Re: [PATCH 2/2] resolve_ref_unsafe(): limit the number of "stat_ref" retries

> From: Michael Haggerty <mhagger@alum.mit.edu>
> 
> If there is a broken symlink where a loose reference file is expected,
> then the attempt to open() it fails with ENOENT. This error is
> misinterpreted to mean that the loose reference file itself has
> disappeared due to a race, causing the lookup to be retried. But in
> this scenario, the retries all suffer from the same problem, causing
> an infinite loop.
> 
> So put a limit (of 5) on the number of times that the stat_ref step
> can be retried.
> 
> Based-on-patch-by: Petr Stodulka <pstodulk@redhat.com>
> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
> ---
>  refs/files-backend.c | 6 ++++--
>  refs/refs-internal.h | 6 ++++++
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index d16feb1..245a0b5 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -1353,6 +1353,7 @@ static int files_read_raw_ref(struct ref_store *ref_store,
>  	int fd;
>  	int ret = -1;
>  	int save_errno;
> +	int retries = 0;
>  
>  	*type = 0;
>  	strbuf_reset(&sb_path);
> @@ -1390,7 +1391,8 @@ static int files_read_raw_ref(struct ref_store *ref_store,
>  	if (S_ISLNK(st.st_mode)) {
>  		strbuf_reset(&sb_contents);
>  		if (strbuf_readlink(&sb_contents, path, 0) < 0) {
> -			if (errno == ENOENT || errno == EINVAL)
> +			if ((errno == ENOENT || errno == EINVAL) &&
> +				retries++ < MAXRETRIES) 
>  				/* inconsistent with lstat; retry */
>  				goto stat_ref;
>  			else
> @@ -1426,7 +1428,7 @@ static int files_read_raw_ref(struct ref_store *ref_store,
>  	 */
>  	fd = open(path, O_RDONLY);
>  	if (fd < 0) {
> -		if (errno == ENOENT)
> +		if (errno == ENOENT && retries++ < MAXRETRIES)
>  			/* inconsistent with lstat; retry */
>  			goto stat_ref;
>  		else
> diff --git a/refs/refs-internal.h b/refs/refs-internal.h
> index 708b260..37e6b99 100644
> --- a/refs/refs-internal.h
> +++ b/refs/refs-internal.h
> @@ -255,6 +255,12 @@ int rename_ref_available(const char *old_refname, const char *new_refname);
>  /* We allow "recursive" symbolic refs. Only within reason, though */
>  #define SYMREF_MAXDEPTH 5
>  
> +/* 
> + * We allow only MAXRETRIES tries to jump on stat_ref, because of possible
> + * infinite loop
> + */
> +#define MAXRETRIES 5
> +
>  /* Include broken references in a do_for_each_ref*() iteration: */
>  #define DO_FOR_EACH_INCLUDE_BROKEN 0x01
>  
>