Add test for Cython module usage in subinterpreters. In Py3.5, we should now...

Add test for Cython module usage in subinterpreters. In Py3.5, we should now raise an exception when trying to re-import a module into a different subinterpreter.

tests/run/reimport_from_subinterpreter.srctree

+# mode: run
+# tag: pep489, subinterpreter
+
+PYTHON setup.py build_ext --inplace
+PYTHON -c 'import subtest; subtest.run_main()'
+PYTHON -c 'import subtest; subtest.run_sub()'
+PYTHON -c 'import subtest; subtest.run_main(); subtest.run_sub()'
+
+######## setup.py ########
+
+from Cython.Build.Dependencies import cythonize
+from distutils.core import setup
+
+setup(
+    ext_modules = cythonize("**/*.pyx"),
+    )
+
+######## subtest.pyx ########
+
+cdef extern from *:
+    """
+    /* Copied from CPython's _testcapi.c module */
+    static PyObject *run_in_subinterpreter(const char *code) {
+        int r;
+        PyThreadState *substate, *mainstate;
+
+        mainstate = PyThreadState_Get();
+
+        PyThreadState_Swap(NULL);
+
+        substate = Py_NewInterpreter();
+        if (substate == NULL) {
+            /* Since no new thread state was created, there is no exception to
+               propagate; raise a fresh one after swapping in the old thread
+               state. */
+            PyThreadState_Swap(mainstate);
+            PyErr_SetString(PyExc_RuntimeError, "sub-interpreter creation failed");
+            return NULL;
+        }
+        r = PyRun_SimpleString(code);
+        Py_EndInterpreter(substate);
+
+        PyThreadState_Swap(mainstate);
+        return PyLong_FromLong(r);
+    }
+    """
+    object run_in_subinterpreter(const char *code)
+
+
+MAIN_HAS_IMPORTED = False
+
+def run_main():
+    global MAIN_HAS_IMPORTED
+    MAIN_HAS_IMPORTED = True
+    import package.subtest
+    from package import subtest
+
+def run_sub():
+    assert 0 == run_in_subinterpreter(b'1+1')
+    assert 0 == run_in_subinterpreter(b'2+2')
+
+    assert 0 == run_in_subinterpreter(b'import package')
+    assert 0 == run_in_subinterpreter(b'import package')
+
+    import sys
+    result = run_in_subinterpreter(b'import package.subtest')
+    if not MAIN_HAS_IMPORTED:
+        assert result == 0, result  # imports only in subinterpreters are ok
+    elif sys.version_info >= (3, 5):
+        assert result == -1, result  # re-import in a different subinterpreter fails in Py3.5+ (with PEP-489)
+    else:
+        assert result == 0, result  # re-import in a different subinterpreter reuses the module in Py<3.5
+
+
+######## package/__init__.py ########
+
+######## package/subtest.pyx ########
+
+print("Module loaded: %s" % __name__)