Revert "Always track frozenset in the GC"

This reverts commit 37ab323.

Author: vstinner

Lib/test/test_capi/test_set.py

@@ -392,10 +392,11 @@ def test_update_array_steal(self):
         self.check_update_method('update_array_steal')
 
     def test_finish(self):
-        # Test PyFrozenSetWriter_Finish(): track the frozenset in the GC
+        # Test PyFrozenSetWriter_Finish(): track the frozenset in the GC if at
+        # least one item is tracked by the GC
         writer = self.create_writer()
         writer.update([2, 5, 7])
-        self.assertTrue(gc.is_tracked(writer.finish()))
+        self.assertFalse(gc.is_tracked(writer.finish()))
 
         class MyObject:
             pass
@@ -429,8 +430,8 @@ def test_discard(self):
 
     @threading_helper.requires_working_threading()
     def test_thread(self):
-        # PyFrozenSetWriter can be used by multiple threads: it's up to the
-        # caller to implement a lock to prevent concurrent accesses.
+        # PyFrozenSetWriter can be used by multiple threads: it's up to the caller
+        # to implement a lock to prevent concurrent accesses.
         writer = self.create_writer()
 
         def thread_func(writer):

Objects/setobject.c

@@ -3287,13 +3287,21 @@ PyObject*
 PyFrozenSetWriter_Finish(PyFrozenSetWriter *writer)
 {
     assert(frozensetwriter_check_consistency(writer));
-    PyObject *frozenset = _PyObject_CAST(writer);
+    PySetObject *frozenset = _PySet_CAST(writer);
 
-    // Now that the frozenset is fully initialized, it's safe to track it
-    // in the garbage collector.
-    _PyObject_GC_TRACK(frozenset);
+    // Now that the frozenset is fully initialized, it's safe to track it in
+    // the GC. Track the frozenset if at least one item is tracked by the GC.
+    Py_ssize_t pos = 0;
+    setentry *entry;
+    while (set_next(frozenset, &pos, &entry)) {
+        PyObject *item = entry->key;
+        if (_PyObject_IS_GC(item) && _PyObject_GC_IS_TRACKED(item)) {
+            _PyObject_GC_TRACK(frozenset);
+            break;
+        }
+    }
 
-    return frozenset;
+    return _PyObject_CAST(frozenset);
 }