Re: [Libguestfs] [nbdkit PATCH 3/3] tests: Add regression test for NBDKIT_EMULATE_CACHE fix

On Fri, Jun 10, 2022 at 10:03:23PM +0100, Richard W.M. Jones wrote: > After your patch series, the two lines just do nothing at all. No, they are still useful. If we don't call them during handshake, then the later call to next->can_zero in io.c may fail without setting errno. Any time we want to guarantee a next->can_* function will succeed at all later points where we need a sane errno, we do so by pre-caching it during handshake.

> > Anyway, I will remove them. Please don't. But adding comments for why they are important is okay.

I also fixed another bug today: the eval plugin is smart enough to synthesize several .can_* helpers if the corresponding function exists (for example, with sh, you have to explicitly implement can_flush to get flush called, but not with eval); but the eval plugin wasn't doing a good synthesis for .can_cache.