `fetch_and_consume_refs()` function returns '1',
> which skipped committing the transaction and directly jumped to the
> cleanup section. This mean that no updates were applied.

Okay, this is obviously broken indeed.

> This also extends to backfilling tags when using the now deprecated
> 'branches/' format for remotes.

I'm a bit lost here -- what does backfilling have to do with the
"branches/" directory? The backfill is supposed to create tags that
point into the history that one has just fetched. So:

  - With `--tags` we fetch all tags announced by the remote.

  - With `--no-tags` we fetch no tags.

  - Otherwise we fetch those tags that point into our history.

The last behaviour is a bit more on the esoteric side, but it's
described as such in git-fetch(1):

    By default, any tag that points into the histories being fetched is
    also fetched; the effect is to fetch tags that point at branches
    that you are interested in. This default behavior can be changed by
    using the --tags or --no-tags options or by configuring
    remote.<name>.tagOpt. By using a refspec that fetches tags
    explicitly, you can fetch tags that do not point into branches you
    are interested in as well.

The following test demonstrates this behaviour:

	test_expect_success "fetch single branch without explicit tag option" '
		git init source &&
		git -C source commit --allow-empty --message common &&
		git clone file://"$(pwd)"/source target &&
		(
			cd source &&
			git commit --allow-empty --message discard-me &&
			git tag discard-me &&
			git commit --amend --allow-empty --message fetch-me &&
			git tag fetch-me
		) &&

		# The "discard-me" tag does not point into the history that we are
		# about to fetch, so it should not have been created.
		git -C target fetch origin &&
		git -C target tag -l >actual &&
		echo "fetch-me" >expect &&

		# But with "--tags" we instruct git-fetch(1) to fetch all tags, so we
		# should now see it.
		git -C target fetch origin --tags &&
		git -C target tag -l >actual &&
		cat >expect <<-\EOF &&
		discard-me
		fetch-me
		EOF
		test_cmp expect actual
	'

> diff --git a/builtin/fetch.c b/builtin/fetch.c
> index c7ff3480fb..d5aee5af10 100644
> --- a/builtin/fetch.c
> +++ b/builtin/fetch.c
> @@ -1686,6 +1686,42 @@ static void ref_transaction_rejection_handler(const char *refname,
>  	*data->retcode = 1;
>  }
>  
> +/*
> + * Commit the reference transaction. If it isn't an atomic transaction, handle
> + * rejected updates as part of using batched updates.
> + */
> +static int commit_ref_transaction(struct ref_transaction **transaction,
> +				  bool is_atomic, const char *remote_name,
> +				  struct strbuf *err)
> +{
> +	int retcode = ref_transaction_commit(*transaction, err);
> +	if (retcode) {
> +		/*
> +		 * Explicitly handle transaction cleanup to avoid
> +		 * aborting an already closed transaction.
> +		 */
> +		ref_transaction_free(*transaction);
> +		*transaction = NULL;
> +	}
> +
> +	if (*transaction && !is_atomic) {
> +		struct ref_rejection_data data = {
> +			.conflict_msg_shown = 0,
> +			.remote_name = remote_name,
> +			.retcode = &retcode,
> +		};
> +
> +		ref_transaction_for_each_rejected_update(*transaction,
> +							 ref_transaction_rejection_handler,
> +							 &data);
> +
> +		ref_transaction_free(*transaction);
> +		*transaction = NULL;
> +	}

Okay. Do we need to discern cases where this is called and we haven't
managed to even queue a single reference update?

> +	return retcode;
> +}
> +
>  static int do_fetch(struct transport *transport,
>  		    struct refspec *rs,
>  		    const struct fetch_config *config)

Nit: it might make sense to have a preparatory commit that extracts the
function but that is otherwise a no-op change.

> @@ -1826,6 +1862,10 @@ static int do_fetch(struct transport *transport,
>  
>  	if (fetch_and_consume_refs(&display_state, transport, transaction, ref_map,
>  				   &fetch_head, config)) {
> +		/* As we're using batched updates, commit any pending updates. */
> +		if (!atomic_fetch)
> +			commit_ref_transaction(&transaction, false,
> +					       transport->remote->name, &err);
>  		retcode = 1;
>  		goto cleanup;
>  	}

Hm. Don't we also have to unset the transaction now? Ah, no, you pass
the pointer to the transaction here and set it to `NULL` in
`commit_ref_transaction()`. Makes sense.

> @@ -1848,8 +1888,12 @@ static int do_fetch(struct transport *transport,
>  			 * the transaction and don't commit anything.
>  			 */
>  			if (backfill_tags(&display_state, transport, transaction, tags_ref_map,
> -					  &fetch_head, config))
> +					  &fetch_head, config)) {
> +				if (!atomic_fetch)
> +					commit_ref_transaction(&transaction, false,
> +							       transport->remote->name, &err);
>  				retcode = 1;
> +			}
>  		}
>  
>  		free_refs(tags_ref_map);

We now have three different callsites where we commit the transaction.
It gets better due to the newly introduced function, but it overall
feels somewhat fragile regardless of that.

Thanks!

Patrick