gh-76785: Crossinterp utils additions (gh-111530)

This moves several general internal APIs out of _xxsubinterpretersmodule.c and into the new Python/crossinterp.c (and the corresponding internal headers).

Specifically:

* _Py_excinfo, etc.:  the initial implementation for non-object exception snapshots (in pycore_pyerrors.h and Python/errors.c)
* _PyXI_exception_info, etc.:  helpers for passing an exception beween interpreters (wraps _Py_excinfo)
* _PyXI_namespace, etc.:  helpers for copying a dict of attrs between interpreters
* _PyXI_Enter(), _PyXI_Exit():  functions that abstract out the transitions between one interpreter and a second that will do some work temporarily

Again, these were all abstracted out of _xxsubinterpretersmodule.c as generalizations.  I plan on proposing these as public API at some point.

Include/internal/pycore_crossinterp.h

@@ -8,6 +8,8 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
+#include "pycore_pyerrors.h"
+
 
 /***************************/
 /* cross-interpreter calls */
@@ -124,6 +126,8 @@ struct _xidregitem {
 };
 
 struct _xidregistry {
+    int global;  /* builtin types or heap types */
+    int initialized;
     PyThread_type_lock mutex;
     struct _xidregitem *head;
 };
@@ -133,6 +137,130 @@ PyAPI_FUNC(int) _PyCrossInterpreterData_UnregisterClass(PyTypeObject *);
 PyAPI_FUNC(crossinterpdatafunc) _PyCrossInterpreterData_Lookup(PyObject *);
 
 
+/*****************************/
+/* runtime state & lifecycle */
+/*****************************/
+
+struct _xi_runtime_state {
+    // builtin types
+    // XXX Remove this field once we have a tp_* slot.
+    struct _xidregistry registry;
+};
+
+struct _xi_state {
+    // heap types
+    // XXX Remove this field once we have a tp_* slot.
+    struct _xidregistry registry;
+
+    // heap types
+    PyObject *PyExc_NotShareableError;
+};
+
+extern PyStatus _PyXI_Init(PyInterpreterState *interp);
+extern void _PyXI_Fini(PyInterpreterState *interp);
+
+
+/***************************/
+/* short-term data sharing */
+/***************************/
+
+typedef enum error_code {
+    _PyXI_ERR_NO_ERROR = 0,
+    _PyXI_ERR_UNCAUGHT_EXCEPTION = -1,
+    _PyXI_ERR_OTHER = -2,
+    _PyXI_ERR_NO_MEMORY = -3,
+    _PyXI_ERR_ALREADY_RUNNING = -4,
+    _PyXI_ERR_MAIN_NS_FAILURE = -5,
+    _PyXI_ERR_APPLY_NS_FAILURE = -6,
+    _PyXI_ERR_NOT_SHAREABLE = -7,
+} _PyXI_errcode;
+
+
+typedef struct _sharedexception {
+    // The originating interpreter.
+    PyInterpreterState *interp;
+    // The kind of error to propagate.
+    _PyXI_errcode code;
+    // The exception information to propagate, if applicable.
+    // This is populated only for _PyXI_ERR_UNCAUGHT_EXCEPTION.
+    _Py_excinfo uncaught;
+} _PyXI_exception_info;
+
+PyAPI_FUNC(void) _PyXI_ApplyExceptionInfo(
+    _PyXI_exception_info *info,
+    PyObject *exctype);
+
+typedef struct xi_session _PyXI_session;
+typedef struct _sharedns _PyXI_namespace;
+
+PyAPI_FUNC(void) _PyXI_FreeNamespace(_PyXI_namespace *ns);
+PyAPI_FUNC(_PyXI_namespace *) _PyXI_NamespaceFromNames(PyObject *names);
+PyAPI_FUNC(int) _PyXI_FillNamespaceFromDict(
+    _PyXI_namespace *ns,
+    PyObject *nsobj,
+    _PyXI_session *session);
+PyAPI_FUNC(int) _PyXI_ApplyNamespace(
+    _PyXI_namespace *ns,
+    PyObject *nsobj,
+    PyObject *dflt);
+
+
+// A cross-interpreter session involves entering an interpreter
+// (_PyXI_Enter()), doing some work with it, and finally exiting
+// that interpreter (_PyXI_Exit()).
+//
+// At the boundaries of the session, both entering and exiting,
+// data may be exchanged between the previous interpreter and the
+// target one in a thread-safe way that does not violate the
+// isolation between interpreters.  This includes setting objects
+// in the target's __main__ module on the way in, and capturing
+// uncaught exceptions on the way out.
+struct xi_session {
+    // Once a session has been entered, this is the tstate that was
+    // current before the session.  If it is different from cur_tstate
+    // then we must have switched interpreters.  Either way, this will
+    // be the current tstate once we exit the session.
+    PyThreadState *prev_tstate;
+    // Once a session has been entered, this is the current tstate.
+    // It must be current when the session exits.
+    PyThreadState *init_tstate;
+    // This is true if init_tstate needs cleanup during exit.
+    int own_init_tstate;
+
+    // This is true if, while entering the session, init_thread took
+    // "ownership" of the interpreter's __main__ module.  This means
+    // it is the only thread that is allowed to run code there.
+    // (Caveat: for now, users may still run exec() against the
+    // __main__ module's dict, though that isn't advisable.)
+    int running;
+    // This is a cached reference to the __dict__ of the entered
+    // interpreter's __main__ module.  It is looked up when at the
+    // beginning of the session as a convenience.
+    PyObject *main_ns;
+
+    // This is set if the interpreter is entered and raised an exception
+    // that needs to be handled in some special way during exit.
+    _PyXI_errcode *exc_override;
+    // This is set if exit captured an exception to propagate.
+    _PyXI_exception_info *exc;
+
+    // -- pre-allocated memory --
+    _PyXI_exception_info _exc;
+    _PyXI_errcode _exc_override;
+};
+
+PyAPI_FUNC(int) _PyXI_Enter(
+    _PyXI_session *session,
+    PyInterpreterState *interp,
+    PyObject *nsupdates);
+PyAPI_FUNC(void) _PyXI_Exit(_PyXI_session *session);
+
+PyAPI_FUNC(void) _PyXI_ApplyCapturedException(
+    _PyXI_session *session,
+    PyObject *excwrapper);
+PyAPI_FUNC(int) _PyXI_HasCapturedException(_PyXI_session *session);
+
+
 #ifdef __cplusplus
 }
 #endif

Include/internal/pycore_interp.h

@@ -153,8 +153,8 @@ struct _is {
     Py_ssize_t co_extra_user_count;
     freefunc co_extra_freefuncs[MAX_CO_EXTRA_USERS];
 
-    // XXX Remove this field once we have a tp_* slot.
+    /* cross-interpreter data and utils */
-    struct _xidregistry xidregistry;
+    struct _xi_state xi;
 
 #ifdef HAVE_FORK
     PyObject *before_forkers;

Include/internal/pycore_pyerrors.h

@@ -68,6 +68,30 @@ extern PyStatus _PyErr_InitTypes(PyInterpreterState *);
 extern void _PyErr_FiniTypes(PyInterpreterState *);
 
 
+/* exception snapshots */
+
+// Ultimately we'd like to preserve enough information about the
+// exception and traceback that we could re-constitute (or at least
+// simulate, a la traceback.TracebackException), and even chain, a copy
+// of the exception in the calling interpreter.
+
+typedef struct _excinfo {
+    const char *type;
+    const char *msg;
+} _Py_excinfo;
+
+extern void _Py_excinfo_Clear(_Py_excinfo *info);
+extern int _Py_excinfo_Copy(_Py_excinfo *dest, _Py_excinfo *src);
+extern const char * _Py_excinfo_InitFromException(
+    _Py_excinfo *info,
+    PyObject *exc);
+extern void _Py_excinfo_Apply(_Py_excinfo *info, PyObject *exctype);
+extern const char * _Py_excinfo_AsUTF8(
+    _Py_excinfo *info,
+    char *buf,
+    size_t bufsize);
+
+
 /* other API */
 
 static inline PyObject* _PyErr_Occurred(PyThreadState *tstate)

Include/internal/pycore_runtime.h

@@ -200,8 +200,8 @@ typedef struct pyruntimestate {
      possible to facilitate out-of-process observability
      tools. */
 
-    // XXX Remove this field once we have a tp_* slot.
+    /* cross-interpreter data and utils */
-    struct _xidregistry xidregistry;
+    struct _xi_runtime_state xi;
 
     struct _pymem_allocators allocators;
     struct _obmalloc_global_state obmalloc;

Include/internal/pycore_runtime_init.h

@@ -95,6 +95,11 @@ extern PyTypeObject _PyExc_MemoryError;
               until _PyInterpreterState_Enable() is called. */ \
             .next_id = -1, \
         }, \
+        .xi = { \
+            .registry = { \
+                .global = 1, \
+            }, \
+        }, \
         /* A TSS key must be initialized with Py_tss_NEEDS_INIT \
            in accordance with the specification. */ \
         .autoTSSkey = Py_tss_NEEDS_INIT, \

Lib/test/support/interpreters.py

@@ -92,7 +92,7 @@ class Interpreter:
         return _interpreters.destroy(self._id)
 
     # XXX Rename "run" to "exec"?
-    def run(self, src_str, /, *, channels=None):
+    def run(self, src_str, /, channels=None):
         """Run the given source code in the interpreter.
 
         This is essentially the same as calling the builtin "exec"

Modules/_xxsubinterpretersmodule.c

@@ -7,6 +7,7 @@
 
 #include "Python.h"
 #include "pycore_crossinterp.h"   // struct _xid
+#include "pycore_pyerrors.h"      // _Py_excinfo
 #include "pycore_initconfig.h"    // _PyErr_SetFromPyStatus()
 #include "pycore_modsupport.h"    // _PyArg_BadArgument()
 #include "pycore_pyerrors.h"      // _PyErr_ChainExceptions1()
@@ -19,22 +20,6 @@
 #define MODULE_NAME "_xxsubinterpreters"
 
 
-static const char *
-_copy_raw_string(PyObject *strobj)
-{
-    const char *str = PyUnicode_AsUTF8(strobj);
-    if (str == NULL) {
-        return NULL;
-    }
-    char *copied = PyMem_RawMalloc(strlen(str)+1);
-    if (copied == NULL) {
-        PyErr_NoMemory();
-        return NULL;
-    }
-    strcpy(copied, str);
-    return copied;
-}
-
 static PyInterpreterState *
 _get_current_interp(void)
 {
@@ -62,21 +47,6 @@ add_new_exception(PyObject *mod, const char *name, PyObject *base)
 #define ADD_NEW_EXCEPTION(MOD, NAME, BASE) \
     add_new_exception(MOD, MODULE_NAME "." Py_STRINGIFY(NAME), BASE)
 
-static int
-_release_xid_data(_PyCrossInterpreterData *data)
-{
-    PyObject *exc = PyErr_GetRaisedException();
-    int res = _PyCrossInterpreterData_Release(data);
-    if (res < 0) {
-        /* The owning interpreter is already destroyed. */
-        _PyCrossInterpreterData_Clear(NULL, data);
-        // XXX Emit a warning?
-        PyErr_Clear();
-    }
-    PyErr_SetRaisedException(exc);
-    return res;
-}
-
 
 /* module state *************************************************************/
 
@@ -113,263 +83,6 @@ clear_module_state(module_state *state)
 }
 
 
-/* data-sharing-specific code ***********************************************/
-
-struct _sharednsitem {
-    const char *name;
-    _PyCrossInterpreterData data;
-};
-
-static void _sharednsitem_clear(struct _sharednsitem *);  // forward
-
-static int
-_sharednsitem_init(struct _sharednsitem *item, PyObject *key, PyObject *value)
-{
-    item->name = _copy_raw_string(key);
-    if (item->name == NULL) {
-        return -1;
-    }
-    if (_PyObject_GetCrossInterpreterData(value, &item->data) != 0) {
-        _sharednsitem_clear(item);
-        return -1;
-    }
-    return 0;
-}
-
-static void
-_sharednsitem_clear(struct _sharednsitem *item)
-{
-    if (item->name != NULL) {
-        PyMem_RawFree((void *)item->name);
-        item->name = NULL;
-    }
-    (void)_release_xid_data(&item->data);
-}
-
-static int
-_sharednsitem_apply(struct _sharednsitem *item, PyObject *ns)
-{
-    PyObject *name = PyUnicode_FromString(item->name);
-    if (name == NULL) {
-        return -1;
-    }
-    PyObject *value = _PyCrossInterpreterData_NewObject(&item->data);
-    if (value == NULL) {
-        Py_DECREF(name);
-        return -1;
-    }
-    int res = PyDict_SetItem(ns, name, value);
-    Py_DECREF(name);
-    Py_DECREF(value);
-    return res;
-}
-
-typedef struct _sharedns {
-    Py_ssize_t len;
-    struct _sharednsitem* items;
-} _sharedns;
-
-static _sharedns *
-_sharedns_new(Py_ssize_t len)
-{
-    _sharedns *shared = PyMem_RawCalloc(sizeof(_sharedns), 1);
-    if (shared == NULL) {
-        PyErr_NoMemory();
-        return NULL;
-    }
-    shared->len = len;
-    shared->items = PyMem_RawCalloc(sizeof(struct _sharednsitem), len);
-    if (shared->items == NULL) {
-        PyErr_NoMemory();
-        PyMem_RawFree(shared);
-        return NULL;
-    }
-    return shared;
-}
-
-static void
-_sharedns_free(_sharedns *shared)
-{
-    for (Py_ssize_t i=0; i < shared->len; i++) {
-        _sharednsitem_clear(&shared->items[i]);
-    }
-    PyMem_RawFree(shared->items);
-    PyMem_RawFree(shared);
-}
-
-static _sharedns *
-_get_shared_ns(PyObject *shareable)
-{
-    if (shareable == NULL || shareable == Py_None) {
-        return NULL;
-    }
-    Py_ssize_t len = PyDict_Size(shareable);
-    if (len == 0) {
-        return NULL;
-    }
-
-    _sharedns *shared = _sharedns_new(len);
-    if (shared == NULL) {
-        return NULL;
-    }
-    Py_ssize_t pos = 0;
-    for (Py_ssize_t i=0; i < len; i++) {
-        PyObject *key, *value;
-        if (PyDict_Next(shareable, &pos, &key, &value) == 0) {
-            break;
-        }
-        if (_sharednsitem_init(&shared->items[i], key, value) != 0) {
-            break;
-        }
-    }
-    if (PyErr_Occurred()) {
-        _sharedns_free(shared);
-        return NULL;
-    }
-    return shared;
-}
-
-static int
-_sharedns_apply(_sharedns *shared, PyObject *ns)
-{
-    for (Py_ssize_t i=0; i < shared->len; i++) {
-        if (_sharednsitem_apply(&shared->items[i], ns) != 0) {
-            return -1;
-        }
-    }
-    return 0;
-}
-
-// Ultimately we'd like to preserve enough information about the
-// exception and traceback that we could re-constitute (or at least
-// simulate, a la traceback.TracebackException), and even chain, a copy
-// of the exception in the calling interpreter.
-
-typedef struct _sharedexception {
-    PyInterpreterState *interp;
-#define ERR_NOT_SET 0
-#define ERR_NO_MEMORY 1
-#define ERR_ALREADY_RUNNING 2
-    int code;
-    const char *name;
-    const char *msg;
-} _sharedexception;
-
-static const struct _sharedexception no_exception = {
-    .name = NULL,
-    .msg = NULL,
-};
-
-static void
-_sharedexception_clear(_sharedexception *exc)
-{
-    if (exc->name != NULL) {
-        PyMem_RawFree((void *)exc->name);
-    }
-    if (exc->msg != NULL) {
-        PyMem_RawFree((void *)exc->msg);
-    }
-}
-
-static const char *
-_sharedexception_bind(PyObject *exc, int code, _sharedexception *sharedexc)
-{
-    if (sharedexc->interp == NULL) {
-        sharedexc->interp = PyInterpreterState_Get();
-    }
-
-    if (code != ERR_NOT_SET) {
-        assert(exc == NULL);
-        assert(code > 0);
-        sharedexc->code = code;
-        return NULL;
-    }
-
-    assert(exc != NULL);
-    const char *failure = NULL;
-
-    PyObject *nameobj = PyUnicode_FromString(Py_TYPE(exc)->tp_name);
-    if (nameobj == NULL) {
-        failure = "unable to format exception type name";
-        code = ERR_NO_MEMORY;
-        goto error;
-    }
-    sharedexc->name = _copy_raw_string(nameobj);
-    Py_DECREF(nameobj);
-    if (sharedexc->name == NULL) {
-        if (PyErr_ExceptionMatches(PyExc_MemoryError)) {
-            failure = "out of memory copying exception type name";
-        } else {
-            failure = "unable to encode and copy exception type name";
-        }
-        code = ERR_NO_MEMORY;
-        goto error;
-    }
-
-    if (exc != NULL) {
-        PyObject *msgobj = PyObject_Str(exc);
-        if (msgobj == NULL) {
-            failure = "unable to format exception message";
-            code = ERR_NO_MEMORY;
-            goto error;
-        }
-        sharedexc->msg = _copy_raw_string(msgobj);
-        Py_DECREF(msgobj);
-        if (sharedexc->msg == NULL) {
-            if (PyErr_ExceptionMatches(PyExc_MemoryError)) {
-                failure = "out of memory copying exception message";
-            } else {
-                failure = "unable to encode and copy exception message";
-            }
-            code = ERR_NO_MEMORY;
-            goto error;
-        }
-    }
-
-    return NULL;
-
-error:
-    assert(failure != NULL);
-    PyErr_Clear();
-    _sharedexception_clear(sharedexc);
-    *sharedexc = (_sharedexception){
-        .interp = sharedexc->interp,
-        .code = code,
-    };
-    return failure;
-}
-
-static void
-_sharedexception_apply(_sharedexception *exc, PyObject *wrapperclass)
-{
-    if (exc->name != NULL) {
-        assert(exc->code == ERR_NOT_SET);
-        if (exc->msg != NULL) {
-            PyErr_Format(wrapperclass, "%s: %s",  exc->name, exc->msg);
-        }
-        else {
-            PyErr_SetString(wrapperclass, exc->name);
-        }
-    }
-    else if (exc->msg != NULL) {
-        assert(exc->code == ERR_NOT_SET);
-        PyErr_SetString(wrapperclass, exc->msg);
-    }
-    else if (exc->code == ERR_NO_MEMORY) {
-        PyErr_NoMemory();
-    }
-    else if (exc->code == ERR_ALREADY_RUNNING) {
-        assert(exc->interp != NULL);
-        assert(_PyInterpreterState_IsRunningMain(exc->interp));
-        _PyInterpreterState_FailIfRunningMain(exc->interp);
-    }
-    else {
-        assert(exc->code == ERR_NOT_SET);
-        PyErr_SetNone(wrapperclass);
-    }
-}
-
-
 /* Python code **************************************************************/
 
 static const char *
@@ -489,43 +202,8 @@ exceptions_init(PyObject *mod)
 }
 
 static int
-_run_script(PyInterpreterState *interp,
+_run_script(PyObject *ns, const char *codestr, Py_ssize_t codestrlen, int flags)
-            const char *codestr, Py_ssize_t codestrlen,
-            _sharedns *shared, _sharedexception *sharedexc, int flags)
 {
-    int errcode = ERR_NOT_SET;
-
-    if (_PyInterpreterState_SetRunningMain(interp) < 0) {
-        assert(PyErr_Occurred());
-        // In the case where we didn't switch interpreters, it would
-        // be more efficient to leave the exception in place and return
-        // immediately.  However, life is simpler if we don't.
-        PyErr_Clear();
-        errcode = ERR_ALREADY_RUNNING;
-        goto error;
-    }
-
-    PyObject *excval = NULL;
-    PyObject *main_mod = PyUnstable_InterpreterState_GetMainModule(interp);
-    if (main_mod == NULL) {
-        goto error;
-    }
-    PyObject *ns = PyModule_GetDict(main_mod);  // borrowed
-    Py_DECREF(main_mod);
-    if (ns == NULL) {
-        goto error;
-    }
-    Py_INCREF(ns);
-
-    // Apply the cross-interpreter data.
-    if (shared != NULL) {
-        if (_sharedns_apply(shared, ns) != 0) {
-            Py_DECREF(ns);
-            goto error;
-        }
-    }
-
-    // Run the script/code/etc.
     PyObject *result = NULL;
     if (flags & RUN_TEXT) {
         result = PyRun_StringFlags(codestr, Py_file_input, ns, ns, NULL);
@@ -540,86 +218,46 @@ _run_script(PyInterpreterState *interp,
     else {
         Py_UNREACHABLE();
     }
-    Py_DECREF(ns);
     if (result == NULL) {
-        goto error;
-    }
-    else {
-        Py_DECREF(result);  // We throw away the result.
-    }
-    _PyInterpreterState_SetNotRunningMain(interp);
-
-    *sharedexc = no_exception;
-    return 0;
-
-error:
-    excval = PyErr_GetRaisedException();
-    const char *failure = _sharedexception_bind(excval, errcode, sharedexc);
-    if (failure != NULL) {
-        fprintf(stderr,
-                "RunFailedError: script raised an uncaught exception (%s)",
-                failure);
-    }
-    if (excval != NULL) {
-        // XXX Instead, store the rendered traceback on sharedexc,
-        // attach it to the exception when applied,
-        // and teach PyErr_Display() to print it.
-        PyErr_Display(NULL, excval, NULL);
-        Py_DECREF(excval);
-    }
-    if (errcode != ERR_ALREADY_RUNNING) {
-        _PyInterpreterState_SetNotRunningMain(interp);
-    }
-    assert(!PyErr_Occurred());
         return -1;
     }
+    Py_DECREF(result);  // We throw away the result.
+    return 0;
+}
 
 static int
-_run_in_interpreter(PyObject *mod, PyInterpreterState *interp,
+_run_in_interpreter(PyInterpreterState *interp,
                     const char *codestr, Py_ssize_t codestrlen,
-                    PyObject *shareables, int flags)
+                    PyObject *shareables, int flags,
+                    PyObject *excwrapper)
 {
-    module_state *state = get_module_state(mod);
+    assert(!PyErr_Occurred());
-    assert(state != NULL);
+    _PyXI_session session = {0};
 
-    _sharedns *shared = _get_shared_ns(shareables);
+    // Prep and switch interpreters.
-    if (shared == NULL && PyErr_Occurred()) {
+    if (_PyXI_Enter(&session, interp, shareables) < 0) {
+        assert(!PyErr_Occurred());
+        _PyXI_ApplyExceptionInfo(session.exc, excwrapper);
+        assert(PyErr_Occurred());
         return -1;
     }
 
-    // Switch to interpreter.
-    PyThreadState *save_tstate = NULL;
-    PyThreadState *tstate = NULL;
-    if (interp != PyInterpreterState_Get()) {
-        tstate = PyThreadState_New(interp);
-        tstate->_whence = _PyThreadState_WHENCE_EXEC;
-        // XXX Possible GILState issues?
-        save_tstate = PyThreadState_Swap(tstate);
-    }
-
     // Run the script.
-    _sharedexception exc = (_sharedexception){ .interp = interp };
+    int res = _run_script(session.main_ns, codestr, codestrlen, flags);
-    int result = _run_script(interp, codestr, codestrlen, shared, &exc, flags);
 
-    // Switch back.
+    // Clean up and switch back.
-    if (save_tstate != NULL) {
+    _PyXI_Exit(&session);
-        PyThreadState_Clear(tstate);
-        PyThreadState_Swap(save_tstate);
-        PyThreadState_Delete(tstate);
-    }
 
     // Propagate any exception out to the caller.
-    if (result < 0) {
     assert(!PyErr_Occurred());
-        _sharedexception_apply(&exc, state->RunFailedError);
+    if (res < 0) {
-        assert(PyErr_Occurred());
+        _PyXI_ApplyCapturedException(&session, excwrapper);
+    }
+    else {
+        assert(!_PyXI_HasCapturedException(&session));
     }
 
-    if (shared != NULL) {
+    return res;
-        _sharedns_free(shared);
-    }
-
-    return result;
 }
 
 
@@ -805,7 +443,6 @@ PyDoc_STRVAR(get_main_doc,
 \n\
 Return the ID of main interpreter.");
 
-
 static PyUnicodeObject *
 convert_script_arg(PyObject *arg, const char *fname, const char *displayname,
                    const char *expected)
@@ -903,10 +540,12 @@ _interp_exec(PyObject *self,
     }
 
     // Run the code in the interpreter.
-    int res = _run_in_interpreter(self, interp, codestr, codestrlen,
+    module_state *state = get_module_state(self);
-                                  shared_arg, flags);
+    assert(state != NULL);
+    int res = _run_in_interpreter(interp, codestr, codestrlen,
+                                  shared_arg, flags, state->RunFailedError);
     Py_XDECREF(bytes_obj);
-    if (res != 0) {
+    if (res < 0) {
         return -1;
     }
 
@@ -981,7 +620,7 @@ interp_run_string(PyObject *self, PyObject *args, PyObject *kwds)
         return NULL;
     }
 
-    int res = _interp_exec(self, id, (PyObject *)script, shared);
+    int res = _interp_exec(self, id, script, shared);
     Py_DECREF(script);
     if (res < 0) {
         return NULL;

Python/errors.c

@@ -1945,3 +1945,178 @@ PyErr_ProgramTextObject(PyObject *filename, int lineno)
 {
     return _PyErr_ProgramDecodedTextObject(filename, lineno, NULL);
 }
+
+
+/***********************/
+/* exception snapshots */
+/***********************/
+
+static const char *
+_copy_raw_string(const char *str)
+{
+    char *copied = PyMem_RawMalloc(strlen(str)+1);
+    if (copied == NULL) {
+        return NULL;
+    }
+    strcpy(copied, str);
+    return copied;
+}
+
+static int
+_exc_type_name_as_utf8(PyObject *exc, const char **p_typename)
+{
+    // XXX Use PyObject_GetAttrString(Py_TYPE(exc), '__name__')?
+    PyObject *nameobj = PyUnicode_FromString(Py_TYPE(exc)->tp_name);
+    if (nameobj == NULL) {
+        assert(PyErr_Occurred());
+        *p_typename = "unable to format exception type name";
+        return -1;
+    }
+    const char *name = PyUnicode_AsUTF8(nameobj);
+    if (name == NULL) {
+        assert(PyErr_Occurred());
+        Py_DECREF(nameobj);
+        *p_typename = "unable to encode exception type name";
+        return -1;
+    }
+    name = _copy_raw_string(name);
+    Py_DECREF(nameobj);
+    if (name == NULL) {
+        *p_typename = "out of memory copying exception type name";
+        return -1;
+    }
+    *p_typename = name;
+    return 0;
+}
+
+static int
+_exc_msg_as_utf8(PyObject *exc, const char **p_msg)
+{
+    PyObject *msgobj = PyObject_Str(exc);
+    if (msgobj == NULL) {
+        assert(PyErr_Occurred());
+        *p_msg = "unable to format exception message";
+        return -1;
+    }
+    const char *msg = PyUnicode_AsUTF8(msgobj);
+    if (msg == NULL) {
+        assert(PyErr_Occurred());
+        Py_DECREF(msgobj);
+        *p_msg = "unable to encode exception message";
+        return -1;
+    }
+    msg = _copy_raw_string(msg);
+    Py_DECREF(msgobj);
+    if (msg == NULL) {
+        assert(PyErr_ExceptionMatches(PyExc_MemoryError));
+        *p_msg = "out of memory copying exception message";
+        return -1;
+    }
+    *p_msg = msg;
+    return 0;
+}
+
+void
+_Py_excinfo_Clear(_Py_excinfo *info)
+{
+    if (info->type != NULL) {
+        PyMem_RawFree((void *)info->type);
+    }
+    if (info->msg != NULL) {
+        PyMem_RawFree((void *)info->msg);
+    }
+    *info = (_Py_excinfo){ NULL };
+}
+
+int
+_Py_excinfo_Copy(_Py_excinfo *dest, _Py_excinfo *src)
+{
+    // XXX Clear dest first?
+
+    if (src->type == NULL) {
+        dest->type = NULL;
+    }
+    else {
+        dest->type = _copy_raw_string(src->type);
+        if (dest->type == NULL) {
+            return -1;
+        }
+    }
+
+    if (src->msg == NULL) {
+        dest->msg = NULL;
+    }
+    else {
+        dest->msg = _copy_raw_string(src->msg);
+        if (dest->msg == NULL) {
+            return -1;
+        }
+    }
+
+    return 0;
+}
+
+const char *
+_Py_excinfo_InitFromException(_Py_excinfo *info, PyObject *exc)
+{
+    assert(exc != NULL);
+
+    // Extract the exception type name.
+    const char *typename = NULL;
+    if (_exc_type_name_as_utf8(exc, &typename) < 0) {
+        assert(typename != NULL);
+        return typename;
+    }
+
+    // Extract the exception message.
+    const char *msg = NULL;
+    if (_exc_msg_as_utf8(exc, &msg) < 0) {
+        assert(msg != NULL);
+        return msg;
+    }
+
+    info->type = typename;
+    info->msg = msg;
+    return NULL;
+}
+
+void
+_Py_excinfo_Apply(_Py_excinfo *info, PyObject *exctype)
+{
+    if (info->type != NULL) {
+        if (info->msg != NULL) {
+            PyErr_Format(exctype, "%s: %s",  info->type, info->msg);
+        }
+        else {
+            PyErr_SetString(exctype, info->type);
+        }
+    }
+    else if (info->msg != NULL) {
+        PyErr_SetString(exctype, info->msg);
+    }
+    else {
+        PyErr_SetNone(exctype);
+    }
+}
+
+const char *
+_Py_excinfo_AsUTF8(_Py_excinfo *info, char *buf, size_t bufsize)
+{
+    // XXX Dynamically allocate if no buf provided?
+    assert(buf != NULL);
+    if (info->type != NULL) {
+        if (info->msg != NULL) {
+            snprintf(buf, bufsize, "%s: %s",  info->type, info->msg);
+            return buf;
+        }
+        else {
+            return info->type;
+        }
+    }
+    else if (info->msg != NULL) {
+        return info->msg;
+    }
+    else {
+        return NULL;
+    }
+}

Python/pylifecycle.c

@@ -738,6 +738,7 @@ pycore_init_types(PyInterpreterState *interp)
     if (_PyStatus_EXCEPTION(status)) {
         return status;
     }
+
     return _PyStatus_OK();
 }
 
@@ -854,6 +855,11 @@ pycore_interp_init(PyThreadState *tstate)
         goto done;
     }
 
+    status = _PyXI_Init(interp);
+    if (_PyStatus_EXCEPTION(status)) {
+        goto done;
+    }
+
     const PyConfig *config = _PyInterpreterState_GetConfig(interp);
 
     status = _PyImport_InitCore(tstate, sysmod, config->_install_importlib);
@@ -1772,6 +1778,7 @@ finalize_interp_clear(PyThreadState *tstate)
 {
     int is_main_interp = _Py_IsMainInterpreter(tstate->interp);
 
+    _PyXI_Fini(tstate->interp);
     _PyExc_ClearExceptionGroupType(tstate->interp);
     _Py_clear_generic_types(tstate->interp);
 

Python/pystate.c

@@ -382,7 +382,7 @@ _Py_COMP_DIAG_POP
 #define LOCKS_INIT(runtime) \
     { \
         &(runtime)->interpreters.mutex, \
-        &(runtime)->xidregistry.mutex, \
+        &(runtime)->xi.registry.mutex, \
         &(runtime)->getargs.mutex, \
         &(runtime)->unicode_state.ids.lock, \
         &(runtime)->imports.extensions.mutex, \
@@ -494,9 +494,6 @@ _PyRuntimeState_Init(_PyRuntimeState *runtime)
     return _PyStatus_OK();
 }
 
-// This is defined in crossinterp.c (for now).
-extern void _Py_xidregistry_clear(struct _xidregistry *);
-
 void
 _PyRuntimeState_Fini(_PyRuntimeState *runtime)
 {
@@ -505,8 +502,6 @@ _PyRuntimeState_Fini(_PyRuntimeState *runtime)
     assert(runtime->object_state.interpreter_leaks == 0);
 #endif
 
-    _Py_xidregistry_clear(&runtime->xidregistry);
-
     if (gilstate_tss_initialized(runtime)) {
         gilstate_tss_fini(runtime);
     }
@@ -552,11 +547,6 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
     for (int i = 0; i < NUMLOCKS; i++) {
         reinit_err += _PyThread_at_fork_reinit(lockptrs[i]);
     }
-    /* PyOS_AfterFork_Child(), which calls this function, later calls
-       _PyInterpreterState_DeleteExceptMain(), so we only need to update
-       the main interpreter here. */
-    assert(runtime->interpreters.main != NULL);
-    runtime->interpreters.main->xidregistry.mutex = runtime->xidregistry.mutex;
 
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
@@ -720,9 +710,6 @@ init_interpreter(PyInterpreterState *interp,
     }
     interp->f_opcode_trace_set = false;
 
-    assert(runtime->xidregistry.mutex != NULL);
-    interp->xidregistry.mutex = runtime->xidregistry.mutex;
-
     interp->_initialized = 1;
     return _PyStatus_OK();
 }
@@ -948,10 +935,6 @@ interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate)
     Py_CLEAR(interp->sysdict);
     Py_CLEAR(interp->builtins);
 
-    _Py_xidregistry_clear(&interp->xidregistry);
-    /* The lock is owned by the runtime, so we don't free it here. */
-    interp->xidregistry.mutex = NULL;
-
     if (tstate->interp == interp) {
         /* We are now safe to fix tstate->_status.cleared. */
         // XXX Do this (much) earlier?