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This is a temporary workaround: it assumes nothing else uses these streams later.
This condition is ok for 'export' and test streams, since these are unlikely to
reuse these streams.  However, the proper fix is for the TaskStreams methods
to be smarter- they could open in append mode if the stream was closed.  The
streams associated with a task could be optimistically closed after it finishes executing.
(Any task can write to another task's streams, which is why it is an optimization only.)