Subject: Re: [PATCH 2/2] restore: avoid sparse index expansion

From: Junio C Hamano <[email-masked]>
Date: Mon, 25 May 2026 08:05:30 +0900

"Derrick Stolee via GitGitGadget" <[email-masked]> writes:

> From: Derrick Stolee <[email-masked]>
>
> Teach update_some() to handle sparse directory entries at the tree
> level rather than expanding the entire sparse index. When iterating a
> source tree during checkout/restore operations:
>
>  - If a directory matches a sparse directory entry with the same OID,
>    skip it entirely (no change needed).
>
>  - If the OID differs and we are in non-overlay mode (e.g., restore
>    --staged), update the sparse directory entry's OID in place. This
>    is semantically correct because non-overlay mode removes paths not
>    in the source tree anyway.
>
>  - In overlay mode (e.g., checkout <tree> -- .), fall through to
>    recursive descent so individual file entries are preserved
>    correctly.
>
> Also switch from index_name_pos() to index_name_pos_sparse() for
> individual file lookups to avoid triggering ensure_full_index() when
> the file is already individually tracked in the index.
>
> Update the test expectation in t1092 to assert that 'restore --staged'
> no longer expands the sparse index.
>
> Signed-off-by: Derrick Stolee <[email-masked]>
> ---
>  builtin/checkout.c                       | 57 +++++++++++++++++++++---
>  t/t1092-sparse-checkout-compatibility.sh |  8 ++--
>  2 files changed, 55 insertions(+), 10 deletions(-)
>
> diff --git a/builtin/checkout.c b/builtin/checkout.c
> index 1345e8574a..67f03dea10 100644
> --- a/builtin/checkout.c
> +++ b/builtin/checkout.c
> @@ -31,6 +31,7 @@
>  #include "revision.h"
>  #include "sequencer.h"
>  #include "setup.h"
> +#include "sparse-index.h"
>  #include "strvec.h"
>  #include "submodule.h"
>  #include "symlinks.h"
> @@ -142,14 +143,56 @@ static int post_checkout_hook(struct commit *old_commit, struct commit *new_comm
>  }
>  
>  static int update_some(const struct object_id *oid, struct strbuf *base,
> -		       const char *pathname, unsigned mode, void *context UNUSED)
> +		       const char *pathname, unsigned mode, void *context)
>  {
>  	int len;
>  	struct cache_entry *ce;
>  	int pos;
> +	int overlay_mode = context ? *((int *)context) : 1;
>  
> -	if (S_ISDIR(mode))
> +	if (S_ISDIR(mode)) {
> +		/*
> +		 * If this directory exists as a sparse directory entry in
> +		 * the index, we can handle it at the tree level without
> +		 * descending into individual files.
> +		 */
> +		if (the_repository->index->sparse_index) {

I wonder if this deep nesting is a sign that the newly added code
from here to ...

> +			struct strbuf dirpath = STRBUF_INIT;
> +
> +			strbuf_addbuf(&dirpath, base);
> +			strbuf_addstr(&dirpath, pathname);
> +			strbuf_addch(&dirpath, '/');
> +
> +			pos = index_name_pos_sparse(the_repository->index,
> +						    dirpath.buf, dirpath.len);
> +			if (pos >= 0) {
> +				struct cache_entry *old =
> +					the_repository->index->cache[pos];
> +				if (S_ISSPARSEDIR(old->ce_mode)) {
> +					if (oideq(oid, &old->oid)) {
> +						strbuf_release(&dirpath);
> +						return 0;
> +					}
> +					if (!overlay_mode) {
> +						/*
> +						 * In non-overlay mode (e.g.,
> +						 * restore --staged), we can
> +						 * replace the sparse dir OID
> +						 * directly since files not in
> +						 * the source tree should be
> +						 * removed anyway.
> +						 */
> +						oidcpy(&old->oid, oid);
> +						old->ce_flags |= CE_UPDATE;
> +						strbuf_release(&dirpath);
> +						return 0;
> +					}
> +				}
> +			}
> +			strbuf_release(&dirpath);
> +		}

... here may become easier to understand if it is made into a small
helper function with a descriptive name.

>  		return READ_TREE_RECURSIVE;
> +	}
>  
>  	len = base->len + strlen(pathname);
>  	ce = make_empty_cache_entry(the_repository->index, len);
> @@ -165,7 +208,7 @@ static int update_some(const struct object_id *oid, struct strbuf *base,
>  	 * entry in place. Whether it is UPTODATE or not, checkout_entry will
>  	 * do the right thing.
>  	 */
> -	pos = index_name_pos(the_repository->index, ce->name, ce->ce_namelen);
> +	pos = index_name_pos_sparse(the_repository->index, ce->name, ce->ce_namelen);
>  	if (pos >= 0) {
>  		struct cache_entry *old = the_repository->index->cache[pos];
>  		if (ce->ce_mode == old->ce_mode &&
> @@ -182,10 +225,11 @@ static int update_some(const struct object_id *oid, struct strbuf *base,
>  	return 0;
>  }
>  
> -static int read_tree_some(struct tree *tree, const struct pathspec *pathspec)
> +static int read_tree_some(struct tree *tree, const struct pathspec *pathspec,
> +			  int overlay_mode)
>  {
>  	read_tree(the_repository, tree,
> -		  pathspec, update_some, NULL);
> +		  pathspec, update_some, &overlay_mode);
>  
>  	/* update the index with the given tree's info
>  	 * for all args, expanding wildcards, and exit
> @@ -580,7 +624,8 @@ static int checkout_paths(const struct checkout_opts *opts,
>  		return error(_("index file corrupt"));
>  
>  	if (opts->source_tree)
> -		read_tree_some(opts->source_tree, &opts->pathspec);
> +		read_tree_some(opts->source_tree, &opts->pathspec,
> +			       opts->overlay_mode);
>  	if (opts->merge)
>  		unmerge_index(the_repository->index, &opts->pathspec, CE_MATCHED);
>  
> diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
> index d69434e7ab..8186da5c88 100755
> --- a/t/t1092-sparse-checkout-compatibility.sh
> +++ b/t/t1092-sparse-checkout-compatibility.sh
> @@ -2608,19 +2608,19 @@ test_expect_success 'restore --staged with wildcards' '
>  	test_all_match git diff --cached
>  '
>  
> -test_expect_success 'sparse-index is expanded: restore --staged' '
> +test_expect_success 'sparse-index is not expanded: restore --staged' '
>  	init_repos &&
>  
>  	git -C sparse-index checkout -b restore-staged-exp base &&
>  	git -C sparse-index reset --soft update-folder1 &&
> -	ensure_expanded restore --staged .
> +	ensure_not_expanded restore --staged .
>  '
>  
> -test_expect_success 'sparse-index is expanded: restore --source --staged' '
> +test_expect_success 'sparse-index is not expanded: restore --source --staged' '
>  	init_repos &&
>  
>  	git -C sparse-index checkout -b restore-source-staged base &&
> -	ensure_expanded restore --source update-folder1 --staged .
> +	ensure_not_expanded restore --source update-folder1 --staged .
>  '

Very nice.