Avoid inspecting hidden classes in capture reports

ext/fiber/profiler/capture.c

@@ -627,6 +627,29 @@ static size_t Fiber_Profiler_Capture_absolute_nesting(struct Fiber_Profiler_Capt
 // If a call is within this threshold of the parent call, it will be skipped when printing the call stack - it's considered inconsequential to the performance of the parent call.
 static const double Fiber_Profiler_Capture_SKIP_THRESHOLD = 0.98;
 
+static VALUE Fiber_Profiler_Capture_class_label(VALUE klass) {
+	// Included modules are represented by hidden ICLASS objects:
+	if (RB_TYPE_P(klass, T_ICLASS)) {
+		klass = RBASIC_CLASS(klass);
+	}
+
+	if (RB_TYPE_P(klass, T_CLASS) || RB_TYPE_P(klass, T_MODULE)) {
+		// Hidden classes/modules cannot be inspected, but `rb_class_path` reads their path directly:
+		if (!RBASIC_CLASS(klass)) {
+			VALUE path = rb_class_path(klass);
+
+			if (RB_TYPE_P(path, T_STRING)) return path;
+
+			return rb_str_new_cstr(RB_TYPE_P(klass, T_CLASS) ? "<hidden class>" : "<hidden module>");
+		}
+
+		// Preserve `rb_inspect` formatting for visible classes, especially singleton classes:
+		return rb_inspect(klass);
+	}
+
+	return rb_str_new_cstr("<unknown>");
+}
+
 void Fiber_Profiler_Capture_print_tty(struct Fiber_Profiler_Capture *capture, FILE *restrict stream, double duration) {
 	double start_time = Fiber_Profiler_Time_delta(&capture->start_time, &capture->switch_time);
 
@@ -677,13 +700,13 @@ void Fiber_Profiler_Capture_print_tty(struct Fiber_Profiler_Capture *capture, FI
 			fprintf(stream, "\e[31m");
 		}
 
-		VALUE class_inspect = rb_inspect(call->klass);
+		VALUE class_label = Fiber_Profiler_Capture_class_label(call->klass);
 		const char *name = rb_id2name(call->id);
 
 		struct timespec offset;
 		Fiber_Profiler_Time_elapsed(&capture->switch_time, &call->enter_time, &offset);
 
-		fprintf(stream, "%s:%d in %s '%s#%s' (%0.4fs, T+" Fiber_Profiler_TIME_PRINTF_TIMESPEC ")\n", call->path, call->line, event_flag_name(call->event_flag), RSTRING_PTR(class_inspect), name, call->duration, Fiber_Profiler_TIME_PRINTF_TIMESPEC_ARGUMENTS(offset));
+		fprintf(stream, "%s:%d in %s '%s#%s' (%0.4fs, T+" Fiber_Profiler_TIME_PRINTF_TIMESPEC ")\n", call->path, call->line, event_flag_name(call->event_flag), RSTRING_PTR(class_label), name, call->duration, Fiber_Profiler_TIME_PRINTF_TIMESPEC_ARGUMENTS(offset));
 
 		fprintf(stream, "\e[0m");
 
@@ -731,15 +754,15 @@ void Fiber_Profiler_Capture_print_json(struct Fiber_Profiler_Capture *capture, F
 			call->nesting = call->parent->nesting + 1;
 		}
 
-		VALUE class_inspect = rb_inspect(call->klass);
+		VALUE class_label = Fiber_Profiler_Capture_class_label(call->klass);
 		const char *name = rb_id2name(call->id);
 
 		size_t nesting = Fiber_Profiler_Capture_absolute_nesting(capture, call);
 
 		struct timespec offset;
 		Fiber_Profiler_Time_elapsed(&capture->switch_time, &call->enter_time, &offset);
 
-		fprintf(stream, "%s{\"path\":\"%s\",\"line\":%d,\"class\":\"%s\",\"method\":\"%s\",\"duration\":%0.6f,\"offset\":" Fiber_Profiler_TIME_PRINTF_TIMESPEC ",\"nesting\":%zu,\"skipped\":%zu,\"filtered\":%zu}", first ? "" : ",", call->path, call->line, RSTRING_PTR(class_inspect), name, call->duration, Fiber_Profiler_TIME_PRINTF_TIMESPEC_ARGUMENTS(offset), nesting, skipped, call->filtered);
+		fprintf(stream, "%s{\"path\":\"%s\",\"line\":%d,\"class\":\"%s\",\"method\":\"%s\",\"duration\":%0.6f,\"offset\":" Fiber_Profiler_TIME_PRINTF_TIMESPEC ",\"nesting\":%zu,\"skipped\":%zu,\"filtered\":%zu}", first ? "" : ",", call->path, call->line, RSTRING_PTR(class_label), name, call->duration, Fiber_Profiler_TIME_PRINTF_TIMESPEC_ARGUMENTS(offset), nesting, skipped, call->filtered);
 
 		skipped = 0;
 		first = 0;

test/fiber/profiler/capture.rb

@@ -140,6 +140,51 @@ def nested(n = 100, &block)
 		end
 	end
 
+	with "hidden classes" do
+		let(:capture) {subject.new(stall_threshold: 0, filter_threshold: 0, output: output)}
+
+		it "can report garbage collection inside methods of hidden classes" do
+			capture.start
+
+			Fiber.new do
+				GC.stress = true
+
+				# Lambda literals call a method on the hidden singleton class of RubyVM::FrozenCore:
+				->{}
+			ensure
+				GC.stress = false
+			end.resume
+
+			capture.stop
+
+			stall = JSON.parse(output.string)
+			calls = stall["calls"]
+			expect(calls).to have_value(have_keys(
+				"class" => be =~ /\A#<Class:0x\h+>\z/,
+				"method" => be == "lambda",
+			))
+		end
+	end
+
+	with "singleton classes" do
+		it "reports the singleton class by name" do
+			capture.start
+
+			Fiber.new do
+				Kernel.sleep 0.001
+			end.resume
+
+			capture.stop
+
+			stall = JSON.parse(output.string)
+			calls = stall["calls"]
+			expect(calls).to have_value(have_keys(
+				"class" => be == "#<Class:Kernel>",
+				"method" => be == "sleep",
+			))
+		end
+	end
+
 	with "Process.fork" do
 		it "should disable the profiler in the child process after fork" do
 			capture.start