unwind from Render Node pull

[Ichthyostega]

As a Lumiera developer,
I need the ability for a short-circuit exit from Node processing,
to handle timing and output protocol failures gracefully and without leaks.

change of requirements

From the original top-down analysis, it seemed that all glitches in output handling could be accommodated directly within the OutputSlot. However, the integration, as pursued by the »Playback Vertical Slice« unveiled the possibility of gradually loosing control within the internal mechanism for output delivery.

Due to general, overarching design considerations, I have ruled out the regular use of Exceptions for handling such plausible corner cases — and especially the processing of Render Node invocations is designed under the assumption that there is a single, predictable and reliable code path. And while it is indeed possible to raise an Exception in response to an unexpected event, the result of such an event can only be to shut down processing altogether (to cancel all jobs and to spin down the Work Force).

So this newly discovered failure mode implies that the output handling layer might signal a problematic situation; the Render Node processing should then attempt to short-circuit any further expensive calculations and unwind from the recursive call-down. However, any relevant clean-up steps must be performed reliably.