bpo-40453: Add PyConfig._isolated_subinterpreter (GH-19820)

An isolated subinterpreter cannot spawn threads, spawn a child
process or call os.fork().

* Add private _Py_NewInterpreter(isolated_subinterpreter) function.
* Add isolated=True keyword-only parameter to
  _xxsubinterpreters.create().
* Allow again os.fork() in "non-isolated" subinterpreters.

5 files changed, 37 insertions, 8 deletions

diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
index 60dd78d92a..add2962189 100644
--- a/Modules/_posixsubprocess.c
+++ b/Modules/_posixsubprocess.c
@@ -663,6 +663,14 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
         return NULL;
     }
 
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
+    if (config->_isolated_interpreter) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "subprocess not supported for isolated subinterpreters");
+        return NULL;
+    }
+
     /* We need to call gc.disable() when we'll be calling preexec_fn */
     if (preexec_fn != Py_None) {
         PyObject *result;
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index b3d90b22c5..77baba4847 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -1085,6 +1085,14 @@ thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
                         "optional 3rd arg must be a dictionary");
         return NULL;
     }
+
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (interp->config._isolated_interpreter) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "thread is not supported for isolated subinterpreters");
+        return NULL;
+    }
+
     boot = PyMem_NEW(struct bootstate, 1);
     if (boot == NULL)
         return PyErr_NoMemory();
diff --git a/Modules/_winapi.c b/Modules/_winapi.c
index 1b28adb0b3..e1672c4785 100644
--- a/Modules/_winapi.c
+++ b/Modules/_winapi.c
@@ -1080,6 +1080,14 @@ _winapi_CreateProcess_impl(PyObject *module,
         return NULL;
     }
 
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
+    if (config->_isolated_interpreter) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "subprocess not supported for isolated subinterpreters");
+        return NULL;
+    }
+
     ZeroMemory(&si, sizeof(si));
     si.StartupInfo.cb = sizeof(si);
 
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 15e80559ec..de11c09087 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -1999,16 +1999,20 @@ _global_channels(void) {
 }
 
 static PyObject *
-interp_create(PyObject *self, PyObject *args)
+interp_create(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    if (!PyArg_UnpackTuple(args, "create", 0, 0)) {
+
+    static char *kwlist[] = {"isolated", NULL};
+    int isolated = 1;
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|$i:create", kwlist,
+                                     &isolated)) {
         return NULL;
     }
 
     // Create and initialize the new interpreter.
     PyThreadState *save_tstate = PyThreadState_Swap(NULL);
     // XXX Possible GILState issues?
-    PyThreadState *tstate = Py_NewInterpreter();
+    PyThreadState *tstate = _Py_NewInterpreter(isolated);
     PyThreadState_Swap(save_tstate);
     if (tstate == NULL) {
         /* Since no new thread state was created, there is no exception to
@@ -2547,8 +2551,8 @@ channel__channel_id(PyObject *self, PyObject *args, PyObject *kwds)
 }
 
 static PyMethodDef module_functions[] = {
-    {"create",                    (PyCFunction)interp_create,
-     METH_VARARGS, create_doc},
+    {"create",                    (PyCFunction)(void(*)(void))interp_create,
+     METH_VARARGS | METH_KEYWORDS, create_doc},
     {"destroy",                   (PyCFunction)(void(*)(void))interp_destroy,
      METH_VARARGS | METH_KEYWORDS, destroy_doc},
     {"list_all",                  interp_list_all,
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 3d3f6ac969..0163b0757a 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6243,9 +6243,10 @@ os_fork_impl(PyObject *module)
 /*[clinic end generated code: output=3626c81f98985d49 input=13c956413110eeaa]*/
 {
     pid_t pid;
-
-    if (_PyInterpreterState_GET() != PyInterpreterState_Main()) {
-        PyErr_SetString(PyExc_RuntimeError, "fork not supported for subinterpreters");
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (interp->config._isolated_interpreter) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "fork not supported for isolated subinterpreters");
         return NULL;
     }
     if (PySys_Audit("os.fork", NULL) < 0) {