gh-157377: clamp gc.get_count() return value at zero

Author: ngoldbaum

Lib/test/test_gc.py

@@ -1658,6 +1658,12 @@ def test_indirect_calls_with_gc_disabled(self):
         finally:
             gc.enable()
 
+    def test_get_count_nonnegative(self):
+        xs = [[] for _ in range(1500)]
+        gc.collect()
+        del xs[:1000]
+        self.assertGreaterEqual(gc.get_count()[0], 0)
+
     @gc_threshold(1000, 0, 0)
     def test_get_count_does_not_prevent_collection(self):
         junk = []

Modules/gcmodule.c

@@ -224,6 +224,9 @@ gc_get_count_impl(PyObject *module)
     // Don't flush: record_allocation() checks the threshold only when it fills.
     int young = _Py_atomic_load_int_relaxed(&gcstate->young.count);
     young += (int)gc->alloc_count;
+    if (young < 0) {
+        young = 0;
+    }
 #endif
 
 #ifndef Py_GIL_DISABLED