sync.WorkGroup: Propagate all exception types, not only those derived from Exception

A problem was hit with pytest.fail with raises Failed exception not
being propagated to .wait. As it turned out it was not propagated
because pytest's Failed derives from BaseException, not Exception, and
we were catching only Exception and its children.

Rework the code to propagate all exception types from workers.

Performance change is with noise (it is either a bit faster for one set
of runs, or a bit slower for another set of runs).

golang/sync.py

@@ -121,15 +121,18 @@ class WorkGroup(object):
 
         try:
             f(g._ctx, *argv, **kw)
-        except Exception as exc:
+        except:
+            _, exc, tb = sys.exc_info()
             with g._mu:
                 if g._err is None:
                     # this goroutine is the first failed task
                     g._err = exc
                     if six.PY2:
                         # py3 has __traceback__ automatically
-                        exc.__traceback__ = sys.exc_info()[2]
+                        exc.__traceback__ = tb
                     g._cancel()
+            exc = None
+            tb  = None
 
 
     def wait(g):

golang/sync_test.py

@@ -25,6 +25,7 @@ from golang import sync, context
 import time, threading
 from pytest import raises
 from six.moves import range as xrange
+import six
 
 def test_once():
     once = sync.Once()
@@ -100,6 +101,17 @@ def test_workgroup():
     wg.wait()
     assert l == [1, 2]
 
+    # WorkGroup must catch/propagate all exception classes.
+    # Python2 allows to raise old-style classes not derived from BaseException.
+    # Python3 allows to raise only BaseException derivatives.
+    if six.PY2:
+        class MyError:
+            def __init__(self, *args):
+                self.args = args
+    else:
+        class MyError(BaseException):
+            pass
+
     # t1=fail, t2=ok, does not look at ctx
     wg = sync.WorkGroup(ctx)
     l = [0, 0]
@@ -109,15 +121,15 @@ def test_workgroup():
             with mu:
                 l[i] = i+1
                 if i == 0:
-                    raise RuntimeError('aaa')
+                    raise MyError('aaa')
         def f(ctx, i):
             Iam_f = 0
             _(ctx, i)
 
         wg.go(f, i)
-    with raises(RuntimeError) as exc:
+    with raises(MyError) as exc:
         wg.wait()
-    assert exc.type       is RuntimeError
+    assert exc.type       is MyError
     assert exc.value.args == ('aaa',)
     assert 'Iam__' in exc.traceback[-1].locals
     assert 'Iam_f' in exc.traceback[-2].locals
@@ -132,18 +144,18 @@ def test_workgroup():
             with mu:
                 l[i] = i+1
                 if i == 0:
-                    raise RuntimeError('bbb')
+                    raise MyError('bbb')
                 if i == 1:
                     ctx.done().recv()
-                    raise ValueError('ccc') # != RuntimeError
+                    raise ValueError('ccc') # != MyError
         def f(ctx, i):
             Iam_f = 0
             _(ctx, i)
 
         wg.go(f, i)
-    with raises(RuntimeError) as exc:
+    with raises(MyError) as exc:
         wg.wait()
-    assert exc.type       is RuntimeError
+    assert exc.type       is MyError
     assert exc.value.args == ('bbb',)
     assert 'Iam__' in exc.traceback[-1].locals
     assert 'Iam_f' in exc.traceback[-2].locals