Re: tb/cruft-packs (was Re: What's cooking in git.git (Mar 2022, #01; Thu, 3))

> On Mon, Mar 07, 2022 at 10:06:00AM -0800, Jonathan Nieder wrote:
>> I'm excited about this work!  I just sent a quick review to the
>> design doc.
> 
> Thanks! I haven't had a chance to look at the design doc, but let me
> respond quickly to this message:
> 
>> Before merging to 'next', I'd be interested in two things:
>>
>>  1. Marking the feature as experimental so we can learn from experience.
>>     Clarifying what aspects we consider to be stable / set in stone and
>>     what are subject to modification.
> 
> I'm not sure there is much practical benefit to marking this feature as
> experimental. The only new file format here is the .mtimes one, which
> should make it easy for us to modify the format in a
> backwards-compatible way.
> 
> If there are other benefits you had in mind, I'm curious to hear them.
> But I think we should be fine to "lock in" the first version of the
> .mtimes format since we have an easy-ish mechanism to change it in the
> future.

>>  2. Marking this as a repository format extension so it doesn't interact
>>     poorly with Git implementations (including older versions of Git
>>     itself) that are not aware of the new feature
> 
> The design of cruft packs was done intentionally to avoid needing a
> format extension. The cruft pack is "just a pack" to any older version
> of Git. The only thing an older version of Git wouldn't understand is
> how to interpret the .mtimes file. But that's no different than the
> current behavior without cruft packs, where any unreachable object
> inherits the mtime of its containing pack.
> 
> So an older version of Git might prune a different set of objects than a
> version that understands cruft packs depending on the contents of the
> .mtimes file, the mtime of the cruft pack, and the width of the grace
> period. But I think by downgrading you are more or less buying into the
> existing behavior. So I don't think there is a compelling reason to
> introduce a format extension here.