Igor Djordjevic <igor.d.djordjevic@xxxxxxxxx> writes: > Hi Sergey, > > On 19/03/2018 06:44, Sergey Organov wrote: >> >> > > > > > Second side note: if we can fast-forward, currently we prefer >> > > > > > that, and I think we should keep that behavior with -R, too. >> > > > > >> > > > > I agree. >> > > > >> > > > I'm admittedly somewhat lost in the discussion, but are you >> > > > talking fast-forward on _rebasing_ existing merge? Where would it >> > > > go in any of the suggested algorithms of rebasing and why? >> > > > >> > > > I readily see how it can break merges. E.g., any "git merge >> > > > --ff-only --no-ff" merge will magically disappear. So, even if >> > > > somehow supported, fast-forward should not be performed by default >> > > > during _rebasing_ of a merge. >> > > >> > > Hmm, now that you brought this up, I can only agree, of course. >> > > >> > > What I had in my mind was more similar to "no-rebase-cousins", like >> > > if we can get away without actually rebasing the merge but still >> > > using the original one, do it. But I guess that`s not what Johannes >> > > originally asked about. >> > > >> > > This is another definitive difference between rebasing (`pick`?) and >> > > recreating (`merge`) a merge commit - in the case where we`re rebasing, >> > > of course it doesn`t make sense to drop commit this time (due to >> > > fast-forward). This does make sense in recreating the merge (only). >> > >> > Eh, I might take this back. I think my original interpretation (and >> > agreement) to fast-forwarding is correct. >> > >> > But the confusion here comes from `--no-ff` as used for merging, as >> > opposed to `--no-ff` as used for rebasing. I _think_ Johannes meant >> > the latter one. >> > >> > In rebasing, `--no-ff` means that even if a commit inside todo list >> > isn`t to be changed, do not reuse it but create a new one. Here`s >> > excerpt from the docs[1]: >> > >> > --no-ff >> > With --interactive, cherry-pick all rebased commits instead of >> > fast-forwarding over the unchanged ones. This ensures that the >> > entire history of the rebased branch is composed of new commits. >> > >> > Without --interactive, this is a synonym for --force-rebase. >> > >> > >> > So fast-forwarding in case of rebasing (merge commits as well) is >> > something you would want by default, as it wouldn`t drop/lose >> > anything, but merely reuse existing commit (if unchanged), instead of >> > cherry-picking (rebasing) it into a new (merge) commit anyway. >> >> This sounds like breakage. E.g., it seems to be breaking every "-x ours" >> merge out there. > > Either you are not understanding how rebase fast-forward works, or > I`m missing what you are pointing to... Mind explaining how can > something that`s left unchanged suddenly become a breakage? It was misunderstanding on my side indeed, sorry. > >> Fast-forwarding existing merge, one way or another, still seems to be >> wrong idea to me, as merge commit is not only about content change, but >> also about joint point at particular place in the DAG. > > Not sure what this has to do with rebase fast-forwarding, either - > nothing changes for fast-forwarded (merge or non-merge) commit in > question, both content, joint point and everything else stays exactly > the same. If anything changed, then it can`t/won`t be fast-forwarded, > being unchanged is a prerequisite. > > Let me elaborate a bit. Here`s a starting diagram: [... detailed explanation skipped for brevity ...] > Does this settle your concerns, or I`m missing something? Yes, it does, thank you! Leaving as many leading commits as possible unchanged during rebase is what fast-forward mean in this case then, and it's pretty OK with me. >> As for fast-forwarding re-merge, explicitly requested, I'm not sure. On >> one hand, it's inline with the default "git merge" behavior, on the >> other hand, it still feels wrong, somehow. > > Regarding fast-forwarding in context of merging, in case where we are > recreating merges (not rebasing them), following existing `git merge` > logic might make sense, where I would expect rebasing todo list `merge` > command to pick-up tricks from `git merge` as needed, like learning > to accept `--no-ff` option, for example, thus not fast-forwarding > merges (on request) even when possible. > > Though, I do agree that in case you want to recreate an existing merge > (instead of just rebasing it), `merge` command fast-forwarding might > probably not be what you want for the most of the time, but I`m afraid > having rebase todo list `merge` command default behavior different than > `git merge` default one (in regards to fast-forwarding) would be > confusing... or not? > > From what I could grasp so far, usually Git commands` default > behavior is (explained to be) chosen per "most common use case", so > might be non fast-forwarding would be fine as default for rebase todo > list `merge` command, even though different than `git merge` > itself...? As far as I can tell, fast-forward as default for "git merge" has been chosen to avoid excessive unintended merges in typical workflows, and therefore this decision is not actually applicable to rebasing, I think. I'm inclined to disable merges to fast-forward during history rebasing, at least by default. As far as I can tell, current "--preserve-merges" does fast-forward and breaks "git merge --ff-only --no-ff" merges, among other things it breaks, and that was my primary concern here. Didn't check what "--recreate-merges" does though. -- Sergey
