bpo-39164: Add private _PyErr_GetExcInfo() function (GH-17752)

This adds a new function named _PyErr_GetExcInfo() that is a variation of the
original PyErr_GetExcInfo() taking a PyThreadState as its first argument.
That function allows to retrieve the exceptions information of any Python
thread -- not only the current one.

Include/cpython/pyerrors.h

@@ -76,6 +76,7 @@ typedef PyOSErrorObject PyWindowsErrorObject;
 
 PyAPI_FUNC(void) _PyErr_SetKeyError(PyObject *);
 _PyErr_StackItem *_PyErr_GetTopmostException(PyThreadState *tstate);
+PyAPI_FUNC(void) _PyErr_GetExcInfo(PyThreadState *, PyObject **, PyObject **, PyObject **);
 
 /* Context manipulation (PEP 3134) */
 

Misc/NEWS.d/next/C API/2019-12-30-10-43-52.bpo-39164.WEV0uu.rst

@@ -0,0 +1 @@
+Add a private ``_PyErr_GetExcInfo()`` function to retrieve exception information of the specified Python thread state.

Python/errors.c

@@ -433,21 +433,27 @@ PyErr_Clear(void)
 
 
 void
-PyErr_GetExcInfo(PyObject **p_type, PyObject **p_value, PyObject **p_traceback)
+_PyErr_GetExcInfo(PyThreadState *tstate,
+                  PyObject **p_type, PyObject **p_value, PyObject **p_traceback)
 {
-    PyThreadState *tstate = _PyThreadState_GET();
-
     _PyErr_StackItem *exc_info = _PyErr_GetTopmostException(tstate);
     *p_type = exc_info->exc_type;
     *p_value = exc_info->exc_value;
     *p_traceback = exc_info->exc_traceback;
 
-
     Py_XINCREF(*p_type);
     Py_XINCREF(*p_value);
     Py_XINCREF(*p_traceback);
 }
 
+
+void
+PyErr_GetExcInfo(PyObject **p_type, PyObject **p_value, PyObject **p_traceback)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    return _PyErr_GetExcInfo(tstate, p_type, p_value, p_traceback);
+}
+
 void
 PyErr_SetExcInfo(PyObject *p_type, PyObject *p_value, PyObject *p_traceback)
 {