read-tree: integrate with sparse index

Add repository setting setup for enabling sparse index usage with `read-tree`.
In order to maintain behavior (and allow for focused adjustment of
implementation in later commits), index is expanded when `--no-sparse-checkout`
or `--prefix` is specified.

Substantial performance improvement for sparse index indicated by `p2000`
results:

Test                                 before            after
-----------------------------------------------------------------------------
git read-tree -mu HEAD (full-v3)     1.21(0.94+0.30)   1.37(1.06+0.33) +13.2%
git read-tree -mu HEAD (full-v4)     1.18(0.90+0.28)   1.35(1.06+0.31) +14.4%
git read-tree -mu HEAD (sparse-v3)   1.61(1.23+0.40)   0.14(0.07+0.05) -91.3%
git read-tree -mu HEAD (sparse-v4)   2.16(1.52+0.48)   0.14(0.07+0.05) -93.5%

Signed-off-by: Victoria Dye <[email protected]>

Author: vdye

builtin/read-tree.c

@@ -168,12 +168,15 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 	argc = parse_options(argc, argv, cmd_prefix, read_tree_options,
 			     read_tree_usage, 0);
 
-	hold_locked_index(&lock_file, LOCK_DIE_ON_ERROR);
-
 	prefix_set = opts.prefix ? 1 : 0;
 	if (1 < opts.merge + opts.reset + prefix_set)
 		die("Which one? -m, --reset, or --prefix?");
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
+	hold_locked_index(&lock_file, LOCK_DIE_ON_ERROR);
+
 	/*
 	 * NEEDSWORK
 	 *
@@ -214,6 +217,10 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 	if (opts.merge && !opts.index_only)
 		setup_work_tree();
 
+	/* TODO: audit sparse index behavior in unpack_trees */
+	if (opts.skip_sparse_checkout || opts.prefix)
+		ensure_full_index(&the_index);
+
 	if (opts.merge) {
 		switch (stage - 1) {
 		case 0:
@@ -223,11 +230,21 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 			opts.fn = opts.prefix ? bind_merge : oneway_merge;
 			break;
 		case 2:
+			/*
+			 * TODO: update twoway_merge to handle edit/edit conflicts in
+			 * sparse directories.
+			 */
+			ensure_full_index(&the_index);
 			opts.fn = twoway_merge;
 			opts.initial_checkout = is_cache_unborn();
 			break;
 		case 3:
 		default:
+			/*
+			 * TODO: update threeway_merge to handle edit/edit conflicts in
+			 * sparse directories.
+			 */
+			ensure_full_index(&the_index);
 			opts.fn = threeway_merge;
 			break;
 		}

t/t1092-sparse-checkout-compatibility.sh

@@ -1250,6 +1250,17 @@ test_expect_success 'sparse index is not expanded: update-index' '
 	ensure_not_expanded update-index --add --remove --again
 '
 
+test_expect_success 'sparse index is not expanded: read-tree' '
+	init_repos &&
+
+	ensure_not_expanded checkout -b test-branch update-folder1 &&
+	for MERGE_TREES in "update-folder2"
+	do
+		ensure_not_expanded read-tree -mu $MERGE_TREES &&
+		ensure_not_expanded reset --hard HEAD || return 1
+	done
+'
+
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout
 # in this scenario, but it shouldn't.
 test_expect_success 'reset mixed and checkout orphan' '