Merge branch 'js/empty-index-fixes'

A few places failed to differenciate the case where the index is
truly empty (nothing added) and we haven't yet read from the
on-disk index file, which have been corrected.

* js/empty-index-fixes:
  commit -a -m: allow the top-level tree to become empty again
  split-index: accept that a base index can be empty
  do_read_index(): always mark index as initialized unless erroring out

builtin/commit.c

@@ -1002,11 +1002,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		struct object_id oid;
 		const char *parent = "HEAD";
 
-		if (!the_index.cache_nr) {
-			discard_index(&the_index);
-			if (repo_read_index(the_repository) < 0)
-				die(_("Cannot read index"));
-		}
+		if (!the_index.initialized && repo_read_index(the_repository) < 0)
+			die(_("Cannot read index"));
 
 		if (amend)
 			parent = "HEAD^1";

read-cache.c

@@ -2236,6 +2236,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	if (fd < 0) {
 		if (!must_exist && errno == ENOENT) {
 			set_new_index_sparsity(istate);
+			istate->initialized = 1;
 			return 0;
 		}
 		die_errno(_("%s: index file open failed"), path);
@@ -2405,12 +2406,14 @@ int read_index_from(struct index_state *istate, const char *path,
 
 	base_oid_hex = oid_to_hex(&split_index->base_oid);
 	base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
-	trace2_region_enter_printf("index", "shared/do_read_index",
-				   the_repository, "%s", base_path);
-	ret = do_read_index(split_index->base, base_path, 0);
-	trace2_region_leave_printf("index", "shared/do_read_index",
-				   the_repository, "%s", base_path);
-	if (!ret) {
+	if (file_exists(base_path)) {
+		trace2_region_enter_printf("index", "shared/do_read_index",
+					the_repository, "%s", base_path);
+
+		ret = do_read_index(split_index->base, base_path, 0);
+		trace2_region_leave_printf("index", "shared/do_read_index",
+					the_repository, "%s", base_path);
+	} else {
 		char *path_copy = xstrdup(path);
 		char *base_path2 = xstrfmt("%s/sharedindex.%s",
 					   dirname(path_copy), base_oid_hex);

t/t2200-add-update.sh

@@ -197,4 +197,15 @@ test_expect_success '"add -u non-existent" should fail' '
 	! grep "non-existent" actual
 '
 
+test_expect_success '"commit -a" implies "add -u" if index becomes empty' '
+	git rm -rf \* &&
+	git commit -m clean-slate &&
+	test_commit file1 &&
+	rm file1.t &&
+	test_tick &&
+	git commit -a -m remove &&
+	git ls-tree HEAD: >out &&
+	test_must_be_empty out
+'
+
 test_done