fixes #4851 [backport] (#24954)

(cherry picked from commit 1e602490e9)

lib/system/cellsets.nim

@@ -252,12 +252,12 @@ iterator elementsExcept(t, s: CellSet): PCell {.inline.} =
   var r = t.head
   while r != nil:
     let ss = cellSetGet(s, r.key)
-    var i:uint = 0
+    var i = 0'u
     while int(i) <= high(r.bits):
       var w = r.bits[i]
       if ss != nil:
         w = w and not ss.bits[i]
-      var j:uint = 0
+      var j = 0'u
       while w != 0:
         if (w and 1) != 0:
           yield cast[PCell]((r.key shl PageShift) or

lib/system/gc.nim

@@ -597,7 +597,13 @@ proc sweep(gch: var GcHeap) =
     if isCell(x):
       # cast to PCell is correct here:
       var c = cast[PCell](x)
-      if c notin gch.marked: freeCyclicCell(gch, c)
+      if c notin gch.marked:
+        # Don't free objects that have the ZctFlag set (created in finalizers)
+        if (c.refcount and ZctFlag) == 0:
+          freeCyclicCell(gch, c)
+        else:
+          # Clear the ZctFlag for the next collection cycle
+          c.refcount = c.refcount and not ZctFlag
 
 proc markS(gch: var GcHeap, c: PCell) =
   gcAssert isAllocatedPtr(gch.region, c), "markS: foreign heap root detected A!"

testament/categories.nim

@@ -83,7 +83,7 @@ proc runBasicDLLTest(c, r: var TResults, cat: Category, options: string, isOrc =
 
   if "boehm" notin options:
     # hcr tests
-    
+
     var basicHcrTest = makeTest("tests/dll/nimhcr_basic.nim", options & " --threads:off --forceBuild --hotCodeReloading:on " & rpath, cat)
     # test segfaults for now but compiles:
     if isOrc: basicHcrTest.spec.action = actionCompile
@@ -165,6 +165,7 @@ proc gcTests(r: var TResults, cat: Category, options: string) =
   test "stackrefleak"
   test "cyclecollector"
   testWithoutBoehm "trace_globals"
+  test "tfinalizers"
 
 # ------------------------- threading tests -----------------------------------
 

tests/gc/tfinalizers.nim

@@ -0,0 +1,19 @@
+
+type
+  PNode = ref TNode
+  TNode = object
+    le: PNode
+
+proc finalizeNode(n: PNode) =
+  var s = @[0]
+
+proc returnTree() =
+  var cycle: PNode
+  new(cycle, finalizeNode)
+  cycle.le = cycle
+
+for i in 1..100:
+  returnTree()
+
+GC_fullCollect()
+GC_fullCollect()