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.

1 files changed, 24 insertions, 0 deletions

diff --git a/Include/internal/pycore_pyerrors.h b/Include/internal/pycore_pyerrors.h
index 184eb35..67ef71c 100644
--- a/Include/internal/pycore_pyerrors.h
+++ b/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)