Subject: Re: [PATCH 2/2] refs/files: handle F/D conflicts in case-insensitive FS

From: Patrick Steinhardt <[email-masked]>
Date: Wed, 3 Sep 2025 09:40:42 +0200

On Tue, Sep 02, 2025 at 10:34:26AM +0200, Karthik Nayak wrote:
> Similar to the previous commit, when using the files-backend on
> case-insensitive filesystems, there is possibility of hitting F/D
> conflicts when creating references within a single transaction, such as:
> 
>   - 'refs/heads/foo'
>   - 'refs/heads/Foo/bar'

Great, I wanted to ask about this scenario.

> Ideally such conflicts are caught in `refs_verify_refnames_available()`
> which is responsible for checking F/D conflicts within a given
> transaction. This utility function is shared across the reference
> backends. As such, it doesn't consider the issues of using a
> case-insensitive, which only affects the files-backend.
> 
> While one solution would be to make the function aware of such issues.
> This feels like leaking implementation details of file-backend specific
> issues into the utility function. So opt for the more simpler option, of
> lowercasing all references sent to this function when on a
> case-insensitive filesystem and operating on the files-backend.
> 
> To do this, simply use a `struct strbuf` to convert the refname to a
> lower case and append it to the list of refnames to be checked. Since we
> use a `struct strbuf` and the memory is cleared right after, make sure
> that the string list duplicates all provided string.
> 
> Without this change, the user would simply be left with a repository
> with '.lock' files which were created in the 'prepare' phase of the
> transaction, as the 'commit' phase would simply abort and not do the
> necessary cleanup.

Oh, that's a clever hack. Does this also work for the case where we have
preexisting refs already that differ only in casing? I guess it should
given that the lookups we perform should yield those refs regardless of
their casing.

In any case, if we don't already have such a test it would be great to
also verify that this works as expected.

> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 9f58ea4858..466cdfe121 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -869,8 +869,23 @@ static enum ref_transaction_error lock_raw_ref(struct files_ref_store *refs,
>  		 * If the ref did not exist and we are creating it, we have to
>  		 * make sure there is no existing packed ref that conflicts
>  		 * with refname. This check is deferred so that we can batch it.
> +		 *
> +		 * For case-insensitive filesystems, we should also check for F/D
> +		 * conflicts between 'foo' and 'Foo/bar'. So let's lowercase
> +		 * the refname.
>  		 */
> -		item = string_list_append(refnames_to_check, refname);
> +		if (ignore_case) {
> +			struct strbuf lower = STRBUF_INIT;
> +
> +			strbuf_addstr(&lower, refname);
> +			strbuf_tolower(&lower);
> +
> +			item = string_list_append(refnames_to_check, lower.buf);
> +			strbuf_release(&lower);

Can we use `string_list_append_nodup()` together with `strbuf_detach()`
here to avoid one memory allocation?

> +		} else {
> +			item = string_list_append(refnames_to_check, refname);
> +		}
> +
>  		item->util = xmalloc(sizeof(update_idx));
>  		memcpy(item->util, &update_idx, sizeof(update_idx));
>  	}
> diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
> index 57f60da81b..84dc68e5f3 100755
> --- a/t/t5510-fetch.sh
> +++ b/t/t5510-fetch.sh
> @@ -53,6 +53,12 @@ test_expect_success "clone and setup child repos" '
>  		cd case_sensitive &&
>  		git branch branch1 &&
>  		git branch bRanch1
> +	) &&
> +	git clone --ref-format=reftable . case_sensitive_fd &&
> +	(
> +		cd case_sensitive_fd &&
> +		git branch foo/bar &&
> +		git branch Foo
>  	)
>  '
>  

Nice idea to use the reftable format here.

> @@ -1546,6 +1552,20 @@ test_expect_success CASE_INSENSITIVE_FS,REFFILES 'existing references in a case
>  	)
>  '
>  
> +test_expect_success CASE_INSENSITIVE_FS,REFFILES 'F/D conflict on case insensitive filesystem' '
> +	test_when_finished rm -rf case_insensitive &&
> +	(
> +		git init --bare case_insensitive &&
> +		cd case_insensitive &&
> +		git remote add origin -- ../case_sensitive_fd &&
> +		test_must_fail git fetch -f origin "refs/heads/*:refs/heads/*" 2>err &&
> +		test_grep "failed: refname conflict" err &&
> +		git rev-parse refs/heads/main >expect &&
> +		git rev-parse refs/heads/foo/bar >actual &&
> +		test_cmp expect actual
> +	)
> +'

Okay, so we again only end up with one of these references, which is the
best we can do.

atrick