Add test case to sgen-bridge-pathologies and add an assert to catch the original bug

Author: filipnavara

src/mono/mono/metadata/sgen-tarjan-bridge.c

@@ -794,8 +794,11 @@ create_scc (ScanData *data)
 
 		// Maybe we should make sure we are not adding duplicates here. It is not really a problem
 		// since we will get rid of duplicates before submitting the SCCs to the client in gather_xrefs
-		if (color_data)
+		if (color_data) {
 			add_other_colors (color_data, &other->xrefs);
+		} else {
+			g_assert (dyn_array_ptr_size (&other->xrefs) == 0);
+		}
 		dyn_array_ptr_uninit (&other->xrefs);
 
 		if (other == data) {

src/mono/mono/tests/sgen-bridge-pathologies.cs

@@ -18,6 +18,11 @@ public class NonBridge
 	public object Link;
 }
 
+public class NonBridge2 : NonBridge
+{
+	public object Link2;
+}
+
 class Driver {
 	const int OBJ_COUNT = 200 * 1000;
 	const int LINK_COUNT = 2;
@@ -207,6 +212,32 @@ static void Spider () {
 		c.Links.Add (last_level);
 	}
 
+	/*
+	 * Simulates a graph with two nested cycles that is produces by
+	 * the async state machine when `async Task M()` method gets its
+	 * continuation rooted by an Action held by RunnableImplementor
+	 * (ie. the task continuation is hooked through the SynchronizationContext
+	 * implentation and rooted only by Android bridge objects).
+	 */
+	static void NestedCycles ()
+	{ 
+		Bridge runnableImplementor = new Bridge ();
+		Bridge byteArrayOutputStream = new Bridge ();
+		NonBridge2 action = new NonBridge2 ();
+		NonBridge displayClass = new NonBridge ();
+		NonBridge2 asyncStateMachineBox = new NonBridge2 ();
+		NonBridge2 asyncStreamWriter = new NonBridge2 ();
+
+		runnableImplementor.Links.Add(action);
+		action.Link = displayClass;
+		action.Link2 = asyncStateMachineBox;
+		displayClass.Link = action;
+		asyncStateMachineBox.Link = asyncStreamWriter;
+		asyncStateMachineBox.Link2 = action;
+		asyncStreamWriter.Link = byteArrayOutputStream;
+		asyncStreamWriter.Link2 = asyncStateMachineBox;
+	}
+
 	static void RunTest (ThreadStart setup)
 	{
 		var t = new Thread (setup);
@@ -231,6 +262,7 @@ static int Main ()
 		RunTest (SetupDeadList);
 		RunTest (SetupSelfLinks);
 		RunTest (Spider);
+		RunTest (NestedCycles);
 
 		for (int i = 0; i < 0; ++i) {
 			GC.Collect ();