https://git.ruby-lang.org/ruby.git/commit/?id=e5fe48646c

From e5fe48646c68be0bd45fb8a1d7de36da00eaec4d Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@p...>
Date: Tue, 29 Jun 2021 14:32:54 -0400
Subject: [Bug #18014] Add assertion to verify freelist

This commit adds an assertion has been added after `gc_page_sweep` to
verify that the freelist length is equal to the number of free slots in
the page.
---
 gc.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/gc.c b/gc.c
index 9af83de..9ccd26d 100644
--- a/gc.c
+++ b/gc.c
@@ -5483,6 +5483,18 @@ gc_page_sweep(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_ https://github.com/ruby/ruby/blob/trunk/gc.c#L5483
         }
     }
 
+#if RGENGC_CHECK_MODE
+    short freelist_len = 0;
+    RVALUE *ptr = sweep_page->freelist;
+    while (ptr) {
+        freelist_len++;
+        ptr = ptr->as.free.next;
+    }
+    if (freelist_len != sweep_page->free_slots) {
+        rb_bug("inconsistent freelist length: expected %d but was %d", sweep_page->free_slots, freelist_len);
+    }
+#endif
+
     gc_report(2, objspace, "page_sweep: end.\n");
 
     return ctx.freed_slots + ctx.empty_slots;
-- 
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