Subject: Re: [PATCH v2 12/17] mktree: create tree using an in-core index

From: Junio C Hamano <[email-masked]>
Date: Thu, 20 Jun 2024 15:26:09 -0700

"Victoria Dye via GitGitGadget" <[email-masked]> writes:

> @@ -60,17 +66,25 @@ static void append_to_tree(unsigned mode, struct object_id *oid, const char *pat
>  	if (literally) {
>  		FLEX_ALLOC_MEM(ent, name, path, len);
>  	} else {
> +		size_t len_to_copy = len;
> +
>  		/* Normalize and validate entry path */
>  		if (S_ISDIR(mode)) {
> -			while(len > 0 && is_dir_sep(path[len - 1]))
> -				len--;
> +			while(len_to_copy > 0 && is_dir_sep(path[len_to_copy - 1]))

Let's fix the style issue while at it, as we are doing other changes
in this step anyway.  "while(" -> "while (".

> +				len_to_copy--;
> +			len = len_to_copy + 1; /* add space for trailing slash */

Do we need to do st_add() here?  Perhaps not, but I just noticed the
careful use of st_add3() below, so...

> +		ent = xcalloc(1, st_add3(sizeof(struct tree_entry), len, 1));

> +		memcpy(ent->name, path, len_to_copy);
>  
>  		if (!verify_path(ent->name, mode))
>  			die(_("invalid path '%s'"), path);
>  		if (strchr(ent->name, '/'))
>  			die("path %s contains slash", path);
> +
> +		/* Add trailing slash to dir */
> +		if (S_ISDIR(mode))
> +			ent->name[len - 1] = '/';

OK.

> @@ -88,11 +102,14 @@ static int ent_compare(const void *a_, const void *b_, void *ctx)
>  	struct tree_entry *b = *(struct tree_entry **)b_;
>  	int ignore_mode = *((int *)ctx);
>  
> -	if (ignore_mode)
> -		cmp = name_compare(a->name, a->len, b->name, b->len);
> -	else
> -		cmp = base_name_compare(a->name, a->len, a->mode,
> -					b->name, b->len, b->mode);
> +	size_t a_len = a->len, b_len = b->len;
> +
> +	if (ignore_mode) {
> +		a_len = df_path_len(a_len, a->mode);
> +		b_len = df_path_len(b_len, b->mode);
> +	}
> +
> +	cmp = name_compare(a->name, a_len, b->name, b_len);
>  	return cmp ? cmp : b->order - a->order;
>  }

OK, now the "mode" is sort of "encoded" already in the "name" by the
slash at the end, the way "ignore-mode" works needs to be redesigned.

If we are ignoring mode, we are dropping the trailing '/' and
otherwise we just feed the name with possible trailing '/', and the
same name_compare() can be used.  OK.

> @@ -108,8 +125,8 @@ static void sort_and_dedup_tree_entry_array(struct tree_entry_array *arr)
>  	for (size_t i = 0; i < count; i++) {
>  		struct tree_entry *curr = arr->entries[i];
>  		if (prev &&
> -		    !name_compare(prev->name, prev->len,
> -				  curr->name, curr->len)) {
> +		    !name_compare(prev->name, df_path_len(prev->len, prev->mode),
> +				  curr->name, df_path_len(curr->len, curr->mode))) {
>  			FREE_AND_NULL(curr);

And here is the matching adjustment for the dedup comparison, which
makes sense.

> @@ -122,24 +139,43 @@ static void sort_and_dedup_tree_entry_array(struct tree_entry_array *arr)
>  	QSORT_S(arr->entries, arr->nr, ent_compare, &ignore_mode);
>  }
>  
> +static int add_tree_entry_to_index(struct index_state *istate,
> +				   struct tree_entry *ent)
> +{
> +	struct cache_entry *ce;
> +	struct strbuf ce_name = STRBUF_INIT;
> +	strbuf_add(&ce_name, ent->name, ent->len);
> +

Perhaps swap the first statement (which is strbuf_add()) and the
blank line that ought to separate the decls and the first statement?

> +	ce = make_cache_entry(istate, ent->mode, &ent->oid, ent->name, 0, 0);
> +	if (!ce)
> +		return error(_("make_cache_entry failed for path '%s'"), ent->name);
> +
> +	add_index_entry(istate, ce, ADD_CACHE_JUST_APPEND);
> +	strbuf_release(&ce_name);
> +	return 0;
> +}

This is only to append; presumably the caller drives this function
out of a sorted list.

>  static void write_tree(struct tree_entry_array *arr, struct object_id *oid)
>  {
> +	struct index_state istate = INDEX_STATE_INIT(the_repository);
> +	istate.sparse_index = 1;
>  
>  	sort_and_dedup_tree_entry_array(arr);
>  
> +	/* Construct an in-memory index from the provided entries */
>  	for (size_t i = 0; i < arr->nr; i++) {
>  		struct tree_entry *ent = arr->entries[i];
> +
> +		if (add_tree_entry_to_index(&istate, ent))
> +			die(_("failed to add tree entry '%s'"), ent->name);
>  	}
> +	/* Write out new tree */
> +	if (cache_tree_update(&istate, WRITE_TREE_SILENT | WRITE_TREE_MISSING_OK))
> +		die(_("failed to write tree"));

Hmph.  Are we doing any run-time verification of what we produce
(e.g., if sort_and_dedup_tree_entry_array() fails to dedup or sort
correctly due to a bug or two, would cache_tree_update() notice that
the in-core index array is fishy)?  I am not suggesting to add an
unconditional "we appended to the index, so we should sort the
entries in it" step before cache_tree_update() call.  It is the
opposite---if we have extra checks in cache_tree_udpate() to slow us
down and if we are confident that the loop that added tree entries
to the index is correct, if we can bypass such checks.

> +	oidcpy(oid, &istate.cache_tree->oid);
> +
> +	release_index(&istate);
>  }

This is the gem of the whole series.  Clever.

What is so satisfying is that it takes not that much of code to
replace the "here is a flat buffer of what the contents of a single
tree object ought to look like" with "let's build in-core index and
write it out just like write-tree would".  Nice.