Accumulating into output_items[0] across multiple general_work()

Hi all,

I have a block whose output items are running averages over a long integration (for the sake of simplicity say a power spectrum accumulated over millions of FFT frames, way too many to hold as one input buffer).

The implementation that fell out naturally is: in general_work(), add the next batch of partial contributions directly into output_items[0][n_emitted], return 0 while the integration is still incomplete, and only return n_emitted > 0 when one or more integrations are done. It's pretty tempting to do as this would practically avoid an extra memory allocation for an internal buffer and a memcpy back to output_items when accumulation is ready.

However, this relies on the assumption that returning 0 leaves the write pointer unadvanced, so the next general_work() call sees the same memory at output_items[0][n_emitted] and I can keep adding into it. 

As far as I can tell, this works on current GR (no-op when produce_each(0) -> write pointer not updated), but it’s more like an implementation detail rather than it being documented as part of the scheduler API.

Two questions:

1) Is the "accumulate into output_items[0] across calls" pattern supported, or am I in undocumented/unidentified scheduler/buffer behavior territory?

2) If it's not supported, is there any reason beyond "no API guarantee”; e.g. would it break under certain custom buffers or futuristically GR4?

If the answer is just "use an internal buffer," happy to refactor.

Thanks!

Wael