[PATCH] checkout: add --sparse for restoring files in sparse checkout mode

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

"git checkout -- <paths>" is usually used to restore all modified
files in <paths>. In sparse checkout mode, this command is overloaded
with another meaning: to add back all files in <paths> that are
excluded by sparse patterns.

Add "--sparse" option to do what normal mode does: restore all
modified files and nothing else.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 I'm not sure if rejecting --sparse when it has no effects (e.g.
 switching branches or interactive mode) like this patch does is a
 good idea. I think it's good in general to spot unused options. On
 the other hand, people may want to add an alias
 "co = checkout --sparse" to make this behavior "default". In that
 case silently ignoring --sparse may be a good idea.

 Documentation/git-checkout.txt       |  7 +++++++
 builtin/checkout.c                   | 18 ++++++++++++++++--
 t/t1011-read-tree-sparse-checkout.sh | 12 ++++++++++++
 3 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 8edcdca..45a2b53 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -180,6 +180,13 @@ branch by running "git rm -rf ." from the top level of the working tree.
 Afterwards you will be ready to prepare your new files, repopulating the
 working tree, by copying them from elsewhere, extracting a tarball, etc.
 
+
+--sparse::
+	In sparse checkout mode, `git checkout -- <paths>` would
+	update all entries matched by <paths> regardless sparse
+	patterns. This option only updates entries matched by <paths>
+	and sparse patterns.
+
 -m::
 --merge::
 	When switching branches,
diff --git a/builtin/checkout.c b/builtin/checkout.c
index a9c1b5a..3a32145 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -35,6 +35,7 @@ struct checkout_opts {
 	int force_detach;
 	int writeout_stage;
 	int overwrite_ignore;
+	int sparse;
 
 	const char *new_branch;
 	const char *new_branch_force;
@@ -224,7 +225,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 	struct checkout state;
 	static char *ps_matched;
 	unsigned char rev[20];
-	int flag;
+	int flag, sparse;
 	struct commit *head;
 	int errs = 0;
 	int stage = opts->writeout_stage;
@@ -254,9 +255,13 @@ static int checkout_paths(const struct checkout_opts *opts,
 		die(_("Cannot update paths and switch to branch '%s' at the same time."),
 		    opts->new_branch);
 
-	if (opts->patch_mode)
+	if (opts->patch_mode) {
+		if (opts->sparse != -1)
+			die(_("--[no-]sparse cannot be used in interactive mode"));
 		return run_add_interactive(revision, "--patch=checkout",
 					   opts->pathspec);
+	}
+	sparse = opts->sparse != -1 ? opts->sparse : 0;
 
 	lock_file = xcalloc(1, sizeof(struct lock_file));
 
@@ -275,6 +280,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 		struct cache_entry *ce = active_cache[pos];
 		if (opts->source_tree && !(ce->ce_flags & CE_UPDATE))
 			continue;
+		if (sparse && ce_skip_worktree(ce))
+			continue;
 		match_pathspec(opts->pathspec, ce->name, ce_namelen(ce), 0, ps_matched);
 	}
 
@@ -315,6 +322,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 		struct cache_entry *ce = active_cache[pos];
 		if (opts->source_tree && !(ce->ce_flags & CE_UPDATE))
 			continue;
+		if (sparse && ce_skip_worktree(ce))
+			continue;
 		if (match_pathspec(opts->pathspec, ce->name, ce_namelen(ce), 0, NULL)) {
 			if (!ce_stage(ce)) {
 				errs |= checkout_entry(ce, &state, NULL);
@@ -963,6 +972,9 @@ static int checkout_branch(struct checkout_opts *opts,
 	if (opts->pathspec)
 		die(_("paths cannot be used with switching branches"));
 
+	if (opts->sparse != -1)
+		die(_("--[no-]sparse can only be used with paths"));
+
 	if (opts->patch_mode)
 		die(_("'%s' cannot be used with switching branches"),
 		    "--patch");
@@ -1029,6 +1041,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		OPT_STRING(0, "conflict", &conflict_style, N_("style"),
 			   N_("conflict style (merge or diff3)")),
 		OPT_BOOLEAN('p', "patch", &opts.patch_mode, N_("select hunks interactively")),
+		OPT_BOOL(0, "sparse", &opts.sparse, N_("limit pathspecs to sparse entries only")),
 		{ OPTION_BOOLEAN, 0, "guess", &dwim_new_local_branch, NULL,
 		  N_("second guess 'git checkout no-such-branch'"),
 		  PARSE_OPT_NOARG | PARSE_OPT_HIDDEN },
@@ -1039,6 +1052,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	memset(&new, 0, sizeof(new));
 	opts.overwrite_ignore = 1;
 	opts.prefix = prefix;
+	opts.sparse = -1;
 
 	gitmodules_config();
 	git_config(git_checkout_config, &opts);
diff --git a/t/t1011-read-tree-sparse-checkout.sh b/t/t1011-read-tree-sparse-checkout.sh
index 5c0053a..1441fbe 100755
--- a/t/t1011-read-tree-sparse-checkout.sh
+++ b/t/t1011-read-tree-sparse-checkout.sh
@@ -250,4 +250,16 @@ EOF
 	test_cmp expected actual
 '
 
+test_expect_success 'checkout --sparse' '
+	echo "*" >.git/info/sparse-checkout &&
+	git checkout -f top &&
+	test -f init.t &&
+	echo sub >.git/info/sparse-checkout &&
+	git checkout &&
+	echo modified >> sub/added &&
+	git checkout --sparse . &&
+	! test -f init.t &&
+	git diff --exit-code HEAD
+'
+
 test_done
-- 
1.8.2.83.gc99314b