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.)

1 files changed, 32 insertions, 13 deletions

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 4195a9dbca8..334abfb191f 100644
--- a/Python/pylifecycle.c
+++ b/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