bigfile/py: Stop caring about sys.exc_{type,value,traceback} variables

Before py3k python stores exception information not only in thread-local state
but also globally in sys.exc_* variables (wrt sys.exc_info()) for
"backward compatibility". However using them is not thread-safe as the
following example demonstrates:

---- 8< ----
from threading import Thread
import sys

def T1():
    print 'T1'
    while 1:
        exc_type = sys.exc_type
        if exc_type is not None:
            print 'AAA: %r' % exc_type

def f(): g()
def g(): h()
def h(): 1/0

def T2():
    print 'T2'
    while 1:
        try:
            f()
        except:
            pass

t1, t2 = Thread(target=T1), Thread(target=T2)
t1.start(); t2.start()
---- 8< ----

---- 8< ----
kirr@deco:~/tmp/trashme/t$ ./excthreads.py
T1
T2

AAA: <type 'exceptions.ZeroDivisionError'>
AAA: <type 'exceptions.ZeroDivisionError'>
AAA: <type 'exceptions.ZeroDivisionError'>
AAA: <type 'exceptions.ZeroDivisionError'>
AAA: <type 'exceptions.ZeroDivisionError'>
AAA: <type 'exceptions.ZeroDivisionError'>
^\Выход
---- 8< ----

Because of the above nothing modern (I've explicitly checked at least CPython
itself and Zope) uses this variables - wherever needed per-thread exception
state is retrieved with sys.exc_info().

So on wendelin.core side it is thus thankless job to try to preserve sys.exc_*
vars state because on a busy server they are literally changing all the -
arbitrary from the point of view of particular thread - time while its python
code runs.

bigfile/_bigfile.c

@@ -428,19 +428,6 @@ static /*const*/ PyTypeObject PyBigFileH_Type = {
  ****************/
 
 
-/* like PySys_SetObject but
- *  1) aborts on failure and
- *  2) return set object
- */
-static PyObject *XPySys_SetObject(char *name, PyObject *v)
-{
-    int err;
-    err = PySys_SetObject(name, v);
-    BUG_ON(err);
-    return v;
-}
-
-
 static int pybigfile_loadblk(BigFile *file, blk_t blk, void *buf)
 {
     PyBigFile *pyfile = upcast(PyBigFile *, file);
@@ -453,7 +440,6 @@ static int pybigfile_loadblk(BigFile *file, blk_t blk, void *buf)
     PyObject  *save_curexc_type, *save_curexc_value, *save_curexc_traceback;
     PyObject  *save_exc_type,    *save_exc_value,    *save_exc_traceback;
     PyObject  *save_async_exc;
-    PyObject  *sys_exc_type,     *sys_exc_value,     *sys_exc_traceback;
     // XXX save/restore trash_delete_{nesting,later} ?
 
 
@@ -508,10 +494,9 @@ static int pybigfile_loadblk(BigFile *file, blk_t blk, void *buf)
     XINC( save_async_exc          = set0(&ts->async_exc)          );
 
     /* before py3k python also stores exception in sys.exc_* variables (wrt
-     * sys.exc_info()) for "backward compatibility" */
-    sys_exc_type        = PySys_XInc_SetNone("exc_type");
-    sys_exc_value       = PySys_XInc_SetNone("exc_value");
-    sys_exc_traceback   = PySys_XInc_SetNone("exc_traceback");
+     * sys.exc_info()) for "backward compatibility", but we do not care about it
+     * as sys.exc_* variables are not thread safe and from a thread point of view
+     * can be changing at arbitrary times during while its python code runs. */
 
 #if BIGFILE_USE_OLD_BUFFER
     pybuf = PyBuffer_FromReadWriteMemory(buf, file->blksize);
@@ -679,9 +664,6 @@ out:
     BUG_ON(ts->curexc_type  || ts->curexc_value || ts->curexc_traceback);
     BUG_ON(ts->exc_type     || ts->exc_value    || ts->exc_traceback);
     BUG_ON(ts->async_exc);
-    BUG_ON(PySys_GetObject("exc_type")      != Py_None ||
-           PySys_GetObject("exc_value")     != Py_None ||
-           PySys_GetObject("exc_traceback") != Py_None);
 
     /* now restore exception state to original */
     ts->curexc_type         = save_curexc_type;
@@ -692,10 +674,6 @@ out:
     ts->exc_traceback       = save_exc_traceback;
     ts->async_exc           = save_async_exc;
 
-    XPySys_SetObject("exc_type",       sys_exc_type);
-    XPySys_SetObject("exc_value",      sys_exc_value);
-    XPySys_SetObject("exc_traceback",  sys_exc_traceback);
-
     Py_XDECREF( save_curexc_type        );
     Py_XDECREF( save_curexc_value       );
     Py_XDECREF( save_curexc_traceback   );
@@ -704,10 +682,6 @@ out:
     Py_XDECREF( save_exc_traceback      );
     Py_XDECREF( save_async_exc          );
 
-    Py_XDECREF( sys_exc_type            );
-    Py_XDECREF( sys_exc_value           );
-    Py_XDECREF( sys_exc_traceback       );
-
     /* optionally release the GIL, if it was not ours initially */
     PyGILState_Release(gstate);
 
@@ -1067,10 +1041,6 @@ XPyErr_FullClear(void)
     Py_XDECREF(x_exc_value);
     Py_XDECREF(x_exc_traceback);
     Py_XDECREF(x_async_exc);
-
-    PySys_SetObject("exc_type",       Py_None);
-    PySys_SetObject("exc_value",      Py_None);
-    PySys_SetObject("exc_traceback",  Py_None);
 }
 
 static PyObject* /* PyListObject* */