Combine multiple exceptions into a linked one

When multiple tasks running at the same time
throw exceptions previously we would not be
able to rethrow all the combined ones which
was suboptimal. Instead of doing that create
a combined linked exception that can retain
all of the exceptions that were thrown and
rethrow that in the situation where more than
one task fails.

Change-Id: I8f882e0d58caa189d6bff2e33b0bc30c4cee553d

taskflow/exceptions.py

@@ -16,6 +16,10 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import StringIO
+
+import traceback
+
 
 class TaskFlowException(Exception):
     """Base class for exceptions emitted from this library."""
@@ -27,6 +31,33 @@ class Duplicate(TaskFlowException):
     pass
 
 
+class LinkedException(TaskFlowException):
+    """A linked chain of many exceptions."""
+    def __init__(self, message, cause, tb):
+        super(LinkedException, self).__init__(message)
+        self.cause = cause
+        self.tb = tb
+        self.next = None
+
+    @classmethod
+    def link(cls, exc_infos):
+        first = None
+        previous = None
+        for exc_info in exc_infos:
+            if not all(exc_info) or not len(exc_infos) == 3:
+                raise ValueError("Invalid exc_info")
+            buf = StringIO.StringIO()
+            traceback.print_exception(exc_info[0], exc_info[1], exc_info[2],
+                                      file=buf)
+            exc = cls(str(exc_info[1]), exc_info[1], buf.getvalue())
+            if previous is not None:
+                previous.next = exc
+            else:
+                first = exc
+            previous = exc
+        return first
+
+
 class StorageError(TaskFlowException):
     """Raised when logbook can not be read/saved/deleted."""
 

taskflow/patterns/threaded_flow.py

@@ -349,11 +349,12 @@ class Flow(flow.Flow):
             except exc.InvalidStateException:
                 pass
             finally:
-                # TODO(harlowja): re-raise a combined exception when
-                # there are more than one failures??
-                for f in failures:
-                    if all(f.exc_info):
-                        raise f.exc_info[0], f.exc_info[1], f.exc_info[2]
+                if len(failures) > 1:
+                    exc_infos = [f.exc_info for f in failures]
+                    raise exc.LinkedException.link(exc_infos)
+                else:
+                    f = failures[0]
+                    raise f.exc_info[0], f.exc_info[1], f.exc_info[2]
 
         def handle_results():
             # Isolate each runner state into groups so that we can easily tell