Subject: Re: [PATCH v2 5/5] Accelerate clear_skip_worktree_from_present_files() by caching

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 16 Feb 2022 10:32:12 +0100

On Fri, Jan 14 2022, Elijah Newren via GitGitGadget wrote:

> From: Elijah Newren <[email-masked]>
> [...]
> +static int path_found(const char *path, const char **dirname, size_t *dir_len,
> +		      int *dir_found)
> +{
> +	struct stat st;
> +	char *newdir;
> +	char *tmp;
> +
> +	/*
> +	 * If dirname corresponds to a directory that doesn't exist, and this
> +	 * path starts with dirname, then path can't exist.
> +	 */
> +	if (!*dir_found && !memcmp(path, *dirname, *dir_len))
> +		return 0;
> +
> +	/*
> +	 * If path itself exists, return 1.
> +	 */
> +	if (!lstat(path, &st))
> +		return 1;
> +
> +	/*
> +	 * Otherwise, path does not exist so we'll return 0...but we'll first
> +	 * determine some info about its parent directory so we can avoid
> +	 * lstat calls for future cache entries.
> +	 */
> +	newdir = strrchr(path, '/');
> +	if (!newdir)
> +		return 0; /* Didn't find a parent dir; just return 0 now. */
> +
> +	/*
> +	 * If path starts with directory (which we already lstat'ed and found),
> +	 * then no need to lstat parent directory again.
> +	 */
> +	if (*dir_found && *dirname && memcmp(path, *dirname, *dir_len))
> +		return 0;

I really don't care/just asking, but there was a discussion on another
topic about guarding calls to the mem*() family when n=0:
https://lore.kernel.org/git/[email-masked]/

Is this the same sort of redundancy where we could lose the "&&
*dirname" part, or is it still important because a "\0" dirname would
have corresponding non-0 *dir_len?

More generally ... (see below)...

> +
> +	/* Free previous dirname, and cache path's dirname */
> +	*dirname = path;
> +	*dir_len = newdir - path + 1;
> +
> +	tmp = xstrndup(path, *dir_len);
> +	*dir_found = !lstat(tmp, &st);

In most other places we're a bit more careful about lstat() error handling, e.g.:
    
    builtin/init-db.c:              if (lstat(path->buf, &st_git)) {
    builtin/init-db.c-                      if (errno != ENOENT)
    builtin/init-db.c-                              die_errno(_("cannot stat '%s'"), path->buf);
    builtin/init-db.c-              }
    
Shouldn't we do the same here and at least error() on return values of
-1 with an accompanying errno that isn't ENOENT?

> +	free(tmp);
> +
> +	return 0;
> +}
> +
>  void clear_skip_worktree_from_present_files(struct index_state *istate)
>  {
> +	const char *last_dirname = NULL;
> +	size_t dir_len = 0;
> +	int dir_found = 1;
> +
>  	int i;
> +
>  	if (!core_apply_sparse_checkout)
>  		return;
>  
>  restart:
>  	for (i = 0; i < istate->cache_nr; i++) {
>  		struct cache_entry *ce = istate->cache[i];
> -		struct stat st;
>  
> -		if (ce_skip_worktree(ce) && !lstat(ce->name, &st)) {
> +		if (ce_skip_worktree(ce) &&
> +		    path_found(ce->name, &last_dirname, &dir_len, &dir_found)) {

...(continued from above) is the "path is zero" part of this even
reachable? I tried with this on top and ran your tests (and the rest of
t*sparse*.sh) successfully:
	
	diff --git a/sparse-index.c b/sparse-index.c
	index eed170cd8f7..f89c944d8cd 100644
	--- a/sparse-index.c
	+++ b/sparse-index.c
	@@ -403,6 +403,7 @@ void clear_skip_worktree_from_present_files(struct index_state *istate)
	 	for (i = 0; i < istate->cache_nr; i++) {
	 		struct cache_entry *ce = istate->cache[i];
	 
	+		assert(*ce->name);
	 		if (ce_skip_worktree(ce) &&
	 		    path_found(ce->name, &last_dirname, &dir_len, &dir_found)) {
	 			if (S_ISSPARSEDIR(ce->ce_mode)) {

I.e. isn't this undue paranoia about the cache API giving us zero-length
paths?