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.)

7 files changed, 119 insertions, 7 deletions

diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
index 44e60d7..8275b11 100644
--- a/Modules/_posixsubprocess.c
+++ b/Modules/_posixsubprocess.c
@@ -842,8 +842,7 @@ subprocess_fork_exec(PyObject *module, PyObject *args)
     }
 
     PyInterpreterState *interp = PyInterpreterState_Get();
-    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
-    if (config->_isolated_interpreter) {
+    if (!_PyInterpreterState_HasFeature(interp, Py_RTFLAGS_SUBPROCESS)) {
         PyErr_SetString(PyExc_RuntimeError,
                         "subprocess not supported for isolated subinterpreters");
         return NULL;
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index e792a2c..fdf2f20 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -3225,6 +3225,66 @@ run_in_subinterp(PyObject *self, PyObject *args)
     return PyLong_FromLong(r);
 }
 
+/* To run some code in a sub-interpreter. */
+static PyObject *
+run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    const char *code;
+    int allow_fork = -1;
+    int allow_subprocess = -1;
+    int allow_threads = -1;
+    int r;
+    PyThreadState *substate, *mainstate;
+    /* only initialise 'cflags.cf_flags' to test backwards compatibility */
+    PyCompilerFlags cflags = {0};
+
+    static char *kwlist[] = {"code",
+                             "allow_fork", "allow_subprocess", "allow_threads",
+                             NULL};
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs,
+                    "s$ppp:run_in_subinterp_with_config", kwlist,
+                    &code, &allow_fork, &allow_subprocess, &allow_threads)) {
+        return NULL;
+    }
+    if (allow_fork < 0) {
+        PyErr_SetString(PyExc_ValueError, "missing allow_fork");
+        return NULL;
+    }
+    if (allow_subprocess < 0) {
+        PyErr_SetString(PyExc_ValueError, "missing allow_subprocess");
+        return NULL;
+    }
+    if (allow_threads < 0) {
+        PyErr_SetString(PyExc_ValueError, "missing allow_threads");
+        return NULL;
+    }
+
+    mainstate = PyThreadState_Get();
+
+    PyThreadState_Swap(NULL);
+
+    const _PyInterpreterConfig config = {
+        .allow_fork = allow_fork,
+        .allow_subprocess = allow_subprocess,
+        .allow_threads = allow_threads,
+    };
+    substate = _Py_NewInterpreterFromConfig(&config);
+    if (substate == NULL) {
+        /* Since no new thread state was created, there is no exception to
+           propagate; raise a fresh one after swapping in the old thread
+           state. */
+        PyThreadState_Swap(mainstate);
+        PyErr_SetString(PyExc_RuntimeError, "sub-interpreter creation failed");
+        return NULL;
+    }
+    r = PyRun_SimpleStringFlags(code, &cflags);
+    Py_EndInterpreter(substate);
+
+    PyThreadState_Swap(mainstate);
+
+    return PyLong_FromLong(r);
+}
+
 static int
 check_time_rounding(int round)
 {
@@ -5998,6 +6058,9 @@ static PyMethodDef TestMethods[] = {
      METH_NOARGS},
     {"crash_no_current_thread", crash_no_current_thread,         METH_NOARGS},
     {"run_in_subinterp",        run_in_subinterp,                METH_VARARGS},
+    {"run_in_subinterp_with_config",
+     _PyCFunction_CAST(run_in_subinterp_with_config),
+     METH_VARARGS | METH_KEYWORDS},
     {"pytime_object_to_time_t", test_pytime_object_to_time_t,  METH_VARARGS},
     {"pytime_object_to_timeval", test_pytime_object_to_timeval,  METH_VARARGS},
     {"pytime_object_to_timespec", test_pytime_object_to_timespec,  METH_VARARGS},
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 5724bd5..d74f928 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -550,6 +550,51 @@ _testinternalcapi_optimize_cfg_impl(PyObject *module, PyObject *instructions,
 }
 
 
+static PyObject *
+get_interp_settings(PyObject *self, PyObject *args)
+{
+    int interpid = -1;
+    if (!PyArg_ParseTuple(args, "|i:get_interp_settings", &interpid)) {
+        return NULL;
+    }
+
+    PyInterpreterState *interp = NULL;
+    if (interpid < 0) {
+        PyThreadState *tstate = _PyThreadState_GET();
+        interp = tstate ? tstate->interp : _PyInterpreterState_Main();
+    }
+    else if (interpid == 0) {
+        interp = _PyInterpreterState_Main();
+    }
+    else {
+        PyErr_Format(PyExc_NotImplementedError,
+                     "%zd", interpid);
+        return NULL;
+    }
+    assert(interp != NULL);
+
+    PyObject *settings = PyDict_New();
+    if (settings == NULL) {
+        return NULL;
+    }
+
+    /* Add the feature flags. */
+    PyObject *flags = PyLong_FromUnsignedLong(interp->feature_flags);
+    if (flags == NULL) {
+        Py_DECREF(settings);
+        return NULL;
+    }
+    int res = PyDict_SetItemString(settings, "feature_flags", flags);
+    Py_DECREF(flags);
+    if (res != 0) {
+        Py_DECREF(settings);
+        return NULL;
+    }
+
+    return settings;
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"get_configs", get_configs, METH_NOARGS},
     {"get_recursion_depth", get_recursion_depth, METH_NOARGS},
@@ -569,6 +614,7 @@ static PyMethodDef TestMethods[] = {
     {"set_eval_frame_default", set_eval_frame_default, METH_NOARGS, NULL},
     {"set_eval_frame_record", set_eval_frame_record, METH_O, NULL},
     _TESTINTERNALCAPI_OPTIMIZE_CFG_METHODDEF
+    {"get_interp_settings", get_interp_settings, METH_VARARGS, NULL},
     {NULL, NULL} /* sentinel */
 };
 
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index 4ac90dc..93b3b8d 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -1128,7 +1128,7 @@ thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
     }
 
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (interp->config._isolated_interpreter) {
+    if (!_PyInterpreterState_HasFeature(interp, Py_RTFLAGS_THREADS)) {
         PyErr_SetString(PyExc_RuntimeError,
                         "thread is not supported for isolated subinterpreters");
         return NULL;
diff --git a/Modules/_winapi.c b/Modules/_winapi.c
index 4845b4e..2a916cc 100644
--- a/Modules/_winapi.c
+++ b/Modules/_winapi.c
@@ -1090,8 +1090,7 @@ _winapi_CreateProcess_impl(PyObject *module,
     }
 
     PyInterpreterState *interp = PyInterpreterState_Get();
-    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
-    if (config->_isolated_interpreter) {
+    if (!_PyInterpreterState_HasFeature(interp, Py_RTFLAGS_SUBPROCESS)) {
         PyErr_SetString(PyExc_RuntimeError,
                         "subprocess not supported for isolated subinterpreters");
         return NULL;
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index f40601a..f38de57 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -2003,8 +2003,13 @@ interp_create(PyObject *self, PyObject *args, PyObject *kwds)
 
     // Create and initialize the new interpreter.
     PyThreadState *save_tstate = _PyThreadState_GET();
+    const _PyInterpreterConfig config = {
+        .allow_fork = !isolated,
+        .allow_subprocess = !isolated,
+        .allow_threads = !isolated,
+    };
     // XXX Possible GILState issues?
-    PyThreadState *tstate = _Py_NewInterpreter(isolated);
+    PyThreadState *tstate = _Py_NewInterpreterFromConfig(&config);
     PyThreadState_Swap(save_tstate);
     if (tstate == NULL) {
         /* Since no new thread state was created, there is no exception to
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 56ea319..a5eb866 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6760,7 +6760,7 @@ os_fork_impl(PyObject *module)
 {
     pid_t pid;
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (interp->config._isolated_interpreter) {
+    if (!_PyInterpreterState_HasFeature(interp, Py_RTFLAGS_FORK)) {
         PyErr_SetString(PyExc_RuntimeError,
                         "fork not supported for isolated subinterpreters");
         return NULL;