Ignore empty samples in retained collector

Author: jhawthorn

ext/vernier/vernier.cc

@@ -1074,6 +1074,12 @@ class RetainedCollector : public BaseCollector {
     void record(VALUE obj) {
         RawSample sample;
         sample.sample();
+        if (sample.empty()) {
+            // During thread allocation we allocate one object without a frame
+            // (as of Ruby 3.3)
+            // Ideally we'd allow empty samples to be represented
+            return;
+        }
         int stack_index = frame_list.stack_index(sample);
 
         object_list.push_back(obj);

test/test_retained_memory.rb

@@ -73,6 +73,12 @@ def test_nested_collections
     assert_operator result1.samples.size, :>, result2.samples.size
   end
 
+  def test_thread_allocation
+    result = Vernier.trace_retained do
+      Thread.new { }.join
+    end
+  end
+
   def test_nothing_retained
     result = Vernier.trace_retained do
       100.times {