[PATCH 4/4] restore: add --intent-to-add (restoring worktree only)

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

"git restore --source" (without --staged) could create new files
(i.e. not present in index) on worktree to match the given source. But
the new files are not tracked, so both "git diff" and "git diff
<source>" ignore new files. "git commit -a" will not recreate a commit
exactly as the given source either.

Add --intent-to-add to help track new files in this case, which is the
default on the least surprise principle.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Documentation/git-restore.txt |  7 ++++
 builtin/checkout.c            | 78 +++++++++++++++++++++++++++++++++++
 t/t2070-restore.sh            | 17 ++++++++
 3 files changed, 102 insertions(+)

diff --git a/Documentation/git-restore.txt b/Documentation/git-restore.txt
index d90093f195..43a7f43b2b 100644
--- a/Documentation/git-restore.txt
+++ b/Documentation/git-restore.txt
@@ -93,6 +93,13 @@ in linkgit:git-checkout[1] for details.
 	are "merge" (default) and "diff3" (in addition to what is
 	shown by "merge" style, shows the original contents).
 
+--intent-to-add::
+--no-intent-to-add::
+	When restoring files only on working tree with `--source`,
+	new files are marked as "intent to add" (see
+	linkgit:git-add[1]). This is the default behavior. Use
+	`--no-intent-to-add` to disable it.
+
 --ignore-unmerged::
 	When restoring files on the working tree from the index, do
 	not abort the operation if there are unmerged entries and
diff --git a/builtin/checkout.c b/builtin/checkout.c
index f884d27f1f..c519067d3d 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -70,6 +70,7 @@ struct checkout_opts {
 	int checkout_worktree;
 	const char *ignore_unmerged_opt;
 	int ignore_unmerged;
+	int intent_to_add;
 
 	const char *new_branch;
 	const char *new_branch_force;
@@ -392,6 +393,69 @@ static int checkout_worktree(const struct checkout_opts *opts)
 	return errs;
 }
 
+/*
+ * Input condition: r->index contains the file list matching worktree.
+ *
+ * r->index is reloaded with $GIT_DIR/index. Files that exist in the
+ * current worktree but not in $GIT_DIR/index are added back as
+ * intent-to-add.
+ */
+static int add_intent_to_add_files(struct repository *r)
+{
+	char **file_list;
+	int pos, worktree_nr, ita_nr = 0;
+	int ret = 0;
+
+	worktree_nr = r->index->cache_nr;
+	ALLOC_ARRAY(file_list, worktree_nr);
+	for (pos = 0; pos < worktree_nr; pos++)
+		file_list[pos] = xstrdup(r->index->cache[pos]->name);
+
+	discard_index(r->index);
+	if (repo_read_index(r) < 0) {
+		ret = error(_("index file corrupt"));
+		goto done;
+	}
+
+	for (pos = 0; pos < worktree_nr; ) {
+		const char *worktree = file_list[pos];
+		int index_pos = index_name_pos(r->index, worktree, strlen(worktree));
+
+		if (index_pos < 0) {
+			if (add_file_to_index(r->index, worktree, ADD_CACHE_INTENT))
+				ret = error(_("failed to add %s"), worktree);
+			else
+				ita_nr++;
+			pos++;
+			continue;
+		}
+
+		/*
+		 * Try to speed up the scanning process a bit.
+		 *
+		 * The assumption here is file_list[] and r->index->cache[]
+		 * are 90% the same. We can just skip a big chunk of the same
+		 * entries and reduce the number of binary lookups.
+		 */
+		pos++;
+		index_pos++;
+		while (pos < worktree_nr && index_pos < r->index->cache_nr &&
+		       !fspathcmp(file_list[pos], r->index->cache[index_pos]->name)) {
+			pos++;
+			index_pos++;
+		}
+	}
+
+	if (!ret)
+		ret = ita_nr;
+
+done:
+	for (pos = 0; pos < worktree_nr; pos++)
+		free(file_list[pos]);
+	free(file_list);
+	return ret;
+}
+
 static int checkout_paths(const struct checkout_opts *opts,
 			  const char *revision)
 {
@@ -531,6 +595,16 @@ static int checkout_paths(const struct checkout_opts *opts,
 	else
 		checkout_index = opts->checkout_index;
 
+	if (opts->intent_to_add && opts->from_treeish &&
+	    !opts->checkout_index && opts->checkout_worktree) {
+		int ita_nr = add_intent_to_add_files(the_repository);
+
+		if (ita_nr > 0)
+			checkout_index = 1;
+		if (ita_nr < 0)
+			errs = -1;
+	}
+
 	if (checkout_index) {
 		if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 			die(_("unable to write new index file"));
@@ -1697,6 +1771,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	opts.overlay_mode = -1;
 	opts.checkout_index = -2;    /* default on */
 	opts.checkout_worktree = -2; /* default on */
+	opts.intent_to_add = 0;
 
 	options = parse_options_dup(checkout_options);
 	options = add_common_options(&opts, options);
@@ -1758,6 +1833,8 @@ int cmd_restore(int argc, const char **argv, const char *prefix)
 			   N_("restore the index")),
 		OPT_BOOL('W', "worktree", &opts.checkout_worktree,
 			   N_("restore the working tree (default)")),
+		OPT_BOOL(0, "intent-to-add", &opts.intent_to_add,
+			 N_("mark new files on working tree as intent-to-add (default)")),
 		OPT_BOOL(0, "ignore-unmerged", &opts.ignore_unmerged,
 			 N_("ignore unmerged entries")),
 		OPT_BOOL(0, "overlay", &opts.overlay_mode, N_("use overlay mode")),
@@ -1773,6 +1850,7 @@ int cmd_restore(int argc, const char **argv, const char *prefix)
 	opts.checkout_index = -1;    /* default off */
 	opts.checkout_worktree = -2; /* default on */
 	opts.ignore_unmerged_opt = "--ignore-unmerged";
+	opts.intent_to_add = 1;
 
 	options = parse_options_dup(restore_options);
 	options = add_common_options(&opts, options);
diff --git a/t/t2070-restore.sh b/t/t2070-restore.sh
index 2650df1966..acbd80c1cd 100755
--- a/t/t2070-restore.sh
+++ b/t/t2070-restore.sh
@@ -95,4 +95,21 @@ test_expect_success 'restore --ignore-unmerged ignores unmerged entries' '
 	)
 '
 
+test_expect_success 'restore worktree only adds new files back as intent-to-add' '
+	git init ita &&
+	(
+		cd ita &&
+		test_commit one &&
+		test_commit two &&
+		git rm one.t &&
+		git commit -m one-is-gone &&
+		git restore --source one one.t &&
+		git diff --summary >actual &&
+		echo " create mode 100644 one.t" >expected &&
+		test_cmp expected actual &&
+		git diff --cached >empty &&
+		test_must_be_empty empty
+	)
+'
+
 test_done
-- 
2.22.0.rc0.322.g2b0371e29a