sparse-checkout: stop blocking empty workdirs

Remove the error condition when updating the sparse-checkout leaves
an empty working directory.

This behavior was added in 9e1afb167 (sparse checkout: inhibit empty
worktree, 2009-08-20). The comment was added in a7bc906f2 (Add
explanation why we do not allow to sparse checkout to empty working
tree, 2011-09-22) in response to a "dubious" comment in 84563a624
(unpack-trees.c: cosmetic fix, 2010-12-22).

With the recent "cone mode" and "git sparse-checkout init [--cone]"
command, it is common to set a reasonable sparse-checkout pattern
set of

	/*
	!/*/

which matches only files at root. If the repository has no such files,
then their "git sparse-checkout init" command will fail.

Now that we expect this to be a common pattern, we should not have the
commands fail on an empty working directory. If it is a confusing
result, then the user can recover with "git sparse-checkout disable"
or "git sparse-checkout set". This is especially simple when using cone
mode.

Reported-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1011-read-tree-sparse-checkout.sh

@@ -74,13 +74,19 @@ test_expect_success 'read-tree --no-sparse-checkout with empty .git/info/sparse-
 test_expect_success 'read-tree with empty .git/info/sparse-checkout' '
 	git config core.sparsecheckout true &&
 	echo >.git/info/sparse-checkout &&
-	read_tree_u_must_fail -m -u HEAD &&
+	read_tree_u_must_succeed -m -u HEAD &&
 	git ls-files --stage >result &&
 	test_cmp expected result &&
 	git ls-files -t >result &&
+	cat >expected.swt <<-\EOF &&
+	S init.t
+	S sub/added
+	S sub/addedtoo
+	S subsub/added
+	EOF
 	test_cmp expected.swt result &&
-	test -f init.t &&
+	! test -f init.t &&
-	test -f sub/added
+	! test -f sub/added
 '
 
 test_expect_success 'match directories with trailing slash' '

t/t1091-sparse-checkout-builtin.sh

@@ -106,10 +106,8 @@ test_expect_success 'set enables config' '
 		cd empty-config &&
 		test_commit test file &&
 		test_path_is_missing .git/config.worktree &&
-		test_must_fail git sparse-checkout set nothing &&
+		git sparse-checkout set nothing &&
 		test_path_is_file .git/config.worktree &&
-		test_must_fail git config core.sparseCheckout &&
-		git sparse-checkout set "/*" &&
 		test_cmp_config true core.sparseCheckout
 	)
 '
@@ -302,8 +300,8 @@ test_expect_success 'revert to old sparse-checkout on empty update' '
 		echo >file &&
 		git add file &&
 		git commit -m "test" &&
-		test_must_fail git sparse-checkout set nothing 2>err &&
+		git sparse-checkout set nothing 2>err &&
-		test_i18ngrep "Sparse checkout leaves no entry on working directory" err &&
+		test_i18ngrep ! "Sparse checkout leaves no entry on working directory" err &&
 		test_i18ngrep ! ".git/index.lock" err &&
 		git sparse-checkout set file
 	)

unpack-trees.c

@@ -1677,8 +1677,6 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	}
 
 	if (!o->skip_sparse_checkout) {
-		int empty_worktree = 1;
-
 		/*
 		 * Sparse checkout loop #2: set NEW_SKIP_WORKTREE on entries not in loop #1
 		 * If they will have NEW_SKIP_WORKTREE, also set CE_SKIP_WORKTREE
@@ -1706,19 +1704,6 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 
 			if (apply_sparse_checkout(&o->result, ce, o))
 				ret = 1;
-
-			if (!ce_skip_worktree(ce))
-				empty_worktree = 0;
-		}
-		/*
-		 * Sparse checkout is meant to narrow down checkout area
-		 * but it does not make sense to narrow down to empty working
-		 * tree. This is usually a mistake in sparse checkout rules.
-		 * Do not allow users to do that.
-		 */
-		if (o->result.cache_nr && empty_worktree) {
-			ret = unpack_failed(o, "Sparse checkout leaves no entry on working directory");
-			goto done;
 		}
 		if (ret == 1) {
 			/*
@@ -1779,7 +1764,7 @@ enum update_sparsity_result update_sparsity(struct unpack_trees_options *o)
 {
 	enum update_sparsity_result ret = UPDATE_SPARSITY_SUCCESS;
 	struct pattern_list pl;
-	int i, empty_worktree;
+	int i;
 	unsigned old_show_all_errors;
 	int free_pattern_list = 0;
 
@@ -1810,7 +1795,6 @@ enum update_sparsity_result update_sparsity(struct unpack_trees_options *o)
 
 	/* Then loop over entries and update/remove as needed */
 	ret = UPDATE_SPARSITY_SUCCESS;
-	empty_worktree = 1;
 	for (i = 0; i < o->src_index->cache_nr; i++) {
 		struct cache_entry *ce = o->src_index->cache[i];
 
@@ -1824,28 +1808,12 @@ enum update_sparsity_result update_sparsity(struct unpack_trees_options *o)
 
 		if (apply_sparse_checkout(o->src_index, ce, o))
 			ret = UPDATE_SPARSITY_WARNINGS;
-
-		if (!ce_skip_worktree(ce))
-			empty_worktree = 0;
-	}
-
-	/*
-	 * Sparse checkout is meant to narrow down checkout area
-	 * but it does not make sense to narrow down to empty working
-	 * tree. This is usually a mistake in sparse checkout rules.
-	 * Do not allow users to do that.
-	 */
-	if (o->src_index->cache_nr && empty_worktree) {
-		unpack_failed(o, "Sparse checkout leaves no entry on working directory");
-		ret = UPDATE_SPARSITY_INDEX_UPDATE_FAILURES;
-		goto done;
 	}
 
 skip_sparse_checkout:
 	if (check_updates(o, o->src_index))
 		ret = UPDATE_SPARSITY_WORKTREE_UPDATE_FAILURES;
 
-done:
 	display_warning_msgs(o);
 	o->show_all_errors = old_show_all_errors;
 	if (free_pattern_list)