On Sun, Nov 06, 2016 at 12:17:10PM -0800, Jacob Keller wrote: > On Sun, Nov 6, 2016 at 9:33 AM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: > > On Sun, Nov 06, 2016 at 09:14:56AM -0800, Junio C Hamano wrote: > >> Josh Triplett <josh@xxxxxxxxxxxxxxxx> writes: > >> > We could, but if we (or one of the many third-party git implementations) > >> > miss a case, gitlinks+reachability may appear to work in many cases with > >> > dataloss afterward, while gitrefs will fail early and not appear > >> > functional. > >> > >> I wonder what happens if we do not introduce the "gitref" but > >> instead change the behaviour of "gitlink" to imply an optional > >> reachability. That is, when enumerating what is reachable in your > >> repository, if you see a gitlink and if you notice that you locally > >> have the target of that gitlink, you follow, but if you know you > >> lack it, you do not error out. This may be making things too > >> complex to feasibily implement by simplify them ;-) and I see a few > >> immediate fallout that needs to be thought through (i.e. downsides) > >> and a few upsides, too. I am feeling feverish and not thinking > >> straight, so I won't try to weigh pros-and-cons. > >> > >> This would definitely need protocol extension when transferring > >> objects across repositories. > > > > It'd also need a repository format extension locally. Otherwise, if you > > ever touched that repository with an older git (or a tool built on an > > older libgit2 or JGit or other library), you could lose data. > > > > It does seem conceptually appealing, though. In an ideal world, the > > original version of gitlink would have had opt-out reachability (and > > .gitmodules with an external repository reference could count as opting > > out). > > > > But I can't think of any case where it's OK for a git implementation to > > not know about this reachability extension and still operate on the > > gitlink. And given that, it might as well use a new object type that > > the old version definitely won't think it understands. > > > > - Josh Triplett > > That's still only true if the receiving end runs fsck, isn't it? I > suppose that's a large number of receivers, and at least there are > ways post-push to determine that objects don't make sense to that > version of git. > > I think using a new mode is the safest way, and it allows easily > implementing RefTrees as well as other projects. Additionally, if we > *wanted* additional "opt-in / opt-out" support we could add this by > default to gitrefs,and they could (possibly) replace gitlinks in the > future? Once we have gitrefs, you have both alternatives: reachable (gitref) or not reachable (gitlink). However, if you want some way to mark reachable objects as not reachable, such as for a sparse checkout, external large-object storage, or similar, then you can use a single unified mechanism for that whether working with gitrefs, trees, or blobs.