Fix leak when importing value

Summary:
When importing a lazy value if we can trigger this lookup while already doing an import. One chain of events is:
`globals().items()` -> forced import of all values -> (import of package, not defined, does `hasattr` to see if it's there -> triggers import) x 10.

As we unwind through the chain of getattrs we continuously leak a reference to the value as the inner most one changed it. So we need to be more aggressive about what we do when checking if the dictionary changed. This causes us to retry the lookup if keys/kind/or the current key have changed. If only the value has changed we return the new value instead of the one we just imported.

Reviewed By: Kronuz

Differential Revision: D68735993

fbshipit-source-id: 03365011e0eccb2297bfb10886be9a013e331140

Objects/dictobject.c

@@ -1182,16 +1182,27 @@ _Py_dict_lookup(PyDictObject *mp, PyObject *key, Py_hash_t hash, PyObject **valu
             }
             return DKIX_EMPTY;
         }
-        /* If the dict hasn't mutated, update resolved_value */
-        if (dk == mp->ma_keys && kind == dk->dk_kind) {
-            if (kind != DICT_KEYS_GENERAL
-                ? DK_UNICODE_ENTRIES(dk)[ix].me_key == startkey
-                : DK_ENTRIES(dk)[ix].me_key == startkey) {
-                if (*value_ptr == value) {
-                    Py_DECREF(*value_ptr);
-                    *value_ptr = resolved_value;
-                }
-            }
+        /* If the dict hasn't mutated, update resolved_value, otherwise
+         * retry the lookup */
+        if (dk != mp->ma_keys || kind != dk->dk_kind) {
+            Py_DECREF(resolved_value);
+            goto start;
+        }
+
+        PyObject *key = kind != DICT_KEYS_GENERAL
+            ? DK_UNICODE_ENTRIES(dk)[ix].me_key : DK_ENTRIES(dk)[ix].me_key;
+        if (key != startkey) {
+            Py_DECREF(resolved_value);
+            goto start;
+        }
+
+        if (*value_ptr == value) {
+            Py_DECREF(*value_ptr);
+            *value_ptr = resolved_value;
+        } else {
+            // Just the value was replaced, so return the new value.
+            Py_DECREF(resolved_value);
+            resolved_value = *value_ptr;
         }
         value = resolved_value;
     }