fix(runtime): snapshot of a story with a sequence fails to load

inkcpp/globals_impl.cpp

@@ -285,20 +285,27 @@ const unsigned char* globals_impl::snap_load(const unsigned char* ptr, const loa
 	);
 	for (size_t i = 0; i < old_capacity; ++i) {
 		hash_t path;
-		ptr                      = snap_read(ptr, path);
+		ptr = snap_read(ptr, path);
+		if (! loader.migratable) {
+			// Not migrating, so this is the story the snapshot was taken from and the counts were loaded
+			// in container order. The stored hash only confirms that; it cannot be used to look the
+			// container up again. A tracked container without a name of its own (a sequence) stores
+			// hash 0, and named containers below an indexed one can share a name, so the lookup finds
+			// nothing or a different container.
+			inkAssert(
+			    _owner->container_data(i)._hash == path,
+			    "Snapshot does not match this story: tracked container %u differs.", ( unsigned ) i
+			);
+			continue;
+		}
 		container_t c_id         = ~0U;
 		ip_t        container_ip = _owner->find_offset_for(path);
 		bool        found        = container_ip != nullptr
 		          && _owner->find_container_id(
 		              static_cast<uint32_t>(container_ip - _owner->instructions()), c_id
 		          );
-		if (! loader.migratable) {
-			inkAssert(found, "Invalid container id reference.");
-			inkAssert(c_id == i, "tracked containere are not allowed to move, expect we migrate");
-		} else {
-			if (found) {
-				_visit_counts.set(c_id, _visit_counts.get_old(i));
-			}
+		if (found) {
+			_visit_counts.set(c_id, _visit_counts.get_old(i));
 		}
 	}
 	if (loader.migratable) {

inkcpp_test/Fixes.cpp

@@ -767,3 +767,38 @@ SCENARIO(
 		}
 	}
 }
+
+SCENARIO(
+    "snapshot restores visit counts of unnamed and same-named containers",
+    "[regression][snapshot][runtime]"
+)
+{
+	// The sequence's container is tracked but has no name of its own, and the labelled gather sits
+	// below a choice's indexed container. Restoring a snapshot used to look each tracked container up
+	// again by name, which found nothing for the first and could find another container for the
+	// second, and threw.
+	GIVEN("a story with a sequence and a labelled gather inside a choice")
+	{
+		std::unique_ptr<story> ink{story::from_file(INK_TEST_RESOURCE_DIR "SnapshotVisitCounts.bin")};
+		runner                 thread = ink->new_runner();
+
+		WHEN("it is played to the first choice and a snapshot is taken")
+		{
+			std::string text = thread->getall();
+			REQUIRE(text == "First visit.\n");
+			REQUIRE(thread->num_choices() == 2);
+			std::unique_ptr<snapshot> snap{thread->create_snapshot()};
+
+			THEN("the snapshot loads into a new runner, which continues the sequence")
+			{
+				runner loaded;
+				REQUIRE_NOTHROW(loaded = ink->new_runner_from_snapshot(*snap));
+				REQUIRE(loaded->num_choices() == 2);
+				loaded->choose(0);
+				REQUIRE(loaded->getall() == "Inside.\n");
+				loaded->choose(0);
+				REQUIRE(loaded->getall() == "Later visit.\n");
+			}
+		}
+	}
+}

inkcpp_test/ink/SnapshotVisitCounts.ink

@@ -0,0 +1,7 @@
+-> knot
+=== knot ===
+{First visit.|Later visit.}
+* [Again]
+  - - (inner) Inside.
+  ** [Back] -> knot
+* [Leave] -> END