https://git.ruby-lang.org/ruby.git/commit/?id=e1bf29314f

From e1bf29314feee6aaf535917da0178e868e7ff3fa Mon Sep 17 00:00:00 2001
From: Aaron Patterson <tenderlove@r...>
Date: Wed, 30 Oct 2019 16:28:21 -0700
Subject: Fix zero free objects assertion

This commit is to attempt fixing this error:

  http://ci.rvm.jp/results/trunk-gc-asserts@ruby-sky1/2353281

Each non-full heap_page struct contains a reference to the next page
that contains free slots.  Compaction could fill any page, including
pages that happen to be linked to as "pages which contain free slots".

To fix this, we'll iterate each page, and rebuild the "free page list"
depending on the number of actual free slots on that page.  If there are
no free slots on the page, we'll set the free_next pointer to NULL.

Finally we'll pop one page off the "free page list" and set it as the
"using page" for the next allocation.

diff --git a/gc.c b/gc.c
index 97133e6..22a3d1b 100644
--- a/gc.c
+++ b/gc.c
@@ -8553,14 +8553,14 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl https://github.com/ruby/ruby/blob/trunk/gc.c#L8553
         heap_add_pages(objspace, heap_eden, heap_allocated_pages);
     }
 
-    VALUE moved_list;
+    VALUE moved_list_head;
     VALUE disabled = rb_gc_disable();
 
     if (use_toward_empty) {
-        moved_list = gc_compact_heap(objspace, compare_free_slots);
+        moved_list_head = gc_compact_heap(objspace, compare_free_slots);
     }
     else {
-        moved_list = gc_compact_heap(objspace, compare_pinned);
+        moved_list_head = gc_compact_heap(objspace, compare_pinned);
     }
     heap_eden->freelist = NULL;
 
@@ -8573,32 +8573,46 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl https://github.com/ruby/ruby/blob/trunk/gc.c#L8573
 
     rb_clear_method_cache_by_class(rb_cObject);
     rb_clear_constant_cache();
-    heap_eden->free_pages = NULL;
-    heap_eden->using_page = NULL;
 
-    while (moved_list) {
+    /* Clear RMOVED manual rather than rely on GC */
+    while (moved_list_head) {
         VALUE next_moved;
         struct heap_page *page;
 
-        page = GET_HEAP_PAGE(moved_list);
-        next_moved = RMOVED(moved_list)->next;
+        page = GET_HEAP_PAGE(moved_list_head);
+        next_moved = RMOVED(moved_list_head)->next;
 
-        RMOVED(moved_list)->flags = 0;
-        RMOVED(moved_list)->destination = 0;
-        RMOVED(moved_list)->next = 0;
+        RMOVED(moved_list_head)->flags = 0;
+        RMOVED(moved_list_head)->destination = 0;
+        RMOVED(moved_list_head)->next = 0;
         page->free_slots++;
-        heap_page_add_freeobj(objspace, page, moved_list);
+        heap_page_add_freeobj(objspace, page, moved_list_head);
         if (page->free_slots == page->total_slots && heap_pages_freeable_pages > 0) {
             heap_pages_freeable_pages--;
 	    heap_unlink_page(objspace, heap_eden, page);
 	    heap_add_page(objspace, heap_tomb, page);
-        } else if (page->free_slots == page->total_slots) {
-            page->free_next = NULL;
         }
         objspace->profile.total_freed_objects++;
-        moved_list = next_moved;
+        moved_list_head = next_moved;
     }
 
+    heap_eden->free_pages = NULL;
+
+    /* Rebuild free_pages linked list */
+    size_t i;
+    for (i = 0; i < heap_allocated_pages; ++i) {
+        struct heap_page *page = heap_pages_sorted[i];
+        if (page->free_slots > 0) {
+            page->free_next = heap_eden->free_pages;
+            heap_eden->free_pages = page;
+        } else {
+            page->free_next = NULL;
+        }
+    }
+
+    heap_eden->using_page = heap_eden->free_pages;
+    heap_eden->free_pages = heap_eden->free_pages->free_next;
+
     if (use_verifier) {
         gc_verify_internal_consistency(objspace);
     }
-- 
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