gh-98608: Change _Py_NewInterpreter() to _Py_NewInterpreterFromConfig() (gh-98609)

(see https://github.com/python/cpython/issues/98608)

This change does the following:

1. change the argument to a new `_PyInterpreterConfig` struct
2. rename the function to `_Py_NewInterpreterFromConfig()`, inspired by `Py_InitializeFromConfig()` (takes a `_PyInterpreterConfig`  instead of `isolated_subinterpreter`)
3. split up the boolean `isolated_subinterpreter` into the corresponding multiple granular settings
   * allow_fork
   * allow_subprocess
   * allow_threads
4. add `PyInterpreterState.feature_flags` to store those settings
5. add a function for checking if a feature is enabled on an opaque `PyInterpreterState *`
6. drop `PyConfig._isolated_interpreter`

The existing default (see `Py_NewInterpeter()` and `Py_Initialize*()`) allows fork, subprocess, and threads and the optional "isolated" interpreter (see the `_xxsubinterpreters` module) disables all three.  None of that changes here; the defaults are preserved.

Note that the given `_PyInterpreterConfig` will not be used outside `_Py_NewInterpreterFromConfig()`, nor preserved.  This contrasts with how `PyConfig` is currently preserved, used, and even modified outside `Py_InitializeFromConfig()`.  I'd rather just avoid that mess from the start for `_PyInterpreterConfig`.  We can preserve it later if we find an actual need.

This change allows us to follow up with a number of improvements (e.g. stop disallowing subprocess and support disallowing exec instead).

(Note that this PR adds "private" symbols.  We'll probably make them public, and add docs, in a separate change.)

Python/initconfig.c

@@ -780,7 +780,6 @@ _PyConfig_InitCompatConfig(PyConfig *config)
     config->check_hash_pycs_mode = NULL;
     config->pathconfig_warnings = -1;
     config->_init_main = 1;
-    config->_isolated_interpreter = 0;
 #ifdef MS_WINDOWS
     config->legacy_windows_stdio = -1;
 #endif
@@ -1015,7 +1014,6 @@ _PyConfig_Copy(PyConfig *config, const PyConfig *config2)
     COPY_WSTR_ATTR(check_hash_pycs_mode);
     COPY_ATTR(pathconfig_warnings);
     COPY_ATTR(_init_main);
-    COPY_ATTR(_isolated_interpreter);
     COPY_ATTR(use_frozen_modules);
     COPY_ATTR(safe_path);
     COPY_WSTRLIST(orig_argv);
@@ -1123,7 +1121,6 @@ _PyConfig_AsDict(const PyConfig *config)
     SET_ITEM_WSTR(check_hash_pycs_mode);
     SET_ITEM_INT(pathconfig_warnings);
     SET_ITEM_INT(_init_main);
-    SET_ITEM_INT(_isolated_interpreter);
     SET_ITEM_WSTRLIST(orig_argv);
     SET_ITEM_INT(use_frozen_modules);
     SET_ITEM_INT(safe_path);
@@ -1418,7 +1415,6 @@ _PyConfig_FromDict(PyConfig *config, PyObject *dict)
 
     GET_UINT(_install_importlib);
     GET_UINT(_init_main);
-    GET_UINT(_isolated_interpreter);
     GET_UINT(use_frozen_modules);
     GET_UINT(safe_path);
     GET_UINT(_is_python_build);

Python/pylifecycle.c

@@ -611,6 +611,22 @@ pycore_init_runtime(_PyRuntimeState *runtime,
 }
 
 
+static void
+init_interp_settings(PyInterpreterState *interp, const _PyInterpreterConfig *config)
+{
+    assert(interp->feature_flags == 0);
+    if (config->allow_fork) {
+        interp->feature_flags |= Py_RTFLAGS_FORK;
+    }
+    if (config->allow_subprocess) {
+        interp->feature_flags |= Py_RTFLAGS_SUBPROCESS;
+    }
+    if (config->allow_threads) {
+        interp->feature_flags |= Py_RTFLAGS_THREADS;
+    }
+}
+
+
 static PyStatus
 init_interp_create_gil(PyThreadState *tstate)
 {
@@ -638,7 +654,7 @@ init_interp_create_gil(PyThreadState *tstate)
 
 static PyStatus
 pycore_create_interpreter(_PyRuntimeState *runtime,
-                          const PyConfig *config,
+                          const PyConfig *src_config,
                           PyThreadState **tstate_p)
 {
     /* Auto-thread-state API */
@@ -653,11 +669,14 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
     }
     assert(_Py_IsMainInterpreter(interp));
 
-    status = _PyConfig_Copy(&interp->config, config);
+    status = _PyConfig_Copy(&interp->config, src_config);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
     }
 
+    const _PyInterpreterConfig config = _PyInterpreterConfig_LEGACY_INIT;
+    init_interp_settings(interp, &config);
+
     PyThreadState *tstate = PyThreadState_New(interp);
     if (tstate == NULL) {
         return _PyStatus_ERR("can't make first thread");
@@ -1961,7 +1980,7 @@ Py_Finalize(void)
 */
 
 static PyStatus
-new_interpreter(PyThreadState **tstate_p, int isolated_subinterpreter)
+new_interpreter(PyThreadState **tstate_p, const _PyInterpreterConfig *config)
 {
     PyStatus status;
 
@@ -1995,23 +2014,23 @@ new_interpreter(PyThreadState **tstate_p, int isolated_subinterpreter)
     PyThreadState *save_tstate = PyThreadState_Swap(tstate);
 
     /* Copy the current interpreter config into the new interpreter */
-    const PyConfig *config;
+    const PyConfig *src_config;
     if (save_tstate != NULL) {
-        config = _PyInterpreterState_GetConfig(save_tstate->interp);
+        src_config = _PyInterpreterState_GetConfig(save_tstate->interp);
     }
     else
     {
         /* No current thread state, copy from the main interpreter */
         PyInterpreterState *main_interp = _PyInterpreterState_Main();
-        config = _PyInterpreterState_GetConfig(main_interp);
+        src_config = _PyInterpreterState_GetConfig(main_interp);
     }
 
-
-    status = _PyConfig_Copy(&interp->config, config);
+    status = _PyConfig_Copy(&interp->config, src_config);
     if (_PyStatus_EXCEPTION(status)) {
         goto error;
     }
-    interp->config._isolated_interpreter = isolated_subinterpreter;
+
+    init_interp_settings(interp, config);
 
     status = init_interp_create_gil(tstate);
     if (_PyStatus_EXCEPTION(status)) {
@@ -2045,21 +2064,21 @@ error:
 }
 
 PyThreadState *
-_Py_NewInterpreter(int isolated_subinterpreter)
+_Py_NewInterpreterFromConfig(const _PyInterpreterConfig *config)
 {
     PyThreadState *tstate = NULL;
-    PyStatus status = new_interpreter(&tstate, isolated_subinterpreter);
+    PyStatus status = new_interpreter(&tstate, config);
     if (_PyStatus_EXCEPTION(status)) {
         Py_ExitStatusException(status);
     }
     return tstate;
-
 }
 
 PyThreadState *
 Py_NewInterpreter(void)
 {
-    return _Py_NewInterpreter(0);
+    const _PyInterpreterConfig config = _PyInterpreterConfig_LEGACY_INIT;
+    return _Py_NewInterpreterFromConfig(&config);
 }
 
 /* Delete an interpreter and its last thread.  This requires that the

Python/pystate.c

@@ -2177,6 +2177,14 @@ _Py_GetConfig(void)
     return _PyInterpreterState_GetConfig(tstate->interp);
 }
 
+
+int
+_PyInterpreterState_HasFeature(PyInterpreterState *interp, unsigned long feature)
+{
+    return ((interp->feature_flags & feature) != 0);
+}
+
+
 #define MINIMUM_OVERHEAD 1000
 
 static PyObject **