Bugfix/segfault when getting surrounding interval of empty cache (#116)

include/message_filters/cache.hpp

@@ -209,6 +209,12 @@ class Cache : public SimpleFilter<M>
     namespace mt = message_filters::message_traits;
 
     std::lock_guard<std::mutex> lock(cache_lock_);
+    if (cache_.size() == 0) {
+      // Early return for empty cache
+      // Logic below is only valid for caches with at least one element
+      return {};
+    }
+
     // Find the starting index. (Find the first index after [or at] the start of the interval)
     int start_index = static_cast<int>(cache_.size()) - 1;
     while (start_index > 0 &&

test/msg_cache_unittest.cpp

@@ -75,6 +75,16 @@ void fillCacheEasy(message_filters::Cache<Msg> & cache, unsigned int start, unsi
   }
 }
 
+TEST(Cache, emptySurroundingInterval)
+{
+  message_filters::Cache<Msg> cache(10);
+  const std::vector<std::shared_ptr<Msg const>> interval_data = cache.getSurroundingInterval(
+    rclcpp::Time(5, 0), rclcpp::Time(9, 0));
+
+  // empty cache shall return empty interval
+  EXPECT_EQ(interval_data.size(), static_cast<unsigned int>(0));
+}
+
 TEST(Cache, easyInterval)
 {
   message_filters::Cache<Msg> cache(10);