[Bug #18929] Fix heap creation thrashing in GC

Before this commit, if we don't have enough slots after sweeping but
had pages on the tomb heap, then the GC would frequently allocate and
deallocate pages. This is because after sweeping it would set
allocatable pages (since there were not enough slots) but free the
pages on the tomb heap.

This commit reuses pages on the tomb heap if there's not enough slots
after sweeping.

gc.c

@@ -5810,6 +5810,19 @@ gc_sweep_finish_size_pool(rb_objspace_t *objspace, rb_size_pool_t *size_pool)
         min_free_slots = gc_params.heap_init_slots;
     }
 
+    /* If we don't have enough slots and we have pages on the tomb heap, move
+     * pages from the tomb heap to the eden heap. This may prevent page
+     * creation thrashing (frequently allocating and deallocting pages) and
+     * GC thrashing (running GC more frequently than required). */
+    struct heap_page *resurrected_page;
+    while (swept_slots < min_free_slots &&
+            (resurrected_page = heap_page_resurrect(objspace, size_pool))) {
+        swept_slots += resurrected_page->free_slots;
+
+        heap_add_page(objspace, size_pool, heap, resurrected_page);
+        heap_add_freepage(heap, resurrected_page);
+    }
+
     if (swept_slots < min_free_slots) {
         bool grow_heap = is_full_marking(objspace);