sparse-checkout: refuse to add to bad patterns

When in cone mode sparse-checkout, it is unclear how 'git
sparse-checkout add <dir1> ...' should behave if the existing
sparse-checkout file does not match the cone mode patterns. Change the
behavior to fail with an error message about the existing patterns.

Also, all cone mode patterns start with a '/' character, so add that
restriction. This is necessary for our example test 'cone mode: warn on
bad pattern', but also requires modifying the example sparse-checkout
file we use to test the warnings related to recognizing cone mode
patterns.

This error checking would cause a failure further down the test script
because of a test that adds non-cone mode patterns without cleaning them
up. Perform that cleanup as part of the test now.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/sparse-checkout.c

@@ -598,6 +598,9 @@ static void add_patterns_cone_mode(int argc, const char **argv,
 		die(_("unable to load existing sparse-checkout patterns"));
 	free(sparse_filename);
 
+	if (!existing.use_cone_patterns)
+		die(_("existing sparse-checkout patterns do not use cone mode"));
+
 	hashmap_for_each_entry(&existing.recursive_hashmap, &iter, pe, ent) {
 		if (!hashmap_contains_parent(&pl->recursive_hashmap,
 					pe->pattern, &buffer) ||

dir.c

@@ -727,7 +727,7 @@ static void add_pattern_to_hashsets(struct pattern_list *pl, struct path_pattern
 	}
 
 	if (given->patternlen < 2 ||
-	    *given->pattern == '*' ||
+	    *given->pattern != '/' ||
 	    strstr(given->pattern, "**")) {
 		/* Not a cone pattern. */
 		warning(_("unrecognized pattern: '%s'"), given->pattern);

t/t1091-sparse-checkout-builtin.sh

@@ -110,7 +110,8 @@ test_expect_success 'switching to cone mode with non-cone mode patterns' '
 		git sparse-checkout init &&
 		git sparse-checkout add dir &&
 		git config core.sparseCheckoutCone true &&
-		git sparse-checkout add dir
+		test_must_fail git sparse-checkout add dir 2>err &&
+		grep "existing sparse-checkout patterns do not use cone mode" err
 	)
 '
 
@@ -176,12 +177,14 @@ test_expect_success 'set sparse-checkout using --stdin' '
 '
 
 test_expect_success 'add to sparse-checkout' '
-	cat repo/.git/info/sparse-checkout >expect &&
+	cat repo/.git/info/sparse-checkout >old &&
+	test_when_finished cp old repo/.git/info/sparse-checkout &&
 	cat >add <<-\EOF &&
 	pattern1
 	/folder1/
 	pattern2
 	EOF
+	cat old >expect &&
 	cat add >>expect &&
 	git -C repo sparse-checkout add --stdin <add &&
 	git -C repo sparse-checkout list >actual &&