commit-reach: properly peel tags

The can_all_from_reach_with_flag() algorithm was refactored in 4fbcca4e
"commit-reach: make can_all_from_reach... linear" but incorrectly
assumed that all objects provided were commits. During a fetch
negotiation, ok_to_give_up() in upload-pack.c may provide unpeeled tags
to the 'from' array. The current code creates a segfault.

Add a direct call to can_all_from_reach_with_flag() in 'test-tool reach'
and add a test in t6600-test-reach.sh that demonstrates this segfault.

Correct the issue by peeling tags when investigating the initial list
of objects in the 'from' array.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-reach.c

@@ -544,20 +544,42 @@ int can_all_from_reach_with_flag(struct object_array *from,
 {
 	struct commit **list = NULL;
 	int i;
+	int nr_commits;
 	int result = 1;
 
 	ALLOC_ARRAY(list, from->nr);
+	nr_commits = 0;
 	for (i = 0; i < from->nr; i++) {
-		list[i] = (struct commit *)from->objects[i].item;
+		struct object *from_one = from->objects[i].item;
 
-		if (parse_commit(list[i]) ||
+		if (!from_one || from_one->flags & assign_flag)
-		    list[i]->generation < min_generation)
+			continue;
-			return 0;
+
+		from_one = deref_tag(the_repository, from_one,
+				     "a from object", 0);
+		if (!from_one || from_one->type != OBJ_COMMIT) {
+			/* no way to tell if this is reachable by
+			 * looking at the ancestry chain alone, so
+			 * leave a note to ourselves not to worry about
+			 * this object anymore.
+			 */
+			from->objects[i].item->flags |= assign_flag;
+			continue;
+		}
+
+		list[nr_commits] = (struct commit *)from_one;
+		if (parse_commit(list[nr_commits]) ||
+		    list[nr_commits]->generation < min_generation) {
+			result = 0;
+			goto cleanup;
+		}
+
+		nr_commits++;
 	}
 
-	QSORT(list, from->nr, compare_commits_by_gen);
+	QSORT(list, nr_commits, compare_commits_by_gen);
 
-	for (i = 0; i < from->nr; i++) {
+	for (i = 0; i < nr_commits; i++) {
 		/* DFS from list[i] */
 		struct commit_list *stack = NULL;
 
@@ -600,7 +622,7 @@ int can_all_from_reach_with_flag(struct object_array *from,
 	}
 
 cleanup:
-	for (i = 0; i < from->nr; i++) {
+	for (i = 0; i < nr_commits; i++) {
 		clear_commit_marks(list[i], RESULT);
 		clear_commit_marks(list[i], assign_flag);
 	}

t/helper/test-reach.c

@@ -31,6 +31,7 @@ int cmd__reach(int ac, const char **av)
 	struct object_id oid_A, oid_B;
 	struct commit *A, *B;
 	struct commit_list *X, *Y;
+	struct object_array X_obj = OBJECT_ARRAY_INIT;
 	struct commit **X_array;
 	int X_nr, X_alloc;
 	struct strbuf buf = STRBUF_INIT;
@@ -49,7 +50,8 @@ int cmd__reach(int ac, const char **av)
 
 	while (strbuf_getline(&buf, stdin) != EOF) {
 		struct object_id oid;
-		struct object *o;
+		struct object *orig;
+		struct object *peeled;
 		struct commit *c;
 		if (buf.len < 3)
 			continue;
@@ -57,14 +59,14 @@ int cmd__reach(int ac, const char **av)
 		if (get_oid_committish(buf.buf + 2, &oid))
 			die("failed to resolve %s", buf.buf + 2);
 
-		o = parse_object(r, &oid);
+		orig = parse_object(r, &oid);
-		o = deref_tag_noverify(o);
+		peeled = deref_tag_noverify(orig);
 
-		if (!o)
+		if (!peeled)
 			die("failed to load commit for input %s resulting in oid %s\n",
 			    buf.buf, oid_to_hex(&oid));
 
-		c = object_as_type(r, o, OBJ_COMMIT, 0);
+		c = object_as_type(r, peeled, OBJ_COMMIT, 0);
 
 		if (!c)
 			die("failed to load commit for input %s resulting in oid %s\n",
@@ -85,6 +87,7 @@ int cmd__reach(int ac, const char **av)
 				commit_list_insert(c, &X);
 				ALLOC_GROW(X_array, X_nr + 1, X_alloc);
 				X_array[X_nr++] = c;
+				add_object_array(orig, NULL, &X_obj);
 				break;
 
 			case 'Y':
@@ -113,6 +116,15 @@ int cmd__reach(int ac, const char **av)
 		print_sorted_commit_ids(list);
 	} else if (!strcmp(av[1], "can_all_from_reach")) {
 		printf("%s(X,Y):%d\n", av[1], can_all_from_reach(X, Y, 1));
+	} else if (!strcmp(av[1], "can_all_from_reach_with_flag")) {
+		struct commit_list *iter = Y;
+
+		while (iter) {
+			iter->item->object.flags |= 2;
+			iter = iter->next;
+		}
+
+		printf("%s(X,_,_,0,0):%d\n", av[1], can_all_from_reach_with_flag(&X_obj, 2, 4, 0, 0));
 	} else if (!strcmp(av[1], "commit_contains")) {
 		struct ref_filter filter;
 		struct contains_cache cache;

t/t6600-test-reach.sh

@@ -31,7 +31,8 @@ test_expect_success 'setup' '
 	for i in $(test_seq 1 10)
 	do
 		test_commit "1-$i" &&
-		git branch -f commit-1-$i
+		git branch -f commit-1-$i &&
+		git tag -a -m "1-$i" tag-1-$i commit-1-$i
 	done &&
 	for j in $(test_seq 1 9)
 	do
@@ -39,11 +40,13 @@ test_expect_success 'setup' '
 		x=$(($j + 1)) &&
 		test_commit "$x-1" &&
 		git branch -f commit-$x-1 &&
+		git tag -a -m "$x-1" tag-$x-1 commit-$x-1 &&
 
 		for i in $(test_seq 2 10)
 		do
 			git merge commit-$j-$i -m "$x-$i" &&
-			git branch -f commit-$x-$i
+			git branch -f commit-$x-$i &&
+			git tag -a -m "$x-$i" tag-$x-$i commit-$x-$i
 		done
 	done &&
 	git commit-graph write --reachable &&
@@ -205,6 +208,29 @@ test_expect_success 'can_all_from_reach:miss' '
 	test_three_modes can_all_from_reach
 '
 
+test_expect_success 'can_all_from_reach_with_flag: tags case' '
+	cat >input <<-\EOF &&
+	X:tag-2-10
+	X:tag-3-9
+	X:tag-4-8
+	X:commit-5-7
+	X:commit-6-6
+	X:commit-7-5
+	X:commit-8-4
+	X:commit-9-3
+	Y:tag-1-9
+	Y:tag-2-8
+	Y:tag-3-7
+	Y:commit-4-6
+	Y:commit-5-5
+	Y:commit-6-4
+	Y:commit-7-3
+	Y:commit-8-1
+	EOF
+	echo "can_all_from_reach_with_flag(X,_,_,0,0):1" >expect &&
+	test_three_modes can_all_from_reach_with_flag
+'
+
 test_expect_success 'commit_contains:hit' '
 	cat >input <<-\EOF &&
 	A:commit-7-7