re-implemented dotnet#2103 on top of refactoring (it was hard to merge)

src/coreclr/src/gc/gc.cpp

@@ -34004,7 +34004,7 @@ void gc_heap::background_sweep()
     //block concurrent allocation for large objects
     dprintf (3, ("lh state: planning"));
 
-    for (int i = 0; i < total_generation_count; i++)
+    for (int i = 0; i <= max_generation; i++)
     {
         generation* gen_to_reset = generation_of (i);
         generation_allocator (gen_to_reset)->clear();
@@ -34128,6 +34128,19 @@ void gc_heap::background_sweep()
             start_seg = gen_start_seg;
             prev_seg = NULL;
             align_const = get_alignment_constant (FALSE);
+
+            // UOH allocations are possible while sweeping SOH, so
+            // we defer clearing UOH free lists until we start sweeping them
+            generation_allocator (gen)->clear();
+            generation_free_list_space (gen) = 0;
+            generation_free_obj_space (gen) = 0;
+            generation_free_list_allocated (gen) = 0;
+            generation_end_seg_allocated (gen) = 0;
+            generation_condemned_allocated (gen) = 0;
+            generation_sweep_allocated (gen) = 0;
+            generation_allocation_pointer (gen)= 0;
+            generation_allocation_limit (gen) = 0;
+            generation_allocation_segment (gen) = heap_segment_rw (generation_start_segment (gen));
         }
 
         PREFIX_ASSUME(start_seg != NULL);