bpo-40514: Drop EXPERIMENTAL_ISOLATED_SUBINTERPRETERS (gh-93185)

This was added for bpo-40514 (gh-84694) to test out a per-interpreter GIL. However, it has since proven unnecessary to keep the experiment in the repo. (It can be done as a branch in a fork like normal.) So here we are removing:

* the configure option
* the macro
* the code enabled by the macro

Include/internal/pycore_ceval.h

@@ -81,11 +81,7 @@ _PyEval_Vector(PyThreadState *tstate,
             PyObject* const* args, size_t argcount,
             PyObject *kwnames);
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-extern int _PyEval_ThreadsInitialized(PyInterpreterState *interp);
-#else
 extern int _PyEval_ThreadsInitialized(struct pyruntimestate *runtime);
-#endif
 extern PyStatus _PyEval_InitGIL(PyThreadState *tstate);
 extern void _PyEval_FiniGIL(PyInterpreterState *interp);
 

Include/internal/pycore_interp.h

@@ -51,9 +51,6 @@ struct _ceval_state {
     /* Request for dropping the GIL */
     _Py_atomic_int gil_drop_request;
     struct _pending_calls pending;
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    struct _gil_runtime_state gil;
-#endif
 };
 
 

Include/internal/pycore_pystate.h

@@ -64,18 +64,10 @@ _Py_ThreadCanHandlePendingCalls(void)
 /* Variable and macro for in-line access to current thread
    and interpreter state */
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-PyAPI_FUNC(PyThreadState*) _PyThreadState_GetTSS(void);
-#endif
-
 static inline PyThreadState*
 _PyRuntimeState_GetThreadState(_PyRuntimeState *runtime)
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    return _PyThreadState_GetTSS();
-#else
     return (PyThreadState*)_Py_atomic_load_relaxed(&runtime->gilstate.tstate_current);
-#endif
 }
 
 /* Get the current Python thread state.
@@ -90,11 +82,7 @@ _PyRuntimeState_GetThreadState(_PyRuntimeState *runtime)
 static inline PyThreadState*
 _PyThreadState_GET(void)
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    return _PyThreadState_GetTSS();
-#else
     return _PyRuntimeState_GetThreadState(&_PyRuntime);
-#endif
 }
 
 PyAPI_FUNC(void) _Py_NO_RETURN _Py_FatalError_TstateNULL(const char *func);

Include/internal/pycore_runtime.h

@@ -23,9 +23,7 @@ struct _ceval_runtime_state {
        the main thread of the main interpreter can handle signals: see
        _Py_ThreadCanHandleSignals(). */
     _Py_atomic_int signals_pending;
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     struct _gil_runtime_state gil;
-#endif
 };
 
 /* GIL state */