[MERGE #5296 @atulkatti] MSFT:15975576 Add asserts to help investigating JSHost fuzzing crashes where only the Recycler background thread is around.

Merge pull request #5296 from atulkatti:Bug15975576.ShutdownThreads.1

Author: atulkatti

lib/Common/Memory/Recycler.h

@@ -30,6 +30,10 @@ class JavascriptThreadService;
 struct RecyclerMemoryData;
 #endif
 
+#if DBG
+class ThreadContext;
+#endif
+
 namespace Memory
 {
 // NOTE: There is perf lab test infrastructure that takes a dependency on the events in this enumeration. Any modifications may cause
@@ -650,6 +654,10 @@ struct CollectionParam
 #if ENABLE_CONCURRENT_GC
 class RecyclerParallelThread
 {
+#if DBG
+    friend class ThreadContext;
+#endif
+
 public:
     typedef void (Recycler::* WorkFunc)();
 
@@ -1914,7 +1922,9 @@ class Recycler
     friend class HeapInfo;
     friend class HeapInfoManager;
     friend class LargeHeapBucket;
-
+#if DBG
+    friend class ThreadContext;
+#endif
     template <typename TBlockType>
     friend class HeapBucketT;
     template <typename TBlockType>

lib/Runtime/Base/ThreadContext.cpp

@@ -525,6 +525,12 @@ ThreadContext::~ThreadContext()
 #ifdef ENABLE_SCRIPT_DEBUGGING
         Assert(this->debugManager == nullptr);
 #endif
+
+#if DBG && ENABLE_CONCURRENT_GC && defined(_WIN32)
+        AssertMsg(recycler->concurrentThread == NULL, "Recycler background thread should have been shutdown before destroying Recycler.");
+        AssertMsg((recycler->parallelThread1.concurrentThread == NULL) && (recycler->parallelThread2.concurrentThread == NULL), "Recycler parallelThread(s) should have been shutdown before destroying Recycler.");
+#endif
+
         HeapDelete(recycler);
     }