gh-76785: Move the Cross-Interpreter Code to Its Own File (gh-111502)

This is partly to clear this stuff out of pystate.c, but also in preparation for moving some code out of _xxsubinterpretersmodule.c.  This change also moves this stuff to the internal API (new: Include/internal/pycore_crossinterp.h).  @vstinner did this previously and I undid it.  Now I'm re-doing it. :/

Include/cpython/pystate.h

@@ -258,80 +258,3 @@ PyAPI_FUNC(_PyFrameEvalFunction) _PyInterpreterState_GetEvalFrameFunc(
 PyAPI_FUNC(void) _PyInterpreterState_SetEvalFrameFunc(
     PyInterpreterState *interp,
     _PyFrameEvalFunction eval_frame);
-
-
-/* cross-interpreter data */
-
-// _PyCrossInterpreterData is similar to Py_buffer as an effectively
-// opaque struct that holds data outside the object machinery.  This
-// is necessary to pass safely between interpreters in the same process.
-typedef struct _xid _PyCrossInterpreterData;
-
-typedef PyObject *(*xid_newobjectfunc)(_PyCrossInterpreterData *);
-typedef void (*xid_freefunc)(void *);
-
-struct _xid {
-    // data is the cross-interpreter-safe derivation of a Python object
-    // (see _PyObject_GetCrossInterpreterData).  It will be NULL if the
-    // new_object func (below) encodes the data.
-    void *data;
-    // obj is the Python object from which the data was derived.  This
-    // is non-NULL only if the data remains bound to the object in some
-    // way, such that the object must be "released" (via a decref) when
-    // the data is released.  In that case the code that sets the field,
-    // likely a registered "crossinterpdatafunc", is responsible for
-    // ensuring it owns the reference (i.e. incref).
-    PyObject *obj;
-    // interp is the ID of the owning interpreter of the original
-    // object.  It corresponds to the active interpreter when
-    // _PyObject_GetCrossInterpreterData() was called.  This should only
-    // be set by the cross-interpreter machinery.
-    //
-    // We use the ID rather than the PyInterpreterState to avoid issues
-    // with deleted interpreters.  Note that IDs are never re-used, so
-    // each one will always correspond to a specific interpreter
-    // (whether still alive or not).
-    int64_t interpid;
-    // new_object is a function that returns a new object in the current
-    // interpreter given the data.  The resulting object (a new
-    // reference) will be equivalent to the original object.  This field
-    // is required.
-    xid_newobjectfunc new_object;
-    // free is called when the data is released.  If it is NULL then
-    // nothing will be done to free the data.  For some types this is
-    // okay (e.g. bytes) and for those types this field should be set
-    // to NULL.  However, for most the data was allocated just for
-    // cross-interpreter use, so it must be freed when
-    // _PyCrossInterpreterData_Release is called or the memory will
-    // leak.  In that case, at the very least this field should be set
-    // to PyMem_RawFree (the default if not explicitly set to NULL).
-    // The call will happen with the original interpreter activated.
-    xid_freefunc free;
-};
-
-PyAPI_FUNC(void) _PyCrossInterpreterData_Init(
-        _PyCrossInterpreterData *data,
-        PyInterpreterState *interp, void *shared, PyObject *obj,
-        xid_newobjectfunc new_object);
-PyAPI_FUNC(int) _PyCrossInterpreterData_InitWithSize(
-        _PyCrossInterpreterData *,
-        PyInterpreterState *interp, const size_t, PyObject *,
-        xid_newobjectfunc);
-PyAPI_FUNC(void) _PyCrossInterpreterData_Clear(
-        PyInterpreterState *, _PyCrossInterpreterData *);
-
-PyAPI_FUNC(int) _PyObject_GetCrossInterpreterData(PyObject *, _PyCrossInterpreterData *);
-PyAPI_FUNC(PyObject *) _PyCrossInterpreterData_NewObject(_PyCrossInterpreterData *);
-PyAPI_FUNC(int) _PyCrossInterpreterData_Release(_PyCrossInterpreterData *);
-PyAPI_FUNC(int) _PyCrossInterpreterData_ReleaseAndRawFree(_PyCrossInterpreterData *);
-
-PyAPI_FUNC(int) _PyObject_CheckCrossInterpreterData(PyObject *);
-
-/* cross-interpreter data registry */
-
-typedef int (*crossinterpdatafunc)(PyThreadState *tstate, PyObject *,
-                                   _PyCrossInterpreterData *);
-
-PyAPI_FUNC(int) _PyCrossInterpreterData_RegisterClass(PyTypeObject *, crossinterpdatafunc);
-PyAPI_FUNC(int) _PyCrossInterpreterData_UnregisterClass(PyTypeObject *);
-PyAPI_FUNC(crossinterpdatafunc) _PyCrossInterpreterData_Lookup(PyObject *);

Include/internal/pycore_ceval.h

@@ -54,16 +54,6 @@ PyAPI_FUNC(int) _PyEval_AddPendingCall(
     void *arg,
     int flags);
 
-typedef int (*_Py_simple_func)(void *);
-extern int _Py_CallInInterpreter(
-    PyInterpreterState *interp,
-    _Py_simple_func func,
-    void *arg);
-extern int _Py_CallInInterpreterAndRawFree(
-    PyInterpreterState *interp,
-    _Py_simple_func func,
-    void *arg);
-
 extern void _PyEval_SignalAsyncExc(PyInterpreterState *interp);
 #ifdef HAVE_FORK
 extern PyStatus _PyEval_ReInitThreads(PyThreadState *tstate);

Include/internal/pycore_crossinterp.h

@@ -0,0 +1,139 @@
+#ifndef Py_INTERNAL_CROSSINTERP_H
+#define Py_INTERNAL_CROSSINTERP_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+
+/***************************/
+/* cross-interpreter calls */
+/***************************/
+
+typedef int (*_Py_simple_func)(void *);
+extern int _Py_CallInInterpreter(
+    PyInterpreterState *interp,
+    _Py_simple_func func,
+    void *arg);
+extern int _Py_CallInInterpreterAndRawFree(
+    PyInterpreterState *interp,
+    _Py_simple_func func,
+    void *arg);
+
+
+/**************************/
+/* cross-interpreter data */
+/**************************/
+
+typedef struct _xid _PyCrossInterpreterData;
+typedef PyObject *(*xid_newobjectfunc)(_PyCrossInterpreterData *);
+typedef void (*xid_freefunc)(void *);
+
+// _PyCrossInterpreterData is similar to Py_buffer as an effectively
+// opaque struct that holds data outside the object machinery.  This
+// is necessary to pass safely between interpreters in the same process.
+struct _xid {
+    // data is the cross-interpreter-safe derivation of a Python object
+    // (see _PyObject_GetCrossInterpreterData).  It will be NULL if the
+    // new_object func (below) encodes the data.
+    void *data;
+    // obj is the Python object from which the data was derived.  This
+    // is non-NULL only if the data remains bound to the object in some
+    // way, such that the object must be "released" (via a decref) when
+    // the data is released.  In that case the code that sets the field,
+    // likely a registered "crossinterpdatafunc", is responsible for
+    // ensuring it owns the reference (i.e. incref).
+    PyObject *obj;
+    // interp is the ID of the owning interpreter of the original
+    // object.  It corresponds to the active interpreter when
+    // _PyObject_GetCrossInterpreterData() was called.  This should only
+    // be set by the cross-interpreter machinery.
+    //
+    // We use the ID rather than the PyInterpreterState to avoid issues
+    // with deleted interpreters.  Note that IDs are never re-used, so
+    // each one will always correspond to a specific interpreter
+    // (whether still alive or not).
+    int64_t interpid;
+    // new_object is a function that returns a new object in the current
+    // interpreter given the data.  The resulting object (a new
+    // reference) will be equivalent to the original object.  This field
+    // is required.
+    xid_newobjectfunc new_object;
+    // free is called when the data is released.  If it is NULL then
+    // nothing will be done to free the data.  For some types this is
+    // okay (e.g. bytes) and for those types this field should be set
+    // to NULL.  However, for most the data was allocated just for
+    // cross-interpreter use, so it must be freed when
+    // _PyCrossInterpreterData_Release is called or the memory will
+    // leak.  In that case, at the very least this field should be set
+    // to PyMem_RawFree (the default if not explicitly set to NULL).
+    // The call will happen with the original interpreter activated.
+    xid_freefunc free;
+};
+
+PyAPI_FUNC(_PyCrossInterpreterData *) _PyCrossInterpreterData_New(void);
+PyAPI_FUNC(void) _PyCrossInterpreterData_Free(_PyCrossInterpreterData *data);
+
+
+/* defining cross-interpreter data */
+
+PyAPI_FUNC(void) _PyCrossInterpreterData_Init(
+        _PyCrossInterpreterData *data,
+        PyInterpreterState *interp, void *shared, PyObject *obj,
+        xid_newobjectfunc new_object);
+PyAPI_FUNC(int) _PyCrossInterpreterData_InitWithSize(
+        _PyCrossInterpreterData *,
+        PyInterpreterState *interp, const size_t, PyObject *,
+        xid_newobjectfunc);
+PyAPI_FUNC(void) _PyCrossInterpreterData_Clear(
+        PyInterpreterState *, _PyCrossInterpreterData *);
+
+
+/* using cross-interpreter data */
+
+PyAPI_FUNC(int) _PyObject_CheckCrossInterpreterData(PyObject *);
+PyAPI_FUNC(int) _PyObject_GetCrossInterpreterData(PyObject *, _PyCrossInterpreterData *);
+PyAPI_FUNC(PyObject *) _PyCrossInterpreterData_NewObject(_PyCrossInterpreterData *);
+PyAPI_FUNC(int) _PyCrossInterpreterData_Release(_PyCrossInterpreterData *);
+PyAPI_FUNC(int) _PyCrossInterpreterData_ReleaseAndRawFree(_PyCrossInterpreterData *);
+
+
+/* cross-interpreter data registry */
+
+// For now we use a global registry of shareable classes.  An
+// alternative would be to add a tp_* slot for a class's
+// crossinterpdatafunc. It would be simpler and more efficient.
+
+typedef int (*crossinterpdatafunc)(PyThreadState *tstate, PyObject *,
+                                   _PyCrossInterpreterData *);
+
+struct _xidregitem;
+
+struct _xidregitem {
+    struct _xidregitem *prev;
+    struct _xidregitem *next;
+    /* This can be a dangling pointer, but only if weakref is set. */
+    PyTypeObject *cls;
+    /* This is NULL for builtin types. */
+    PyObject *weakref;
+    size_t refcount;
+    crossinterpdatafunc getdata;
+};
+
+struct _xidregistry {
+    PyThread_type_lock mutex;
+    struct _xidregitem *head;
+};
+
+PyAPI_FUNC(int) _PyCrossInterpreterData_RegisterClass(PyTypeObject *, crossinterpdatafunc);
+PyAPI_FUNC(int) _PyCrossInterpreterData_UnregisterClass(PyTypeObject *);
+PyAPI_FUNC(crossinterpdatafunc) _PyCrossInterpreterData_Lookup(PyObject *);
+
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_CROSSINTERP_H */

Include/internal/pycore_interp.h

@@ -15,6 +15,7 @@ extern "C" {
 #include "pycore_ceval_state.h"   // struct _ceval_state
 #include "pycore_code.h"          // struct callable_cache
 #include "pycore_context.h"       // struct _Py_context_state
+#include "pycore_crossinterp.h"   // struct _xidregistry
 #include "pycore_dict_state.h"    // struct _Py_dict_state
 #include "pycore_dtoa.h"          // struct _dtoa_state
 #include "pycore_exceptions.h"    // struct _Py_exc_state
@@ -41,28 +42,6 @@ struct _Py_long_state {
 
 /* cross-interpreter data registry */
 
-/* For now we use a global registry of shareable classes.  An
-   alternative would be to add a tp_* slot for a class's
-   crossinterpdatafunc. It would be simpler and more efficient. */
-
-struct _xidregitem;
-
-struct _xidregitem {
-    struct _xidregitem *prev;
-    struct _xidregitem *next;
-    /* This can be a dangling pointer, but only if weakref is set. */
-    PyTypeObject *cls;
-    /* This is NULL for builtin types. */
-    PyObject *weakref;
-    size_t refcount;
-    crossinterpdatafunc getdata;
-};
-
-struct _xidregistry {
-    PyThread_type_lock mutex;
-    struct _xidregitem *head;
-};
-
 
 /* interpreter state */
 

Include/internal/pycore_runtime.h

@@ -10,6 +10,7 @@ extern "C" {
 
 #include "pycore_atexit.h"          // struct _atexit_runtime_state
 #include "pycore_ceval_state.h"     // struct _ceval_runtime_state
+#include "pycore_crossinterp.h"   // struct _xidregistry
 #include "pycore_faulthandler.h"    // struct _faulthandler_runtime_state
 #include "pycore_floatobject.h"     // struct _Py_float_runtime_state
 #include "pycore_import.h"          // struct _import_runtime_state

Lib/test/test__xxsubinterpreters.py

@@ -7,7 +7,7 @@ from textwrap import dedent
 import threading
 import unittest
 
-import _testcapi
+import _testinternalcapi
 from test import support
 from test.support import import_helper
 from test.support import script_helper
@@ -146,8 +146,8 @@ class ShareableTypeTests(unittest.TestCase):
     def _assert_values(self, values):
         for obj in values:
             with self.subTest(obj):
-                xid = _testcapi.get_crossinterp_data(obj)
+                xid = _testinternalcapi.get_crossinterp_data(obj)
-                got = _testcapi.restore_crossinterp_data(xid)
+                got = _testinternalcapi.restore_crossinterp_data(xid)
 
                 self.assertEqual(got, obj)
                 self.assertIs(type(got), type(obj))
@@ -155,8 +155,8 @@ class ShareableTypeTests(unittest.TestCase):
     def test_singletons(self):
         for obj in [None]:
             with self.subTest(obj):
-                xid = _testcapi.get_crossinterp_data(obj)
+                xid = _testinternalcapi.get_crossinterp_data(obj)
-                got = _testcapi.restore_crossinterp_data(xid)
+                got = _testinternalcapi.restore_crossinterp_data(xid)
 
                 # XXX What about between interpreters?
                 self.assertIs(got, obj)
@@ -187,7 +187,7 @@ class ShareableTypeTests(unittest.TestCase):
         for i in ints:
             with self.subTest(i):
                 with self.assertRaises(OverflowError):
-                    _testcapi.get_crossinterp_data(i)
+                    _testinternalcapi.get_crossinterp_data(i)
 
 
 class ModuleTests(TestBase):

Makefile.pre.in

@@ -409,6 +409,7 @@ PYTHON_OBJS=	\
 		Python/codecs.o \
 		Python/compile.o \
 		Python/context.o \
+		Python/crossinterp.o \
 		Python/dynamic_annotations.o \
 		Python/errors.o \
 		Python/executor.o \
@@ -1800,6 +1801,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/internal/pycore_complexobject.h \
 		$(srcdir)/Include/internal/pycore_condvar.h \
 		$(srcdir)/Include/internal/pycore_context.h \
+		$(srcdir)/Include/internal/pycore_crossinterp.h \
 		$(srcdir)/Include/internal/pycore_dict.h \
 		$(srcdir)/Include/internal/pycore_dict_state.h \
 		$(srcdir)/Include/internal/pycore_descrobject.h \

Modules/_testcapimodule.c

@@ -1445,58 +1445,6 @@ run_in_subinterp(PyObject *self, PyObject *args)
     return PyLong_FromLong(r);
 }
 
-static void
-_xid_capsule_destructor(PyObject *capsule)
-{
-    _PyCrossInterpreterData *data = \
-            (_PyCrossInterpreterData *)PyCapsule_GetPointer(capsule, NULL);
-    if (data != NULL) {
-        assert(_PyCrossInterpreterData_Release(data) == 0);
-        PyMem_Free(data);
-    }
-}
-
-static PyObject *
-get_crossinterp_data(PyObject *self, PyObject *args)
-{
-    PyObject *obj = NULL;
-    if (!PyArg_ParseTuple(args, "O:get_crossinterp_data", &obj)) {
-        return NULL;
-    }
-
-    _PyCrossInterpreterData *data = PyMem_NEW(_PyCrossInterpreterData, 1);
-    if (data == NULL) {
-        PyErr_NoMemory();
-        return NULL;
-    }
-    if (_PyObject_GetCrossInterpreterData(obj, data) != 0) {
-        PyMem_Free(data);
-        return NULL;
-    }
-    PyObject *capsule = PyCapsule_New(data, NULL, _xid_capsule_destructor);
-    if (capsule == NULL) {
-        assert(_PyCrossInterpreterData_Release(data) == 0);
-        PyMem_Free(data);
-    }
-    return capsule;
-}
-
-static PyObject *
-restore_crossinterp_data(PyObject *self, PyObject *args)
-{
-    PyObject *capsule = NULL;
-    if (!PyArg_ParseTuple(args, "O:restore_crossinterp_data", &capsule)) {
-        return NULL;
-    }
-
-    _PyCrossInterpreterData *data = \
-            (_PyCrossInterpreterData *)PyCapsule_GetPointer(capsule, NULL);
-    if (data == NULL) {
-        return NULL;
-    }
-    return _PyCrossInterpreterData_NewObject(data);
-}
-
 static PyMethodDef ml;
 
 static PyObject *
@@ -3282,8 +3230,6 @@ static PyMethodDef TestMethods[] = {
     {"crash_no_current_thread", crash_no_current_thread,         METH_NOARGS},
     {"test_current_tstate_matches", test_current_tstate_matches, METH_NOARGS},
     {"run_in_subinterp",        run_in_subinterp,                METH_VARARGS},
-    {"get_crossinterp_data",    get_crossinterp_data,            METH_VARARGS},
-    {"restore_crossinterp_data", restore_crossinterp_data,       METH_VARARGS},
     {"create_cfunction",        create_cfunction,                METH_NOARGS},
     {"call_in_temporary_c_thread", call_in_temporary_c_thread, METH_VARARGS,
      PyDoc_STR("set_error_class(error_class) -> None")},

Modules/_testinternalcapi.c

@@ -1466,6 +1466,58 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
 }
 
 
+static void
+_xid_capsule_destructor(PyObject *capsule)
+{
+    _PyCrossInterpreterData *data = \
+            (_PyCrossInterpreterData *)PyCapsule_GetPointer(capsule, NULL);
+    if (data != NULL) {
+        assert(_PyCrossInterpreterData_Release(data) == 0);
+        _PyCrossInterpreterData_Free(data);
+    }
+}
+
+static PyObject *
+get_crossinterp_data(PyObject *self, PyObject *args)
+{
+    PyObject *obj = NULL;
+    if (!PyArg_ParseTuple(args, "O:get_crossinterp_data", &obj)) {
+        return NULL;
+    }
+
+    _PyCrossInterpreterData *data = _PyCrossInterpreterData_New();
+    if (data == NULL) {
+        return NULL;
+    }
+    if (_PyObject_GetCrossInterpreterData(obj, data) != 0) {
+        _PyCrossInterpreterData_Free(data);
+        return NULL;
+    }
+    PyObject *capsule = PyCapsule_New(data, NULL, _xid_capsule_destructor);
+    if (capsule == NULL) {
+        assert(_PyCrossInterpreterData_Release(data) == 0);
+        _PyCrossInterpreterData_Free(data);
+    }
+    return capsule;
+}
+
+static PyObject *
+restore_crossinterp_data(PyObject *self, PyObject *args)
+{
+    PyObject *capsule = NULL;
+    if (!PyArg_ParseTuple(args, "O:restore_crossinterp_data", &capsule)) {
+        return NULL;
+    }
+
+    _PyCrossInterpreterData *data = \
+            (_PyCrossInterpreterData *)PyCapsule_GetPointer(capsule, NULL);
+    if (data == NULL) {
+        return NULL;
+    }
+    return _PyCrossInterpreterData_NewObject(data);
+}
+
+
 /*[clinic input]
 _testinternalcapi.write_unraisable_exc
     exception as exc: object
@@ -1645,6 +1697,8 @@ static PyMethodDef module_functions[] = {
      METH_VARARGS | METH_KEYWORDS},
     {"compile_perf_trampoline_entry", compile_perf_trampoline_entry, METH_VARARGS},
     {"perf_trampoline_set_persist_after_fork", perf_trampoline_set_persist_after_fork, METH_VARARGS},
+    {"get_crossinterp_data",    get_crossinterp_data,            METH_VARARGS},
+    {"restore_crossinterp_data", restore_crossinterp_data,       METH_VARARGS},
     _TESTINTERNALCAPI_WRITE_UNRAISABLE_EXC_METHODDEF
     _TESTINTERNALCAPI_TEST_LONG_NUMBITS_METHODDEF
     {NULL, NULL} /* sentinel */

Modules/_xxinterpchannelsmodule.c

@@ -7,6 +7,7 @@
 
 #include "Python.h"
 #include "interpreteridobject.h"
+#include "pycore_crossinterp.h"   // struct _xid
 #include "pycore_pybuffer.h"      // _PyBuffer_ReleaseInInterpreterAndRawFree()
 #include "pycore_interp.h"        // _PyInterpreterState_LookUpID()
 

Modules/_xxsubinterpretersmodule.c

@@ -6,6 +6,7 @@
 #endif
 
 #include "Python.h"
+#include "pycore_crossinterp.h"   // struct _xid
 #include "pycore_initconfig.h"    // _PyErr_SetFromPyStatus()
 #include "pycore_modsupport.h"    // _PyArg_BadArgument()
 #include "pycore_pyerrors.h"      // _PyErr_ChainExceptions1()

Objects/abstract.c

@@ -5,6 +5,7 @@
 #include "pycore_pybuffer.h"
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
 #include "pycore_ceval.h"         // _Py_EnterRecursiveCallTstate()
+#include "pycore_crossinterp.h"   // _Py_CallInInterpreter()
 #include "pycore_object.h"        // _Py_CheckSlotResult()
 #include "pycore_long.h"          // _Py_IsNegative
 #include "pycore_pyerrors.h"      // _PyErr_Occurred()

PCbuild/_freeze_module.vcxproj

@@ -209,6 +209,7 @@
     <ClCompile Include="..\Python\codecs.c" />
     <ClCompile Include="..\Python\compile.c" />
     <ClCompile Include="..\Python\context.c" />
+    <ClCompile Include="..\Python\crossinterp.c" />
     <ClCompile Include="..\Python\dtoa.c" />
     <ClCompile Include="..\Python\dynamic_annotations.c" />
     <ClCompile Include="..\Python\dynload_win.c" />

PCbuild/_freeze_module.vcxproj.filters

@@ -103,6 +103,9 @@
     <ClCompile Include="..\Python\context.c">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="..\Python\crossinterp.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
     <ClCompile Include="..\Objects\descrobject.c">
       <Filter>Source Files</Filter>
     </ClCompile>

PCbuild/pythoncore.vcxproj

@@ -216,6 +216,7 @@
     <ClInclude Include="..\Include\internal\pycore_complexobject.h" />
     <ClInclude Include="..\Include\internal\pycore_condvar.h" />
     <ClInclude Include="..\Include\internal\pycore_context.h" />
+    <ClInclude Include="..\Include\internal\pycore_crossinterp.h" />
     <ClInclude Include="..\Include\internal\pycore_descrobject.h" />
     <ClInclude Include="..\Include\internal\pycore_dict.h" />
     <ClInclude Include="..\Include\internal\pycore_dict_state.h" />
@@ -561,6 +562,7 @@
     <ClCompile Include="..\Python\codecs.c" />
     <ClCompile Include="..\Python\compile.c" />
     <ClCompile Include="..\Python\context.c" />
+    <ClCompile Include="..\Python\crossinterp.c" />
     <ClCompile Include="..\Python\dynamic_annotations.c" />
     <ClCompile Include="..\Python\dynload_win.c" />
     <ClCompile Include="..\Python\errors.c" />

PCbuild/pythoncore.vcxproj.filters

@@ -579,6 +579,9 @@
     <ClInclude Include="..\Include\internal\pycore_context.h">
       <Filter>Include\internal</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\internal\pycore_crossinterp.h">
+      <Filter>Include\internal</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\internal\pycore_descrobject.h">
       <Filter>Include\internal</Filter>
     </ClInclude>
@@ -1280,6 +1283,9 @@
     <ClCompile Include="..\Python\compile.c">
       <Filter>Python</Filter>
     </ClCompile>
+    <ClCompile Include="..\Python\crossinterp.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
     <ClCompile Include="..\Python\dynamic_annotations.c">
       <Filter>Python</Filter>
     </ClCompile>

Python/crossinterp.c

@@ -0,0 +1,627 @@
+
+/* API for managing interactions between isolated interpreters */
+
+#include "Python.h"
+#include "pycore_ceval.h"         // _Py_simple_func
+#include "pycore_crossinterp.h"   // struct _xid
+#include "pycore_pyerrors.h"      // _PyErr_Clear()
+#include "pycore_pystate.h"       // _PyInterpreterState_GET()
+#include "pycore_weakref.h"       // _PyWeakref_GET_REF()
+
+
+/***************************/
+/* cross-interpreter calls */
+/***************************/
+
+int
+_Py_CallInInterpreter(PyInterpreterState *interp,
+                      _Py_simple_func func, void *arg)
+{
+    if (interp == _PyThreadState_GetCurrent()->interp) {
+        return func(arg);
+    }
+    // XXX Emit a warning if this fails?
+    _PyEval_AddPendingCall(interp, (_Py_pending_call_func)func, arg, 0);
+    return 0;
+}
+
+int
+_Py_CallInInterpreterAndRawFree(PyInterpreterState *interp,
+                                _Py_simple_func func, void *arg)
+{
+    if (interp == _PyThreadState_GetCurrent()->interp) {
+        int res = func(arg);
+        PyMem_RawFree(arg);
+        return res;
+    }
+    // XXX Emit a warning if this fails?
+    _PyEval_AddPendingCall(interp, func, arg, _Py_PENDING_RAWFREE);
+    return 0;
+}
+
+
+/**************************/
+/* cross-interpreter data */
+/**************************/
+
+_PyCrossInterpreterData *
+_PyCrossInterpreterData_New(void)
+{
+    _PyCrossInterpreterData *xid = PyMem_RawMalloc(
+                                            sizeof(_PyCrossInterpreterData));
+    if (xid == NULL) {
+        PyErr_NoMemory();
+    }
+    return xid;
+}
+
+void
+_PyCrossInterpreterData_Free(_PyCrossInterpreterData *xid)
+{
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    _PyCrossInterpreterData_Clear(interp, xid);
+    PyMem_RawFree(xid);
+}
+
+
+/* defining cross-interpreter data */
+
+static inline void
+_xidata_init(_PyCrossInterpreterData *data)
+{
+    // If the value is being reused
+    // then _xidata_clear() should have been called already.
+    assert(data->data == NULL);
+    assert(data->obj == NULL);
+    *data = (_PyCrossInterpreterData){0};
+    data->interpid = -1;
+}
+
+static inline void
+_xidata_clear(_PyCrossInterpreterData *data)
+{
+    // _PyCrossInterpreterData only has two members that need to be
+    // cleaned up, if set: "data" must be freed and "obj" must be decref'ed.
+    // In both cases the original (owning) interpreter must be used,
+    // which is the caller's responsibility to ensure.
+    if (data->data != NULL) {
+        if (data->free != NULL) {
+            data->free(data->data);
+        }
+        data->data = NULL;
+    }
+    Py_CLEAR(data->obj);
+}
+
+void
+_PyCrossInterpreterData_Init(_PyCrossInterpreterData *data,
+                             PyInterpreterState *interp,
+                             void *shared, PyObject *obj,
+                             xid_newobjectfunc new_object)
+{
+    assert(data != NULL);
+    assert(new_object != NULL);
+    _xidata_init(data);
+    data->data = shared;
+    if (obj != NULL) {
+        assert(interp != NULL);
+        // released in _PyCrossInterpreterData_Clear()
+        data->obj = Py_NewRef(obj);
+    }
+    // Ideally every object would know its owning interpreter.
+    // Until then, we have to rely on the caller to identify it
+    // (but we don't need it in all cases).
+    data->interpid = (interp != NULL) ? interp->id : -1;
+    data->new_object = new_object;
+}
+
+int
+_PyCrossInterpreterData_InitWithSize(_PyCrossInterpreterData *data,
+                                     PyInterpreterState *interp,
+                                     const size_t size, PyObject *obj,
+                                     xid_newobjectfunc new_object)
+{
+    assert(size > 0);
+    // For now we always free the shared data in the same interpreter
+    // where it was allocated, so the interpreter is required.
+    assert(interp != NULL);
+    _PyCrossInterpreterData_Init(data, interp, NULL, obj, new_object);
+    data->data = PyMem_RawMalloc(size);
+    if (data->data == NULL) {
+        return -1;
+    }
+    data->free = PyMem_RawFree;
+    return 0;
+}
+
+void
+_PyCrossInterpreterData_Clear(PyInterpreterState *interp,
+                              _PyCrossInterpreterData *data)
+{
+    assert(data != NULL);
+    // This must be called in the owning interpreter.
+    assert(interp == NULL
+           || data->interpid == -1
+           || data->interpid == interp->id);
+    _xidata_clear(data);
+}
+
+
+/* using cross-interpreter data */
+
+static int
+_check_xidata(PyThreadState *tstate, _PyCrossInterpreterData *data)
+{
+    // data->data can be anything, including NULL, so we don't check it.
+
+    // data->obj may be NULL, so we don't check it.
+
+    if (data->interpid < 0) {
+        _PyErr_SetString(tstate, PyExc_SystemError, "missing interp");
+        return -1;
+    }
+
+    if (data->new_object == NULL) {
+        _PyErr_SetString(tstate, PyExc_SystemError, "missing new_object func");
+        return -1;
+    }
+
+    // data->free may be NULL, so we don't check it.
+
+    return 0;
+}
+
+crossinterpdatafunc _PyCrossInterpreterData_Lookup(PyObject *);
+
+/* This is a separate func from _PyCrossInterpreterData_Lookup in order
+   to keep the registry code separate. */
+static crossinterpdatafunc
+_lookup_getdata(PyObject *obj)
+{
+    crossinterpdatafunc getdata = _PyCrossInterpreterData_Lookup(obj);
+    if (getdata == NULL && PyErr_Occurred() == 0)
+        PyErr_Format(PyExc_ValueError,
+                     "%S does not support cross-interpreter data", obj);
+    return getdata;
+}
+
+int
+_PyObject_CheckCrossInterpreterData(PyObject *obj)
+{
+    crossinterpdatafunc getdata = _lookup_getdata(obj);
+    if (getdata == NULL) {
+        return -1;
+    }
+    return 0;
+}
+
+int
+_PyObject_GetCrossInterpreterData(PyObject *obj, _PyCrossInterpreterData *data)
+{
+    PyThreadState *tstate = _PyThreadState_GetCurrent();
+#ifdef Py_DEBUG
+    // The caller must hold the GIL
+    _Py_EnsureTstateNotNULL(tstate);
+#endif
+    PyInterpreterState *interp = tstate->interp;
+
+    // Reset data before re-populating.
+    *data = (_PyCrossInterpreterData){0};
+    data->interpid = -1;
+
+    // Call the "getdata" func for the object.
+    Py_INCREF(obj);
+    crossinterpdatafunc getdata = _lookup_getdata(obj);
+    if (getdata == NULL) {
+        Py_DECREF(obj);
+        return -1;
+    }
+    int res = getdata(tstate, obj, data);
+    Py_DECREF(obj);
+    if (res != 0) {
+        return -1;
+    }
+
+    // Fill in the blanks and validate the result.
+    data->interpid = interp->id;
+    if (_check_xidata(tstate, data) != 0) {
+        (void)_PyCrossInterpreterData_Release(data);
+        return -1;
+    }
+
+    return 0;
+}
+
+PyObject *
+_PyCrossInterpreterData_NewObject(_PyCrossInterpreterData *data)
+{
+    return data->new_object(data);
+}
+
+static int
+_call_clear_xidata(void *data)
+{
+    _xidata_clear((_PyCrossInterpreterData *)data);
+    return 0;
+}
+
+static int
+_xidata_release(_PyCrossInterpreterData *data, int rawfree)
+{
+    if ((data->data == NULL || data->free == NULL) && data->obj == NULL) {
+        // Nothing to release!
+        if (rawfree) {
+            PyMem_RawFree(data);
+        }
+        else {
+            data->data = NULL;
+        }
+        return 0;
+    }
+
+    // Switch to the original interpreter.
+    PyInterpreterState *interp = _PyInterpreterState_LookUpID(data->interpid);
+    if (interp == NULL) {
+        // The interpreter was already destroyed.
+        // This function shouldn't have been called.
+        // XXX Someone leaked some memory...
+        assert(PyErr_Occurred());
+        if (rawfree) {
+            PyMem_RawFree(data);
+        }
+        return -1;
+    }
+
+    // "Release" the data and/or the object.
+    if (rawfree) {
+        return _Py_CallInInterpreterAndRawFree(interp, _call_clear_xidata, data);
+    }
+    else {
+        return _Py_CallInInterpreter(interp, _call_clear_xidata, data);
+    }
+}
+
+int
+_PyCrossInterpreterData_Release(_PyCrossInterpreterData *data)
+{
+    return _xidata_release(data, 0);
+}
+
+int
+_PyCrossInterpreterData_ReleaseAndRawFree(_PyCrossInterpreterData *data)
+{
+    return _xidata_release(data, 1);
+}
+
+
+/* registry of {type -> crossinterpdatafunc} */
+
+/* For now we use a global registry of shareable classes.  An
+   alternative would be to add a tp_* slot for a class's
+   crossinterpdatafunc. It would be simpler and more efficient. */
+
+static int
+_xidregistry_add_type(struct _xidregistry *xidregistry,
+                      PyTypeObject *cls, crossinterpdatafunc getdata)
+{
+    struct _xidregitem *newhead = PyMem_RawMalloc(sizeof(struct _xidregitem));
+    if (newhead == NULL) {
+        return -1;
+    }
+    *newhead = (struct _xidregitem){
+        // We do not keep a reference, to avoid keeping the class alive.
+        .cls = cls,
+        .refcount = 1,
+        .getdata = getdata,
+    };
+    if (cls->tp_flags & Py_TPFLAGS_HEAPTYPE) {
+        // XXX Assign a callback to clear the entry from the registry?
+        newhead->weakref = PyWeakref_NewRef((PyObject *)cls, NULL);
+        if (newhead->weakref == NULL) {
+            PyMem_RawFree(newhead);
+            return -1;
+        }
+    }
+    newhead->next = xidregistry->head;
+    if (newhead->next != NULL) {
+        newhead->next->prev = newhead;
+    }
+    xidregistry->head = newhead;
+    return 0;
+}
+
+static struct _xidregitem *
+_xidregistry_remove_entry(struct _xidregistry *xidregistry,
+                          struct _xidregitem *entry)
+{
+    struct _xidregitem *next = entry->next;
+    if (entry->prev != NULL) {
+        assert(entry->prev->next == entry);
+        entry->prev->next = next;
+    }
+    else {
+        assert(xidregistry->head == entry);
+        xidregistry->head = next;
+    }
+    if (next != NULL) {
+        next->prev = entry->prev;
+    }
+    Py_XDECREF(entry->weakref);
+    PyMem_RawFree(entry);
+    return next;
+}
+
+// This is used in pystate.c (for now).
+void
+_Py_xidregistry_clear(struct _xidregistry *xidregistry)
+{
+    struct _xidregitem *cur = xidregistry->head;
+    xidregistry->head = NULL;
+    while (cur != NULL) {
+        struct _xidregitem *next = cur->next;
+        Py_XDECREF(cur->weakref);
+        PyMem_RawFree(cur);
+        cur = next;
+    }
+}
+
+static struct _xidregitem *
+_xidregistry_find_type(struct _xidregistry *xidregistry, PyTypeObject *cls)
+{
+    struct _xidregitem *cur = xidregistry->head;
+    while (cur != NULL) {
+        if (cur->weakref != NULL) {
+            // cur is/was a heap type.
+            PyObject *registered = _PyWeakref_GET_REF(cur->weakref);
+            if (registered == NULL) {
+                // The weakly ref'ed object was freed.
+                cur = _xidregistry_remove_entry(xidregistry, cur);
+                continue;
+            }
+            assert(PyType_Check(registered));
+            assert(cur->cls == (PyTypeObject *)registered);
+            assert(cur->cls->tp_flags & Py_TPFLAGS_HEAPTYPE);
+            Py_DECREF(registered);
+        }
+        if (cur->cls == cls) {
+            return cur;
+        }
+        cur = cur->next;
+    }
+    return NULL;
+}
+
+static inline struct _xidregistry *
+_get_xidregistry(PyInterpreterState *interp, PyTypeObject *cls)
+{
+    struct _xidregistry *xidregistry = &interp->runtime->xidregistry;
+    if (cls->tp_flags & Py_TPFLAGS_HEAPTYPE) {
+        assert(interp->xidregistry.mutex == xidregistry->mutex);
+        xidregistry = &interp->xidregistry;
+    }
+    return xidregistry;
+}
+
+static void _register_builtins_for_crossinterpreter_data(struct _xidregistry *xidregistry);
+
+static inline void
+_ensure_builtins_xid(PyInterpreterState *interp, struct _xidregistry *xidregistry)
+{
+    if (xidregistry != &interp->xidregistry) {
+        assert(xidregistry == &interp->runtime->xidregistry);
+        if (xidregistry->head == NULL) {
+            _register_builtins_for_crossinterpreter_data(xidregistry);
+        }
+    }
+}
+
+int
+_PyCrossInterpreterData_RegisterClass(PyTypeObject *cls,
+                                      crossinterpdatafunc getdata)
+{
+    if (!PyType_Check(cls)) {
+        PyErr_Format(PyExc_ValueError, "only classes may be registered");
+        return -1;
+    }
+    if (getdata == NULL) {
+        PyErr_Format(PyExc_ValueError, "missing 'getdata' func");
+        return -1;
+    }
+
+    int res = 0;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    struct _xidregistry *xidregistry = _get_xidregistry(interp, cls);
+    PyThread_acquire_lock(xidregistry->mutex, WAIT_LOCK);
+
+    _ensure_builtins_xid(interp, xidregistry);
+
+    struct _xidregitem *matched = _xidregistry_find_type(xidregistry, cls);
+    if (matched != NULL) {
+        assert(matched->getdata == getdata);
+        matched->refcount += 1;
+        goto finally;
+    }
+
+    res = _xidregistry_add_type(xidregistry, cls, getdata);
+
+finally:
+    PyThread_release_lock(xidregistry->mutex);
+    return res;
+}
+
+int
+_PyCrossInterpreterData_UnregisterClass(PyTypeObject *cls)
+{
+    int res = 0;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    struct _xidregistry *xidregistry = _get_xidregistry(interp, cls);
+    PyThread_acquire_lock(xidregistry->mutex, WAIT_LOCK);
+
+    struct _xidregitem *matched = _xidregistry_find_type(xidregistry, cls);
+    if (matched != NULL) {
+        assert(matched->refcount > 0);
+        matched->refcount -= 1;
+        if (matched->refcount == 0) {
+            (void)_xidregistry_remove_entry(xidregistry, matched);
+        }
+        res = 1;
+    }
+
+    PyThread_release_lock(xidregistry->mutex);
+    return res;
+}
+
+
+/* Cross-interpreter objects are looked up by exact match on the class.
+   We can reassess this policy when we move from a global registry to a
+   tp_* slot. */
+
+crossinterpdatafunc
+_PyCrossInterpreterData_Lookup(PyObject *obj)
+{
+    PyTypeObject *cls = Py_TYPE(obj);
+
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    struct _xidregistry *xidregistry = _get_xidregistry(interp, cls);
+    PyThread_acquire_lock(xidregistry->mutex, WAIT_LOCK);
+
+    _ensure_builtins_xid(interp, xidregistry);
+
+    struct _xidregitem *matched = _xidregistry_find_type(xidregistry, cls);
+    crossinterpdatafunc func = matched != NULL ? matched->getdata : NULL;
+
+    PyThread_release_lock(xidregistry->mutex);
+    return func;
+}
+
+/* cross-interpreter data for builtin types */
+
+struct _shared_bytes_data {
+    char *bytes;
+    Py_ssize_t len;
+};
+
+static PyObject *
+_new_bytes_object(_PyCrossInterpreterData *data)
+{
+    struct _shared_bytes_data *shared = (struct _shared_bytes_data *)(data->data);
+    return PyBytes_FromStringAndSize(shared->bytes, shared->len);
+}
+
+static int
+_bytes_shared(PyThreadState *tstate, PyObject *obj,
+              _PyCrossInterpreterData *data)
+{
+    if (_PyCrossInterpreterData_InitWithSize(
+            data, tstate->interp, sizeof(struct _shared_bytes_data), obj,
+            _new_bytes_object
+            ) < 0)
+    {
+        return -1;
+    }
+    struct _shared_bytes_data *shared = (struct _shared_bytes_data *)data->data;
+    if (PyBytes_AsStringAndSize(obj, &shared->bytes, &shared->len) < 0) {
+        _PyCrossInterpreterData_Clear(tstate->interp, data);
+        return -1;
+    }
+    return 0;
+}
+
+struct _shared_str_data {
+    int kind;
+    const void *buffer;
+    Py_ssize_t len;
+};
+
+static PyObject *
+_new_str_object(_PyCrossInterpreterData *data)
+{
+    struct _shared_str_data *shared = (struct _shared_str_data *)(data->data);
+    return PyUnicode_FromKindAndData(shared->kind, shared->buffer, shared->len);
+}
+
+static int
+_str_shared(PyThreadState *tstate, PyObject *obj,
+            _PyCrossInterpreterData *data)
+{
+    if (_PyCrossInterpreterData_InitWithSize(
+            data, tstate->interp, sizeof(struct _shared_str_data), obj,
+            _new_str_object
+            ) < 0)
+    {
+        return -1;
+    }
+    struct _shared_str_data *shared = (struct _shared_str_data *)data->data;
+    shared->kind = PyUnicode_KIND(obj);
+    shared->buffer = PyUnicode_DATA(obj);
+    shared->len = PyUnicode_GET_LENGTH(obj);
+    return 0;
+}
+
+static PyObject *
+_new_long_object(_PyCrossInterpreterData *data)
+{
+    return PyLong_FromSsize_t((Py_ssize_t)(data->data));
+}
+
+static int
+_long_shared(PyThreadState *tstate, PyObject *obj,
+             _PyCrossInterpreterData *data)
+{
+    /* Note that this means the size of shareable ints is bounded by
+     * sys.maxsize.  Hence on 32-bit architectures that is half the
+     * size of maximum shareable ints on 64-bit.
+     */
+    Py_ssize_t value = PyLong_AsSsize_t(obj);
+    if (value == -1 && PyErr_Occurred()) {
+        if (PyErr_ExceptionMatches(PyExc_OverflowError)) {
+            PyErr_SetString(PyExc_OverflowError, "try sending as bytes");
+        }
+        return -1;
+    }
+    _PyCrossInterpreterData_Init(data, tstate->interp, (void *)value, NULL,
+            _new_long_object);
+    // data->obj and data->free remain NULL
+    return 0;
+}
+
+static PyObject *
+_new_none_object(_PyCrossInterpreterData *data)
+{
+    // XXX Singleton refcounts are problematic across interpreters...
+    return Py_NewRef(Py_None);
+}
+
+static int
+_none_shared(PyThreadState *tstate, PyObject *obj,
+             _PyCrossInterpreterData *data)
+{
+    _PyCrossInterpreterData_Init(data, tstate->interp, NULL, NULL,
+            _new_none_object);
+    // data->data, data->obj and data->free remain NULL
+    return 0;
+}
+
+static void
+_register_builtins_for_crossinterpreter_data(struct _xidregistry *xidregistry)
+{
+    // None
+    if (_xidregistry_add_type(xidregistry, (PyTypeObject *)PyObject_Type(Py_None), _none_shared) != 0) {
+        Py_FatalError("could not register None for cross-interpreter sharing");
+    }
+
+    // int
+    if (_xidregistry_add_type(xidregistry, &PyLong_Type, _long_shared) != 0) {
+        Py_FatalError("could not register int for cross-interpreter sharing");
+    }
+
+    // bytes
+    if (_xidregistry_add_type(xidregistry, &PyBytes_Type, _bytes_shared) != 0) {
+        Py_FatalError("could not register bytes for cross-interpreter sharing");
+    }
+
+    // str
+    if (_xidregistry_add_type(xidregistry, &PyUnicode_Type, _str_shared) != 0) {
+        Py_FatalError("could not register str for cross-interpreter sharing");
+    }
+}

Python/pystate.c

@@ -16,7 +16,6 @@
 #include "pycore_pystate.h"
 #include "pycore_runtime_init.h"  // _PyRuntimeState_INIT
 #include "pycore_sysmodule.h"     // _PySys_Audit()
-#include "pycore_weakref.h"       // _PyWeakref_GET_REF()
 
 /* --------------------------------------------------------------------------
 CAUTION
@@ -495,7 +494,8 @@ _PyRuntimeState_Init(_PyRuntimeState *runtime)
     return _PyStatus_OK();
 }
 
-static void _xidregistry_clear(struct _xidregistry *);
+// This is defined in crossinterp.c (for now).
+extern void _Py_xidregistry_clear(struct _xidregistry *);
 
 void
 _PyRuntimeState_Fini(_PyRuntimeState *runtime)
@@ -505,7 +505,7 @@ _PyRuntimeState_Fini(_PyRuntimeState *runtime)
     assert(runtime->object_state.interpreter_leaks == 0);
 #endif
 
-    _xidregistry_clear(&runtime->xidregistry);
+    _Py_xidregistry_clear(&runtime->xidregistry);
 
     if (gilstate_tss_initialized(runtime)) {
         gilstate_tss_fini(runtime);
@@ -948,7 +948,7 @@ interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate)
     Py_CLEAR(interp->sysdict);
     Py_CLEAR(interp->builtins);
 
-    _xidregistry_clear(&interp->xidregistry);
+    _Py_xidregistry_clear(&interp->xidregistry);
     /* The lock is owned by the runtime, so we don't free it here. */
     interp->xidregistry.mutex = NULL;
 
@@ -2415,593 +2415,6 @@ PyGILState_Release(PyGILState_STATE oldstate)
 }
 
 
-/**************************/
-/* cross-interpreter data */
-/**************************/
-
-/* cross-interpreter data */
-
-static inline void
-_xidata_init(_PyCrossInterpreterData *data)
-{
-    // If the value is being reused
-    // then _xidata_clear() should have been called already.
-    assert(data->data == NULL);
-    assert(data->obj == NULL);
-    *data = (_PyCrossInterpreterData){0};
-    data->interpid = -1;
-}
-
-static inline void
-_xidata_clear(_PyCrossInterpreterData *data)
-{
-    // _PyCrossInterpreterData only has two members that need to be
-    // cleaned up, if set: "data" must be freed and "obj" must be decref'ed.
-    // In both cases the original (owning) interpreter must be used,
-    // which is the caller's responsibility to ensure.
-    if (data->data != NULL) {
-        if (data->free != NULL) {
-            data->free(data->data);
-        }
-        data->data = NULL;
-    }
-    Py_CLEAR(data->obj);
-}
-
-void
-_PyCrossInterpreterData_Init(_PyCrossInterpreterData *data,
-                             PyInterpreterState *interp,
-                             void *shared, PyObject *obj,
-                             xid_newobjectfunc new_object)
-{
-    assert(data != NULL);
-    assert(new_object != NULL);
-    _xidata_init(data);
-    data->data = shared;
-    if (obj != NULL) {
-        assert(interp != NULL);
-        // released in _PyCrossInterpreterData_Clear()
-        data->obj = Py_NewRef(obj);
-    }
-    // Ideally every object would know its owning interpreter.
-    // Until then, we have to rely on the caller to identify it
-    // (but we don't need it in all cases).
-    data->interpid = (interp != NULL) ? interp->id : -1;
-    data->new_object = new_object;
-}
-
-int
-_PyCrossInterpreterData_InitWithSize(_PyCrossInterpreterData *data,
-                                     PyInterpreterState *interp,
-                                     const size_t size, PyObject *obj,
-                                     xid_newobjectfunc new_object)
-{
-    assert(size > 0);
-    // For now we always free the shared data in the same interpreter
-    // where it was allocated, so the interpreter is required.
-    assert(interp != NULL);
-    _PyCrossInterpreterData_Init(data, interp, NULL, obj, new_object);
-    data->data = PyMem_RawMalloc(size);
-    if (data->data == NULL) {
-        return -1;
-    }
-    data->free = PyMem_RawFree;
-    return 0;
-}
-
-void
-_PyCrossInterpreterData_Clear(PyInterpreterState *interp,
-                              _PyCrossInterpreterData *data)
-{
-    assert(data != NULL);
-    // This must be called in the owning interpreter.
-    assert(interp == NULL || data->interpid == interp->id);
-    _xidata_clear(data);
-}
-
-static int
-_check_xidata(PyThreadState *tstate, _PyCrossInterpreterData *data)
-{
-    // data->data can be anything, including NULL, so we don't check it.
-
-    // data->obj may be NULL, so we don't check it.
-
-    if (data->interpid < 0) {
-        _PyErr_SetString(tstate, PyExc_SystemError, "missing interp");
-        return -1;
-    }
-
-    if (data->new_object == NULL) {
-        _PyErr_SetString(tstate, PyExc_SystemError, "missing new_object func");
-        return -1;
-    }
-
-    // data->free may be NULL, so we don't check it.
-
-    return 0;
-}
-
-crossinterpdatafunc _PyCrossInterpreterData_Lookup(PyObject *);
-
-/* This is a separate func from _PyCrossInterpreterData_Lookup in order
-   to keep the registry code separate. */
-static crossinterpdatafunc
-_lookup_getdata(PyObject *obj)
-{
-    crossinterpdatafunc getdata = _PyCrossInterpreterData_Lookup(obj);
-    if (getdata == NULL && PyErr_Occurred() == 0)
-        PyErr_Format(PyExc_ValueError,
-                     "%S does not support cross-interpreter data", obj);
-    return getdata;
-}
-
-int
-_PyObject_CheckCrossInterpreterData(PyObject *obj)
-{
-    crossinterpdatafunc getdata = _lookup_getdata(obj);
-    if (getdata == NULL) {
-        return -1;
-    }
-    return 0;
-}
-
-int
-_PyObject_GetCrossInterpreterData(PyObject *obj, _PyCrossInterpreterData *data)
-{
-    _PyRuntimeState *runtime = &_PyRuntime;
-    PyThreadState *tstate = current_fast_get(runtime);
-#ifdef Py_DEBUG
-    // The caller must hold the GIL
-    _Py_EnsureTstateNotNULL(tstate);
-#endif
-    PyInterpreterState *interp = tstate->interp;
-
-    // Reset data before re-populating.
-    *data = (_PyCrossInterpreterData){0};
-    data->interpid = -1;
-
-    // Call the "getdata" func for the object.
-    Py_INCREF(obj);
-    crossinterpdatafunc getdata = _lookup_getdata(obj);
-    if (getdata == NULL) {
-        Py_DECREF(obj);
-        return -1;
-    }
-    int res = getdata(tstate, obj, data);
-    Py_DECREF(obj);
-    if (res != 0) {
-        return -1;
-    }
-
-    // Fill in the blanks and validate the result.
-    data->interpid = interp->id;
-    if (_check_xidata(tstate, data) != 0) {
-        (void)_PyCrossInterpreterData_Release(data);
-        return -1;
-    }
-
-    return 0;
-}
-
-PyObject *
-_PyCrossInterpreterData_NewObject(_PyCrossInterpreterData *data)
-{
-    return data->new_object(data);
-}
-
-int
-_Py_CallInInterpreter(PyInterpreterState *interp,
-                      _Py_simple_func func, void *arg)
-{
-    if (interp == current_fast_get(interp->runtime)->interp) {
-        return func(arg);
-    }
-    // XXX Emit a warning if this fails?
-    _PyEval_AddPendingCall(interp, (_Py_pending_call_func)func, arg, 0);
-    return 0;
-}
-
-int
-_Py_CallInInterpreterAndRawFree(PyInterpreterState *interp,
-                                _Py_simple_func func, void *arg)
-{
-    if (interp == current_fast_get(interp->runtime)->interp) {
-        int res = func(arg);
-        PyMem_RawFree(arg);
-        return res;
-    }
-    // XXX Emit a warning if this fails?
-    _PyEval_AddPendingCall(interp, func, arg, _Py_PENDING_RAWFREE);
-    return 0;
-}
-
-static int
-_call_clear_xidata(void *data)
-{
-    _xidata_clear((_PyCrossInterpreterData *)data);
-    return 0;
-}
-
-static int
-_xidata_release(_PyCrossInterpreterData *data, int rawfree)
-{
-    if ((data->data == NULL || data->free == NULL) && data->obj == NULL) {
-        // Nothing to release!
-        if (rawfree) {
-            PyMem_RawFree(data);
-        }
-        else {
-            data->data = NULL;
-        }
-        return 0;
-    }
-
-    // Switch to the original interpreter.
-    PyInterpreterState *interp = _PyInterpreterState_LookUpID(data->interpid);
-    if (interp == NULL) {
-        // The interpreter was already destroyed.
-        // This function shouldn't have been called.
-        // XXX Someone leaked some memory...
-        assert(PyErr_Occurred());
-        if (rawfree) {
-            PyMem_RawFree(data);
-        }
-        return -1;
-    }
-
-    // "Release" the data and/or the object.
-    if (rawfree) {
-        return _Py_CallInInterpreterAndRawFree(interp, _call_clear_xidata, data);
-    }
-    else {
-        return _Py_CallInInterpreter(interp, _call_clear_xidata, data);
-    }
-}
-
-int
-_PyCrossInterpreterData_Release(_PyCrossInterpreterData *data)
-{
-    return _xidata_release(data, 0);
-}
-
-int
-_PyCrossInterpreterData_ReleaseAndRawFree(_PyCrossInterpreterData *data)
-{
-    return _xidata_release(data, 1);
-}
-
-/* registry of {type -> crossinterpdatafunc} */
-
-/* For now we use a global registry of shareable classes.  An
-   alternative would be to add a tp_* slot for a class's
-   crossinterpdatafunc. It would be simpler and more efficient. */
-
-static int
-_xidregistry_add_type(struct _xidregistry *xidregistry,
-                      PyTypeObject *cls, crossinterpdatafunc getdata)
-{
-    struct _xidregitem *newhead = PyMem_RawMalloc(sizeof(struct _xidregitem));
-    if (newhead == NULL) {
-        return -1;
-    }
-    *newhead = (struct _xidregitem){
-        // We do not keep a reference, to avoid keeping the class alive.
-        .cls = cls,
-        .refcount = 1,
-        .getdata = getdata,
-    };
-    if (cls->tp_flags & Py_TPFLAGS_HEAPTYPE) {
-        // XXX Assign a callback to clear the entry from the registry?
-        newhead->weakref = PyWeakref_NewRef((PyObject *)cls, NULL);
-        if (newhead->weakref == NULL) {
-            PyMem_RawFree(newhead);
-            return -1;
-        }
-    }
-    newhead->next = xidregistry->head;
-    if (newhead->next != NULL) {
-        newhead->next->prev = newhead;
-    }
-    xidregistry->head = newhead;
-    return 0;
-}
-
-static struct _xidregitem *
-_xidregistry_remove_entry(struct _xidregistry *xidregistry,
-                          struct _xidregitem *entry)
-{
-    struct _xidregitem *next = entry->next;
-    if (entry->prev != NULL) {
-        assert(entry->prev->next == entry);
-        entry->prev->next = next;
-    }
-    else {
-        assert(xidregistry->head == entry);
-        xidregistry->head = next;
-    }
-    if (next != NULL) {
-        next->prev = entry->prev;
-    }
-    Py_XDECREF(entry->weakref);
-    PyMem_RawFree(entry);
-    return next;
-}
-
-static void
-_xidregistry_clear(struct _xidregistry *xidregistry)
-{
-    struct _xidregitem *cur = xidregistry->head;
-    xidregistry->head = NULL;
-    while (cur != NULL) {
-        struct _xidregitem *next = cur->next;
-        Py_XDECREF(cur->weakref);
-        PyMem_RawFree(cur);
-        cur = next;
-    }
-}
-
-static struct _xidregitem *
-_xidregistry_find_type(struct _xidregistry *xidregistry, PyTypeObject *cls)
-{
-    struct _xidregitem *cur = xidregistry->head;
-    while (cur != NULL) {
-        if (cur->weakref != NULL) {
-            // cur is/was a heap type.
-            PyObject *registered = _PyWeakref_GET_REF(cur->weakref);
-            if (registered == NULL) {
-                // The weakly ref'ed object was freed.
-                cur = _xidregistry_remove_entry(xidregistry, cur);
-                continue;
-            }
-            assert(PyType_Check(registered));
-            assert(cur->cls == (PyTypeObject *)registered);
-            assert(cur->cls->tp_flags & Py_TPFLAGS_HEAPTYPE);
-            Py_DECREF(registered);
-        }
-        if (cur->cls == cls) {
-            return cur;
-        }
-        cur = cur->next;
-    }
-    return NULL;
-}
-
-static inline struct _xidregistry *
-_get_xidregistry(PyInterpreterState *interp, PyTypeObject *cls)
-{
-    struct _xidregistry *xidregistry = &interp->runtime->xidregistry;
-    if (cls->tp_flags & Py_TPFLAGS_HEAPTYPE) {
-        assert(interp->xidregistry.mutex == xidregistry->mutex);
-        xidregistry = &interp->xidregistry;
-    }
-    return xidregistry;
-}
-
-static void _register_builtins_for_crossinterpreter_data(struct _xidregistry *xidregistry);
-
-static inline void
-_ensure_builtins_xid(PyInterpreterState *interp, struct _xidregistry *xidregistry)
-{
-    if (xidregistry != &interp->xidregistry) {
-        assert(xidregistry == &interp->runtime->xidregistry);
-        if (xidregistry->head == NULL) {
-            _register_builtins_for_crossinterpreter_data(xidregistry);
-        }
-    }
-}
-
-int
-_PyCrossInterpreterData_RegisterClass(PyTypeObject *cls,
-                                      crossinterpdatafunc getdata)
-{
-    if (!PyType_Check(cls)) {
-        PyErr_Format(PyExc_ValueError, "only classes may be registered");
-        return -1;
-    }
-    if (getdata == NULL) {
-        PyErr_Format(PyExc_ValueError, "missing 'getdata' func");
-        return -1;
-    }
-
-    int res = 0;
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    struct _xidregistry *xidregistry = _get_xidregistry(interp, cls);
-    PyThread_acquire_lock(xidregistry->mutex, WAIT_LOCK);
-
-    _ensure_builtins_xid(interp, xidregistry);
-
-    struct _xidregitem *matched = _xidregistry_find_type(xidregistry, cls);
-    if (matched != NULL) {
-        assert(matched->getdata == getdata);
-        matched->refcount += 1;
-        goto finally;
-    }
-
-    res = _xidregistry_add_type(xidregistry, cls, getdata);
-
-finally:
-    PyThread_release_lock(xidregistry->mutex);
-    return res;
-}
-
-int
-_PyCrossInterpreterData_UnregisterClass(PyTypeObject *cls)
-{
-    int res = 0;
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    struct _xidregistry *xidregistry = _get_xidregistry(interp, cls);
-    PyThread_acquire_lock(xidregistry->mutex, WAIT_LOCK);
-
-    struct _xidregitem *matched = _xidregistry_find_type(xidregistry, cls);
-    if (matched != NULL) {
-        assert(matched->refcount > 0);
-        matched->refcount -= 1;
-        if (matched->refcount == 0) {
-            (void)_xidregistry_remove_entry(xidregistry, matched);
-        }
-        res = 1;
-    }
-
-    PyThread_release_lock(xidregistry->mutex);
-    return res;
-}
-
-
-/* Cross-interpreter objects are looked up by exact match on the class.
-   We can reassess this policy when we move from a global registry to a
-   tp_* slot. */
-
-crossinterpdatafunc
-_PyCrossInterpreterData_Lookup(PyObject *obj)
-{
-    PyTypeObject *cls = Py_TYPE(obj);
-
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    struct _xidregistry *xidregistry = _get_xidregistry(interp, cls);
-    PyThread_acquire_lock(xidregistry->mutex, WAIT_LOCK);
-
-    _ensure_builtins_xid(interp, xidregistry);
-
-    struct _xidregitem *matched = _xidregistry_find_type(xidregistry, cls);
-    crossinterpdatafunc func = matched != NULL ? matched->getdata : NULL;
-
-    PyThread_release_lock(xidregistry->mutex);
-    return func;
-}
-
-/* cross-interpreter data for builtin types */
-
-struct _shared_bytes_data {
-    char *bytes;
-    Py_ssize_t len;
-};
-
-static PyObject *
-_new_bytes_object(_PyCrossInterpreterData *data)
-{
-    struct _shared_bytes_data *shared = (struct _shared_bytes_data *)(data->data);
-    return PyBytes_FromStringAndSize(shared->bytes, shared->len);
-}
-
-static int
-_bytes_shared(PyThreadState *tstate, PyObject *obj,
-              _PyCrossInterpreterData *data)
-{
-    if (_PyCrossInterpreterData_InitWithSize(
-            data, tstate->interp, sizeof(struct _shared_bytes_data), obj,
-            _new_bytes_object
-            ) < 0)
-    {
-        return -1;
-    }
-    struct _shared_bytes_data *shared = (struct _shared_bytes_data *)data->data;
-    if (PyBytes_AsStringAndSize(obj, &shared->bytes, &shared->len) < 0) {
-        _PyCrossInterpreterData_Clear(tstate->interp, data);
-        return -1;
-    }
-    return 0;
-}
-
-struct _shared_str_data {
-    int kind;
-    const void *buffer;
-    Py_ssize_t len;
-};
-
-static PyObject *
-_new_str_object(_PyCrossInterpreterData *data)
-{
-    struct _shared_str_data *shared = (struct _shared_str_data *)(data->data);
-    return PyUnicode_FromKindAndData(shared->kind, shared->buffer, shared->len);
-}
-
-static int
-_str_shared(PyThreadState *tstate, PyObject *obj,
-            _PyCrossInterpreterData *data)
-{
-    if (_PyCrossInterpreterData_InitWithSize(
-            data, tstate->interp, sizeof(struct _shared_str_data), obj,
-            _new_str_object
-            ) < 0)
-    {
-        return -1;
-    }
-    struct _shared_str_data *shared = (struct _shared_str_data *)data->data;
-    shared->kind = PyUnicode_KIND(obj);
-    shared->buffer = PyUnicode_DATA(obj);
-    shared->len = PyUnicode_GET_LENGTH(obj);
-    return 0;
-}
-
-static PyObject *
-_new_long_object(_PyCrossInterpreterData *data)
-{
-    return PyLong_FromSsize_t((Py_ssize_t)(data->data));
-}
-
-static int
-_long_shared(PyThreadState *tstate, PyObject *obj,
-             _PyCrossInterpreterData *data)
-{
-    /* Note that this means the size of shareable ints is bounded by
-     * sys.maxsize.  Hence on 32-bit architectures that is half the
-     * size of maximum shareable ints on 64-bit.
-     */
-    Py_ssize_t value = PyLong_AsSsize_t(obj);
-    if (value == -1 && PyErr_Occurred()) {
-        if (PyErr_ExceptionMatches(PyExc_OverflowError)) {
-            PyErr_SetString(PyExc_OverflowError, "try sending as bytes");
-        }
-        return -1;
-    }
-    _PyCrossInterpreterData_Init(data, tstate->interp, (void *)value, NULL,
-            _new_long_object);
-    // data->obj and data->free remain NULL
-    return 0;
-}
-
-static PyObject *
-_new_none_object(_PyCrossInterpreterData *data)
-{
-    // XXX Singleton refcounts are problematic across interpreters...
-    return Py_NewRef(Py_None);
-}
-
-static int
-_none_shared(PyThreadState *tstate, PyObject *obj,
-             _PyCrossInterpreterData *data)
-{
-    _PyCrossInterpreterData_Init(data, tstate->interp, NULL, NULL,
-            _new_none_object);
-    // data->data, data->obj and data->free remain NULL
-    return 0;
-}
-
-static void
-_register_builtins_for_crossinterpreter_data(struct _xidregistry *xidregistry)
-{
-    // None
-    if (_xidregistry_add_type(xidregistry, (PyTypeObject *)PyObject_Type(Py_None), _none_shared) != 0) {
-        Py_FatalError("could not register None for cross-interpreter sharing");
-    }
-
-    // int
-    if (_xidregistry_add_type(xidregistry, &PyLong_Type, _long_shared) != 0) {
-        Py_FatalError("could not register int for cross-interpreter sharing");
-    }
-
-    // bytes
-    if (_xidregistry_add_type(xidregistry, &PyBytes_Type, _bytes_shared) != 0) {
-        Py_FatalError("could not register bytes for cross-interpreter sharing");
-    }
-
-    // str
-    if (_xidregistry_add_type(xidregistry, &PyUnicode_Type, _str_shared) != 0) {
-        Py_FatalError("could not register str for cross-interpreter sharing");
-    }
-}
-
-
 /*************/
 /* Other API */
 /*************/