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

From: Junio C Hamano <[email-masked]>
Date: Wed, 03 Sep 2025 11:16:23 -0700

Karthik Nayak <[email-masked]> writes:

> 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'
>
> 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.

Sounds like a sensible way to separate the issues and
responsibilities between higher and lower layers.

> While one solution would be to make the function aware of such issues.
> This feels like...

The first line alone is only half a sentence.  "such issues. This"
-> "such issues, this".

> ... leaking implementation details of file-backend specific
> issues into the utility function.

Very true.

> 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.

So when you are trying to lock "Foo", you lock "foo", for example?
How would that let the generic code liks verify_refname_available
notice that an existing ref "Foo/bar" would crash with the name you
are trying to take, which is now downcased to be "foo"?  I am not
sure if the above explanation is sufficiently clear to convince
readers why it is sufficient..

> Reported-by: Junio C Hamano <[email-masked]>

Hmph, I do not recall reporting anything, but perhaps it was a long
time ago...

> Signed-off-by: Karthik Nayak <[email-masked]>
> ---
>  refs/files-backend.c | 19 +++++++++++++++++--
>  t/t5510-fetch.sh     | 20 ++++++++++++++++++++
>  2 files changed, 37 insertions(+), 2 deletions(-)
>
> 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);
> +		} else {
> +			item = string_list_append(refnames_to_check, refname);
> +		}
> +
>  		item->util = xmalloc(sizeof(update_idx));
>  		memcpy(item->util, &update_idx, sizeof(update_idx));
>  	}
> @@ -2796,7 +2811,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
>  			       "ref_transaction_prepare");
>  	size_t i;
>  	int ret = 0;
> -	struct string_list refnames_to_check = STRING_LIST_INIT_NODUP;
> +	struct string_list refnames_to_check = STRING_LIST_INIT_DUP;
>  	char *head_ref = NULL;
>  	int head_type;
>  	struct files_transaction_backend_data *backend_data;
> 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
>  	)
>  '
>  
> @@ -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
> +	)
> +'
> +
>  . "$TEST_DIRECTORY"/lib-httpd.sh
>  start_httpd