gh-100227: Make the Global Interned Dict Safe for Isolated Interpreters (gh-102925)

This is effectively two changes.  The first (the bulk of the change) is where we add _Py_AddToGlobalDict() (and _PyRuntime.cached_objects.main_tstate, etc.).  The second (much smaller) change is where we update PyUnicode_InternInPlace() to use _Py_AddToGlobalDict() instead of calling PyDict_SetDefault() directly.

Basically, _Py_AddToGlobalDict() is a wrapper around PyDict_SetDefault() that should be used whenever we need to add a value to a runtime-global dict object (in the few cases where we are leaving the container global rather than moving it to PyInterpreterState, e.g. the interned strings dict).  _Py_AddToGlobalDict() does all the necessary work to make sure the target global dict is shared safely between isolated interpreters.  This is especially important as we move the obmalloc state to each interpreter (gh-101660), as well as, potentially, the GIL (PEP 684).

https://github.com/python/cpython/issues/100227

Include/internal/pycore_global_objects.h

@@ -28,6 +28,10 @@ extern "C" {
 
 struct _Py_cached_objects {
     PyObject *interned_strings;
+    /* A thread state tied to the main interpreter,
+       used exclusively for when a global object (e.g. interned strings)
+       is resized (i.e. deallocated + allocated) from an arbitrary thread. */
+    PyThreadState main_tstate;
 };
 
 #define _Py_GLOBAL_OBJECT(NAME) \

Include/internal/pycore_pystate.h

@@ -127,6 +127,11 @@ PyAPI_FUNC(void) _PyThreadState_Init(
     PyThreadState *tstate);
 PyAPI_FUNC(void) _PyThreadState_DeleteExcept(PyThreadState *tstate);
 
+extern void _PyThreadState_InitDetached(PyThreadState *, PyInterpreterState *);
+extern void _PyThreadState_ClearDetached(PyThreadState *);
+
+extern PyObject * _Py_AddToGlobalDict(PyObject *, PyObject *, PyObject *);
+
 
 static inline void
 _PyThreadState_UpdateTracingState(PyThreadState *tstate)

Include/internal/pycore_runtime_init.h

@@ -59,6 +59,9 @@ extern PyTypeObject _PyExc_MemoryError;
         .types = { \
             .next_version_tag = 1, \
         }, \
+        .cached_objects = { \
+            .main_tstate = _PyThreadState_INIT, \
+        }, \
         .static_objects = { \
             .singletons = { \
                 .small_ints = _Py_small_ints_INIT, \

Include/internal/pycore_unicodeobject.h

@@ -34,6 +34,7 @@ struct _Py_unicode_runtime_ids {
 
 struct _Py_unicode_runtime_state {
     struct _Py_unicode_runtime_ids ids;
+    /* The interned dict is at _PyRuntime.cached_objects.interned_strings. */
 };
 
 /* fs_codec.encoding is initialized to NULL.