Rockbox

This seems like an odd thing to implement to me. Do you have anything in mind for which this will be used? I guess the time stretcher can be re-coded to use this, but apart from that, I can’t think of many effects that need large look-aheads.
And BTW, I see you convert some “while {}” loops to “do {} while” loops. Are you certain the number of samples is always non-zero? I distinctly remember this wasn’t always the case in the past when using the resampler.

Yes, I have something specific in mind.  FS#10199  is a limiter that needs to look ahead a few milliseconds to gradually reduce peaks instead of crushing them as they appear. This greatly reduces distortion. Instead of implementing a private buffer, I created a more general solution in case someone else wants to use it. The 4 KiB memory cost is the same either way.

I think you have the loop change backwards. I actually converted the “do {} while” loops to “while {}” loops. Just as you say, I’m not 100% sure that no zero sample chunks could get through, so I put in a defense. I don’t think any of my changes would cause any zero sample chunks to get through, but you never know if another process might.