bpo-38076 Clear the interpreter state only after clearing module globals (GH-18039)

Currently, during runtime destruction, `_PyImport_Cleanup` is clearing the interpreter state before clearing out the modules themselves. This leads to a segfault on modules that rely on the module state to clear themselves up.

For example, let's take the small snippet added in the issue by @DinoV :
```
import _struct

class C:
    def __init__(self):
        self.pack = _struct.pack
    def __del__(self):
        self.pack('I', -42)

_struct.x = C()
```

The module `_struct` uses the module state to run `pack`. Therefore, the module state has to be alive until after the module has been cleared out to successfully run `C.__del__`. This happens at line 606, when `_PyImport_Cleanup` calls `_PyModule_Clear`. In fact, the loop that calls `_PyModule_Clear` has in its comments: 

> Now, if there are any modules left alive, clear their globals to minimize potential leaks.  All C extension modules actually end up here, since they are kept alive in the interpreter state.

That means that we can't clear the module state (which is used by C Extensions) before we run that loop.

Moving `_PyInterpreterState_ClearModules` until after it, fixes the segfault in the code snippet.

Finally, this updates a test in `io` to correctly assert the error that it now throws (since it now finds the io module state). The test that uses this is: `test_create_at_shutdown_without_encoding`. Given this test is now working is a proof that the module state now stays alive even when `__del__` is called at module destruction time. Thus, I didn't add a new tests for this.


https://bugs.python.org/issue38076

1 files changed, 2 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/C API/2020-01-17-11-37-05.bpo-38076.cxfw2x.rst b/Misc/NEWS.d/next/C API/2020-01-17-11-37-05.bpo-38076.cxfw2x.rst
new file mode 100644
index 0000000..d9f6dc3
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2020-01-17-11-37-05.bpo-38076.cxfw2x.rst
@@ -0,0 +1,2 @@
+Fix to clear the interpreter state only after clearing module globals to
+guarantee module state access from C Extensions during runtime destruction