Fix issue dotnet#74741 - assert failure during weak pointer scanning. (…

…dotnet#74810)

The problem here is that during GCScan::GcWeakPtrScan, we call GCHeap::IsPromoted so we can zero out object pointers to objects that weren't marked. At the same time, one thread may do the same for the sync block table. When the first kind of threads validate the object, they also validate the sync block entry that may have already been zeroed - this causes an assert failure. I don't think there is any bad effect apart from the assert failure though.

The fix is to simply call Object::Validate with a bVerifySyncBlock of FALSE if the object hasn't been marked.

src/coreclr/gc/gc.cpp

@@ -4473,8 +4473,12 @@ class CObjectHeader : public Object
         return ((ArrayBase *)this)->GetNumComponents();
     }
 
-    void Validate(BOOL bDeep=TRUE)
+    void Validate(BOOL bDeep=TRUE, BOOL bVerifyNextHeader = FALSE, BOOL bVerifySyncBlock = FALSE)
     {
+        // declaration of extra parameters just so the call site would need no #ifdefs
+        UNREFERENCED_PARAMETER(bVerifyNextHeader);
+        UNREFERENCED_PARAMETER(bVerifySyncBlock);
+
         MethodTable * pMT = GetMethodTable();
 
         _ASSERTE(pMT->SanityCheck());
@@ -45145,15 +45149,10 @@ HRESULT GCHeap::Initialize()
 // GC callback functions
 bool GCHeap::IsPromoted(Object* object)
 {
-#ifdef _DEBUG
-    if (object)
-    {
-        ((CObjectHeader*)object)->Validate();
-    }
-#endif //_DEBUG
-
     uint8_t* o = (uint8_t*)object;
 
+    bool is_marked;
+
     if (gc_heap::settings.condemned_generation == max_generation)
     {
 #ifdef MULTIPLE_HEAPS
@@ -45165,27 +45164,35 @@ bool GCHeap::IsPromoted(Object* object)
 #ifdef BACKGROUND_GC
         if (gc_heap::settings.concurrent)
         {
-            bool is_marked = (!((o < hp->background_saved_highest_address) && (o >= hp->background_saved_lowest_address))||
+            is_marked = (!((o < hp->background_saved_highest_address) && (o >= hp->background_saved_lowest_address))||
                             hp->background_marked (o));
-            return is_marked;
         }
         else
 #endif //BACKGROUND_GC
         {
-            return (!((o < hp->highest_address) && (o >= hp->lowest_address))
-                    || hp->is_mark_set (o));
+            is_marked = (!((o < hp->highest_address) && (o >= hp->lowest_address))
+                        || hp->is_mark_set (o));
         }
     }
     else
     {
 #ifdef USE_REGIONS
-        return (gc_heap::is_in_gc_range (o) ? (gc_heap::is_in_condemned_gc (o) ? gc_heap::is_mark_set (o) : true) : true);
+        is_marked = (gc_heap::is_in_gc_range (o) ? (gc_heap::is_in_condemned_gc (o) ? gc_heap::is_mark_set (o) : true) : true);
 #else
         gc_heap* hp = gc_heap::heap_of (o);
-        return (!((o < hp->gc_high) && (o >= hp->gc_low))
-                || hp->is_mark_set (o));
+        is_marked = (!((o < hp->gc_high) && (o >= hp->gc_low))
+                   || hp->is_mark_set (o));
 #endif //USE_REGIONS
     }
+
+#ifdef _DEBUG
+    if (o)
+    {
+        ((CObjectHeader*)o)->Validate(TRUE, TRUE, is_marked);
+    }
+#endif //_DEBUG
+
+    return is_marked;
 }
 
 size_t GCHeap::GetPromotedBytes(int heap_index)