unpack-trees(): "enable" sparse checkout and load $GIT_DIR/info/sparse-checkout

This patch introduces core.sparseCheckout, which will control whether
sparse checkout support is enabled in unpack_trees()

It also loads sparse-checkout file that will be used in the next patch.
I split it out so the next patch will be shorter, easier to read.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -439,6 +439,10 @@ On some file system/operating system combinations, this is unreliable.
 Set this config setting to 'rename' there; However, This will remove the
 check that makes sure that existing object files will not get overwritten.
 
+core.sparseCheckout::
+	Enable "sparse checkout" feature. See section "Sparse checkout" in
+	linkgit:git-read-tree[1] for more information.
+
 add.ignore-errors::
 	Tells 'git-add' to continue adding files when some files cannot be
 	added due to indexing errors. Equivalent to the '--ignore-errors'

Documentation/git-read-tree.txt

@@ -401,7 +401,9 @@ follows:
 ----------------
 
 Then you can disable sparse checkout. Sparse checkout support in "git
-read-tree" and similar commands is disabled by default.
+read-tree" and similar commands is disabled by default. You need to
+turn `core.sparseCheckout` on in order to have sparse checkout
+support.
 
 
 SEE ALSO

cache.h

@@ -526,6 +526,7 @@ extern size_t delta_base_cache_limit;
 extern int auto_crlf;
 extern int fsync_object_files;
 extern int core_preload_index;
+extern int core_apply_sparse_checkout;
 
 enum safe_crlf {
 	SAFE_CRLF_FALSE = 0,

config.c

@@ -503,6 +503,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.sparsecheckout")) {
+		core_apply_sparse_checkout = git_config_bool(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }

environment.c

@@ -48,6 +48,7 @@ enum push_default_type push_default = PUSH_DEFAULT_MATCHING;
 #endif
 enum object_creation_mode object_creation_mode = OBJECT_CREATION_MODE;
 int grafts_replace_parents = 1;
+int core_apply_sparse_checkout;
 
 /* Parallel index stat data preload? */
 int core_preload_index = 0;

unpack-trees.c

@@ -378,6 +378,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 {
 	int ret;
 	static struct cache_entry *dfc;
+	struct exclude_list el;
 
 	if (len > MAX_UNPACK_TREES)
 		die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);
@@ -387,6 +388,16 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	state.quiet = 1;
 	state.refresh_cache = 1;
 
+	memset(&el, 0, sizeof(el));
+	if (!core_apply_sparse_checkout || !o->update)
+		o->skip_sparse_checkout = 1;
+	if (!o->skip_sparse_checkout) {
+		if (add_excludes_from_file_to_list(git_path("info/sparse-checkout"), "", 0, NULL, &el, 0) < 0)
+			o->skip_sparse_checkout = 1;
+		else
+			o->el = &el;
+	}
+
 	memset(&o->result, 0, sizeof(o->result));
 	o->result.initialized = 1;
 	if (o->src_index) {
@@ -407,26 +418,39 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		info.fn = unpack_callback;
 		info.data = o;
 
-		if (traverse_trees(len, t, &info) < 0)
-			return unpack_failed(o, NULL);
+		if (traverse_trees(len, t, &info) < 0) {
+			ret = unpack_failed(o, NULL);
+			goto done;
+		}
 	}
 
 	/* Any left-over entries in the index? */
 	if (o->merge) {
 		while (o->pos < o->src_index->cache_nr) {
 			struct cache_entry *ce = o->src_index->cache[o->pos];
-			if (unpack_index_entry(ce, o) < 0)
-				return unpack_failed(o, NULL);
+			if (unpack_index_entry(ce, o) < 0) {
+				ret = unpack_failed(o, NULL);
+				goto done;
+			}
 		}
 	}
 
-	if (o->trivial_merges_only && o->nontrivial_merge)
-		return unpack_failed(o, "Merge requires file-level merging");
+	if (o->trivial_merges_only && o->nontrivial_merge) {
+		ret = unpack_failed(o, "Merge requires file-level merging");
+		goto done;
+	}
 
 	o->src_index = NULL;
 	ret = check_updates(o) ? (-2) : 0;
 	if (o->dst_index)
 		*o->dst_index = o->result;
+
+done:
+	for (i = 0;i < el.nr;i++)
+		free(el.excludes[i]);
+	if (el.excludes)
+		free(el.excludes);
+
 	return ret;
 }
 

unpack-trees.h

@@ -4,6 +4,7 @@
 #define MAX_UNPACK_TREES 8
 
 struct unpack_trees_options;
+struct exclude_list;
 
 typedef int (*merge_fn_t)(struct cache_entry **src,
 		struct unpack_trees_options *options);
@@ -28,6 +29,7 @@ struct unpack_trees_options {
 		     skip_unmerged,
 		     initial_checkout,
 		     diff_index_cached,
+		     skip_sparse_checkout,
 		     gently;
 	const char *prefix;
 	int pos;
@@ -44,6 +46,8 @@ struct unpack_trees_options {
 	struct index_state *dst_index;
 	struct index_state *src_index;
 	struct index_state result;
+
+	struct exclude_list *el; /* for internal use */
 };
 
 extern int unpack_trees(unsigned n, struct tree_desc *t,