GH-126491: GC: Mark objects reachable from roots before doing cycle collection (GH-126502)

* Mark almost all reachable objects before doing collection phase

* Add stats for objects marked

* Visit new frames before each increment

* Remove lazy dict tracking

* Update docs

* Clearer calculation of work to do.

Lib/test/test_gc.py

@@ -31,6 +31,11 @@ except ImportError:
         return C
     ContainerNoGC = None
 
+try:
+    import _testinternalcapi
+except ImportError:
+    _testinternalcapi = None
+
 ### Support code
 ###############################################################################
 
@@ -1130,6 +1135,7 @@ class IncrementalGCTests(unittest.TestCase):
     def tearDown(self):
         gc.disable()
 
+    @unittest.skipIf(_testinternalcapi is None, "requires _testinternalcapi")
     @requires_gil_enabled("Free threading does not support incremental GC")
     # Use small increments to emulate longer running process in a shorter time
     @gc_threshold(200, 10)
@@ -1155,32 +1161,18 @@ class IncrementalGCTests(unittest.TestCase):
             return head
 
         head = make_ll(1000)
-        count = 1000
-
-        # There will be some objects we aren't counting,
-        # e.g. the gc stats dicts. This test checks
-        # that the counts don't grow, so we try to
-        # correct for the uncounted objects
-        # This is just an estimate.
-        CORRECTION = 20
 
         enabled = gc.isenabled()
         gc.enable()
         olds = []
+        initial_heap_size = _testinternalcapi.get_heap_size()
         for i in range(20_000):
             newhead = make_ll(20)
-            count += 20
             newhead.surprise = head
             olds.append(newhead)
             if len(olds) == 20:
-                stats = gc.get_stats()
-                young = stats[0]
-                incremental = stats[1]
-                old = stats[2]
-                collected = young['collected'] + incremental['collected'] + old['collected']
-                count += CORRECTION
-                live = count - collected
-                self.assertLess(live, 25000)
+                new_objects = _testinternalcapi.get_heap_size() - initial_heap_size
+                self.assertLess(new_objects, 25_000)
                 del olds[:]
         if not enabled:
             gc.disable()
@@ -1322,7 +1314,8 @@ class GCCallbackTests(unittest.TestCase):
             from test.support import gc_collect, SuppressCrashReport
 
             a = [1, 2, 3]
-            b = [a]
+            b = [a, a]
+            a.append(b)
 
             # Avoid coredump when Py_FatalError() calls abort()
             SuppressCrashReport().__enter__()
@@ -1332,6 +1325,8 @@ class GCCallbackTests(unittest.TestCase):
             # (to avoid deallocating it):
             import ctypes
             ctypes.pythonapi.Py_DecRef(ctypes.py_object(a))
+            del a
+            del b
 
             # The garbage collector should now have a fatal error
             # when it reaches the broken object
@@ -1360,7 +1355,7 @@ class GCCallbackTests(unittest.TestCase):
         self.assertRegex(stderr,
             br'object type name: list')
         self.assertRegex(stderr,
-            br'object repr     : \[1, 2, 3\]')
+            br'object repr     : \[1, 2, 3, \[\[...\], \[...\]\]\]')
 
 
 class GCTogglingTests(unittest.TestCase):