Revert changes related to traceback

Storing a traceback object in a Future object on Python 2 may introduce
reference cycles, and I need to spend more time to check all cases. I prefer to
defer these enhancemnets to release first current changes.

4 files changed, 12 insertions, 63 deletions

diff --git a/doc/changelog.rst b/doc/changelog.rst
index 407ea60..d402d60 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -27,10 +27,6 @@ Major bugfixes:
 
 Other changes:
 
-* On Python 2, Task waiting on a future now keeps the exception traceback of
-  the future, instead of showing where the exception was re-raised
-* On Python 2, Future.exception() now raises the traceback with the original
-  traceback.
 * Return destructor logs the source traceback in debug mode
 * Python issue #22448: Improve cancelled timer callback handles cleanup. Patch
   by Joshua Moore-Oliva.
diff --git a/trollius/compat.py b/trollius/compat.py
index 7947842..b93070d 100644
--- a/trollius/compat.py
+++ b/trollius/compat.py
@@ -51,11 +51,3 @@ def flatten_bytes(data):
         return data.tobytes()
     else:
         return data
-
-if PY3:
-    def reraise(tp, value, tb=None):
-        if value.__traceback__ is not tb:
-            raise value.with_traceback(tb)
-        raise value
-else:
-    exec("""def reraise(tp, value, tb=None):  raise tp, value, tb""")
diff --git a/trollius/futures.py b/trollius/futures.py
index 68beb99..780c73d 100644
--- a/trollius/futures.py
+++ b/trollius/futures.py
@@ -13,7 +13,6 @@ try:
 except ImportError:
     import repr as reprlib   # Python 2
 
-from . import compat
 from . import events
 from . import executor
 
@@ -138,10 +137,6 @@ class Future(object):
     _exception = None
     _loop = None
 
-    # Used by Python 2 to raise the exception with the original traceback
-    # in the exception() method
-    _exception_tb = None
-
     _log_traceback = False   # Used for Python >= 3.4
     _tb_logger = None        # Used for Python <= 3.3
 
@@ -278,13 +273,8 @@ class Future(object):
         if self._tb_logger is not None:
             self._tb_logger.clear()
             self._tb_logger = None
-        tb = self._exception_tb
-        self._exception_tb = None
         if self._exception is not None:
-            if tb is not None:
-                compat.reraise(type(self._exception), self._exception, tb)
-            else:
-                raise self._exception
+            raise self._exception
         return self._result
 
     def exception(self):
@@ -303,7 +293,6 @@ class Future(object):
         if self._tb_logger is not None:
             self._tb_logger.clear()
             self._tb_logger = None
-        self._exception_tb = None
         return self._exception
 
     def add_done_callback(self, fn):
@@ -351,16 +340,7 @@ class Future(object):
         self._state = _FINISHED
         self._schedule_callbacks()
 
-    def _get_exception_tb(self):
-        """Helper method to call _set_exception_with_tb().
-
-        Use it to get the traceback of a future to copy it to a new future."""
-        return self._exception_tb
-
     def set_exception(self, exception):
-        self._set_exception_with_tb(exception, None)
-
-    def _set_exception_with_tb(self, exception, exc_tb):
         """Mark the future done and set an exception.
 
         If the future is already done when this method is called, raises
@@ -371,17 +351,13 @@ class Future(object):
         if isinstance(exception, type):
             exception = exception()
         self._exception = exception
-        if exc_tb is not None:
-            self._exception_tb = exc_tb
-        elif not compat.PY3:
-            self._exception_tb = sys.exc_info()[2]
         self._state = _FINISHED
         self._schedule_callbacks()
         if _PY34:
             self._log_traceback = True
         else:
             self._tb_logger = _TracebackLogger(self, exception)
-            if compat.PY3:
+            if hasattr(exception, '__traceback__'):
                 # Python 3: exception contains a link to the traceback
 
                 # Arrange for the logger to be activated after all callbacks
@@ -391,7 +367,7 @@ class Future(object):
                 if self._loop.get_debug():
                     frame = sys._getframe(1)
                     tb = ['Traceback (most recent call last):\n']
-                    tb += traceback.format_tb(self._exception_tb)
+                    tb += traceback.format_stack(frame)
                     tb += traceback.format_exception_only(type(exception), exception)
                     self._tb_logger.tb = tb
                 else:
diff --git a/trollius/tasks.py b/trollius/tasks.py
index 0915120..ab5c226 100644
--- a/trollius/tasks.py
+++ b/trollius/tasks.py
@@ -229,7 +229,7 @@ class Task(futures.Future):
         self._must_cancel = True
         return True
 
-    def _step(self, value=None, exc=None, exc_tb=None):
+    def _step(self, value=None, exc=None):
         assert not self.done(), \
             '_step(): already done: {0!r}, {1!r}, {2!r}'.format(self, value, exc)
         if self._must_cancel:
@@ -240,7 +240,6 @@ class Task(futures.Future):
         self._fut_waiter = None
 
         self.__class__._current_tasks[self._loop] = self
-        init_exc = exc
         # Call either coro.throw(exc) or coro.send(value).
         try:
             if exc is not None:
@@ -263,10 +262,7 @@ class Task(futures.Future):
         except futures.CancelledError as exc:
             super(Task, self).cancel()  # I.e., Future.cancel(self).
         except Exception as exc:
-            if exc is init_exc:
-                self._set_exception_with_tb(exc, exc_tb)
-            else:
-                self.set_exception(exc)
+            self.set_exception(exc)
         except BaseException as exc:
             self.set_exception(exc)
             raise
@@ -312,27 +308,16 @@ class Task(futures.Future):
                         'Task got bad yield: {0!r}'.format(result)))
         finally:
             self.__class__._current_tasks.pop(self._loop)
-
-            # Needed to break cycles when an exception occurs.
-            self = None
-            init_exc = None
+            self = None  # Needed to break cycles when an exception occurs.
 
     def _wakeup(self, future):
-        if (future._state == futures._FINISHED
-        and future._exception is not None):
-            # Get the traceback before calling exception(), because the call
-            # to exception() clears the traceback
-            exc_tb = future._get_exception_tb()
-            exc = future.exception()
-            self._step(None, exc, exc_tb)
+        try:
+            value = future.result()
+        except Exception as exc:
+            # This may also be a cancellation.
+            self._step(None, exc)
         else:
-            try:
-                value = future.result()
-            except Exception as exc:
-                # This may also be a cancellation.
-                self._step(None, exc)
-            else:
-                self._step(value, None)
+            self._step(value, None)
         self = None  # Needed to break cycles when an exception occurs.