gh-117662 introduced some refleaks, or, rather, exposed some existing refleaks. The leaks are coming when test.support.os_helper is imported in a "legacy" interpreter. I've updated test.test_interpreters.utils to avoid importing os_helper, which fixes the leaks. I'll address the root cause separately.
This is similar to the situation with threading._DummyThread. The methods (incl. __del__()) of interpreters.Interpreter objects must be careful with interpreters not created by interpreters.create(). The simplest thing to start with is to disable any method that modifies or runs in the interpreter. As part of this, the runtime keeps track of where an interpreter was created. We also handle interpreter "refcounts" properly.
This brings the code under test.support.interpreters, and the corresponding extension modules, in line with recent updates to PEP 734.
(Note: PEP 734 has not been accepted at this time. However, we are using an internal copy of the implementation in the test suite to exercise the existing subinterpreters feature.)
When an exception is uncaught in Interpreter.exec_sync(), it helps to show that exception's error display if uncaught in the calling interpreter. We do so here by generating a TracebackException in the subinterpreter and passing it between interpreters using pickle.
Eric Snow