Handle frame labels being nil after refinement

Co-authored-by: John Hawthorn <[email protected]>

Author: tenderlove

ext/vernier/vernier.cc

@@ -78,6 +78,9 @@ static const char *gvl_event_name(rb_event_flag_t event) {
 struct FrameInfo {
     static const char *label_cstr(VALUE frame) {
         VALUE label = rb_profile_frame_full_label(frame);
+        // Currently (2025-03-22, Ruby 3.4.2) this occurs when an iseq method
+        // entry is replaced with a refinement
+        if (NIL_P(label)) return "(nil)";
         return StringValueCStr(label);
     }
 
@@ -86,7 +89,7 @@ struct FrameInfo {
         if (NIL_P(file))
             file = rb_profile_frame_path(frame);
         if (NIL_P(file)) {
-            return "";
+            return "(nil)";
         } else {
             return StringValueCStr(file);
         }

test/test_stack_table.rb

@@ -144,6 +144,26 @@ def test_convert
     assert_equal expected, actual
   end
 
+  def test_replacing_with_a_refinement
+    klass = Class.new do
+      def foo
+        stack_table = Vernier::StackTable.new
+        stack_table.stack(stack_table.current_stack)
+      end
+    end
+
+    stack = klass.new.foo
+
+    Module.new do
+      refine klass do
+        def foo
+        end
+      end
+    end
+
+    assert_includes stack[0].to_s, "(nil) at (nil)"
+  end
+
   def test_backtrace
     stack_table = Vernier::StackTable.new
     expected = caller_locations(0); index = stack_table.current_stack