Enforce non-null now_fn in TimeControllerClock (#731)

Signed-off-by: Emerson Knapp <[email protected]>

Author: emersonknapp

rosbag2_cpp/src/rosbag2_cpp/clocks/time_controller_clock.cpp

@@ -122,6 +122,9 @@ TimeControllerClock::TimeControllerClock(
   std::chrono::milliseconds sleep_time_while_paused)
 : impl_(std::make_unique<TimeControllerClockImpl>(now_fn, sleep_time_while_paused))
 {
+  if (now_fn == nullptr) {
+    throw std::invalid_argument("TimeControllerClock now_fn must be non-empty.");
+  }
   std::lock_guard<std::mutex> lock(impl_->state_mutex);
   impl_->reference.ros = starting_time;
   impl_->reference.steady = impl_->now_fn();

rosbag2_cpp/test/rosbag2_cpp/test_time_controller_clock.cpp

@@ -43,6 +43,14 @@ class TimeControllerClockTest : public Test
   rcutils_time_point_value_t ros_start_time = 0;
 };
 
+TEST_F(TimeControllerClockTest, must_provide_now_fn)
+{
+  NowFunction empty_now;
+  EXPECT_THROW(
+    rosbag2_cpp::TimeControllerClock(ros_start_time, 1.0, empty_now),
+    std::invalid_argument);
+}
+
 TEST_F(TimeControllerClockTest, steadytime_precision)
 {
   const double playback_rate = 1.0;