gh-109860: Use a New Thread State When Switching Interpreters, When Necessary (gh-110245)

In a few places we switch to another interpreter without knowing if it has a thread state associated with the current thread.  For the main interpreter there wasn't much of a problem, but for subinterpreters we were *mostly* okay re-using the tstate created with the interpreter (located via PyInterpreterState_ThreadHead()).  There was a good chance that tstate wasn't actually in use by another thread.

However, there are no guarantees of that.  Furthermore, re-using an already used tstate is currently fragile.  To address this, now we create a new thread state in each of those places and use it.

One consequence of this change is that PyInterpreterState_ThreadHead() may not return NULL (though that won't happen for the main interpreter).

1 files changed, 1 insertions, 0 deletions

diff --git a/Include/internal/pycore_runtime_init.h b/Include/internal/pycore_runtime_init.h
index 2deba02a89f..574a3c1a9db 100644
--- a/Include/internal/pycore_runtime_init.h
+++ b/Include/internal/pycore_runtime_init.h
@@ -185,6 +185,7 @@ extern PyTypeObject _PyExc_MemoryError;
 
 #define _PyThreadState_INIT \
     { \
+        ._whence = _PyThreadState_WHENCE_NOTSET, \
         .py_recursion_limit = Py_DEFAULT_RECURSION_LIMIT, \
         .context_ver = 1, \
     }