Merge branch 'js/sparse-vs-split-index' into jch

Mark in various places in the code that the sparse index and the
split index features are mutually incompatible.

* js/sparse-vs-split-index:
  split-index: it really is incompatible with the sparse index
  t1091: disable split index
  sparse-index: sparse index is disallowed when split index is active

read-cache.c

@@ -3010,6 +3010,9 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 	    !is_null_oid(&istate->split_index->base_oid)) {
 		struct strbuf sb = STRBUF_INIT;
 
+		if (istate->sparse_index)
+			die(_("cannot write split index for a sparse index"));
+
 		err = write_link_extension(&sb, istate) < 0 ||
 			write_index_ext_header(f, eoie_c, CACHE_EXT_LINK,
 					       sb.len) < 0;

sparse-index.c

@@ -136,7 +136,7 @@ static int is_sparse_index_allowed(struct index_state *istate, int flags)
 		/*
 		 * The sparse index is not (yet) integrated with a split index.
 		 */
-		if (istate->split_index)
+		if (istate->split_index || git_env_bool("GIT_TEST_SPLIT_INDEX", 0))
 			return 0;
 		/*
 		 * The GIT_TEST_SPARSE_INDEX environment variable triggers the

split-index.c

@@ -5,6 +5,9 @@
 struct split_index *init_split_index(struct index_state *istate)
 {
 	if (!istate->split_index) {
+		if (istate->sparse_index)
+			die(_("cannot use split index with a sparse index"));
+
 		CALLOC_ARRAY(istate->split_index, 1);
 		istate->split_index->refcount = 1;
 	}

t/t1091-sparse-checkout-builtin.sh

@@ -5,6 +5,9 @@ test_description='sparse checkout builtin tests'
 GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
+GIT_TEST_SPLIT_INDEX=false
+export GIT_TEST_SPLIT_INDEX
+
 . ./test-lib.sh
 
 list_files() {
@@ -234,36 +237,31 @@ test_expect_success 'sparse-checkout disable' '
 '
 
 test_expect_success 'sparse-index enabled and disabled' '
-	(
-		sane_unset GIT_TEST_SPLIT_INDEX &&
-		git -C repo update-index --no-split-index &&
-
-		git -C repo sparse-checkout init --cone --sparse-index &&
-		test_cmp_config -C repo true index.sparse &&
-		git -C repo ls-files --sparse >sparse &&
-		git -C repo sparse-checkout disable &&
-		git -C repo ls-files --sparse >full &&
-
-		cat >expect <<-\EOF &&
-		@@ -1,4 +1,7 @@
-		 a
-		-deep/
-		-folder1/
-		-folder2/
-		+deep/a
-		+deep/deeper1/a
-		+deep/deeper1/deepest/a
-		+deep/deeper2/a
-		+folder1/a
-		+folder2/a
-		EOF
+	git -C repo sparse-checkout init --cone --sparse-index &&
+	test_cmp_config -C repo true index.sparse &&
+	git -C repo ls-files --sparse >sparse &&
+	git -C repo sparse-checkout disable &&
+	git -C repo ls-files --sparse >full &&
 
-		diff -u sparse full | tail -n +3 >actual &&
-		test_cmp expect actual &&
+	cat >expect <<-\EOF &&
+	@@ -1,4 +1,7 @@
+	 a
+	-deep/
+	-folder1/
+	-folder2/
+	+deep/a
+	+deep/deeper1/a
+	+deep/deeper1/deepest/a
+	+deep/deeper2/a
+	+folder1/a
+	+folder2/a
+	EOF
+
+	diff -u sparse full | tail -n +3 >actual &&
+	test_cmp expect actual &&
 
-		git -C repo config --list >config &&
-		! grep index.sparse config
-	)
+	git -C repo config --list >config &&
+	! grep index.sparse config
 '
 
 test_expect_success 'cone mode: init and set' '