Re: [PATCH v4 7/8] refs: support rejection in batch updates during F/D checks

[Patrick Steinhardt <ps@pks.im>]

On Thu, Mar 20, 2025 at 12:44:02PM +0100, Karthik Nayak wrote:
> diff --git a/refs.c b/refs.c
> index b34ec198f5..f719046f47 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -2540,6 +2540,7 @@ enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs
>  					  const struct string_list *refnames,
>  					  const struct string_list *extras,
>  					  const struct string_list *skip,
> +					  struct ref_transaction *transaction,
>  					  unsigned int initial_transaction,
>  					  struct strbuf *err)
>  {
> @@ -2599,12 +2601,26 @@ enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs
>  			if (!initial_transaction &&
>  			    !refs_read_raw_ref(refs, dirname.buf, &oid, &referent,
>  					       &type, &ignore_errno)) {
> +				if (transaction && ref_transaction_maybe_set_rejected(
> +					    transaction, *update_idx,
> +					    REF_TRANSACTION_ERROR_NAME_CONFLICT)) {
> +					strset_remove(&dirnames, dirname.buf);
> +					continue;
> +				}
> +

Okay. We have to remove the dirname from `dirnames` again so that the
next reference that creates a reference in the same directory would also
be marked as conflicting. It does have the consequence that we now have
to read the dirname N times again, where N is the number of refs that
are created below that directory.

We could probably improve this by using another map that contains the
conflicting names, right?

> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index be758ffff5..1d50d4013c 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -864,7 +868,9 @@ static enum ref_transaction_error lock_raw_ref(struct files_ref_store *refs,
>  		 * make sure there is no existing packed ref that conflicts
>  		 * with refname. This check is deferred so that we can batch it.
>  		 */
> -		string_list_append(refnames_to_check, refname);
> +		item = string_list_append(refnames_to_check, refname);
> +		item->util = xmalloc(sizeof(update_idx));
> +		memcpy(item->util, &update_idx, sizeof(update_idx));
>  	}
>  
>  	ret = 0;

Hm, so we have to allocate the `util` field now to store the update
index, which is a bit unfortunate because all of this is part of the hot
loop. We cannot store a direct pointer though because the array of
updates may be reallocated, which would invalidate any pointers pointing
into the array.

I was wondering whether we could abuse an `uintptr_t` and use it to
store the update index as a pointer. It does feel somewhat dirty though.

Patrick