On Sat, Dec 23 2017, Carl Baldwin jotted: > The big contention among git users is whether to rebase or to merge > changes [2][3] while iterating. I used to firmly believe that merging > was the way to go and rebase was harmful. More recently, I have worked > in some environments where I saw rebase used very effectively while > iterating on changes and I relaxed my stance a lot. Now, I'm on the > fence. I appreciate the strengths and weaknesses of both approaches. I > waffle between the two depending on the situation, the tools being > used, and I guess, to some extent, my mood. > > I think what git needs is something brand new that brings the two > together and has all of the advantages of both approaches. Let me > explain what I've got in mind... > > I've been calling this proposal `git replay` or `git replace` but I'd > like to hear other suggestions for what to name it. It works like > rebase except with one very important difference. Instead of orphaning > the original commit, it keeps a pointer to it in the commit just like > a `parent` entry but calls it `replaces` instead to distinguish it > from regular history. In the resulting commit history, following > `parent` pointers shows exactly the same history as if the commit had > been rebased. Meanwhile, the history of iterating on the change itself > is available by following `replaces` pointers. The new commit replaces > the old one but keeps it around to record how the change evolved. > > The git history now has two dimensions. The first shows a cleaned up > history where fix ups and code review feedback have been rolled into > the original changes and changes can possibly be ordered in a nice > linear progression that is much easier to understand. The second > drills into the history of a change. There is no loss and you don't > change history in a way that will cause problems for others who have > the older commits. > > Replay handles collaboration between multiple authors on a single > change. This is difficult and prone to accidental loss when using > rebase and it results in a complex history when done with merge. With > replay, collaborators could merge while collaborating on a single > change and a record of each one's contributions can be preserved. > Attempting this level of collaboration caused me many headaches when I > worked with the gerrit workflow (which in many ways, I like a lot). > > I blogged about this proposal earlier this year when I first thought > of it [1]. I got busy and didn't think about it for a while. Now with > a little time off of work, I've come back to revisit it. The blog > entry has a few examples showing how it works and how the history will > look in a few examples. Take a look. > > Various git commands will have to learn how to handle this kind of > history. For example, things like fetch, push, gc, and others that > move history around and clean out orphaned history should treat > anything reachable through `replaces` pointers as precious. Log and > related history commands may need new switches to traverse the history > differently in different situations. Bisect is a interesting one. I > tend to think that bisect should prefer the regular commit history but > have the ability to drill into the change history if necessary. > > In my opinion, this proposal would bring together rebase and merge in > a powerful way and could end the contention. Thanks for your > consideration. > > Carl Baldwin > > [1] http://blog.episodicgenius.com/post/merge-or-rebase--neither/ > [2] https://git-scm.com/book/en/v2/Git-Branching-Rebasing > [3] http://changelog.complete.org/archives/586-rebase-considered-harmful I think this is a worthwhile thing to implement, there are certainly use-cases where you'd like to have your cake & eat it too as it were, i.e. have a nice rebased history in "git log", but also have the "raw" history for all the reasons the fossil people like to talk about, or for some compliance reasons. But I don't see why you think this needs a new "replaces" parent pointer orthagonal to parent pointers, i.e. something that would need to be a new field in the commit object (I may have misread the proposal, it's not heavy on technical details). Consider a merge use case like this: A---B---C topic / \ D---E---F---G---H master Here we worked on a topic with commits A,B & C, maybe we regret not squashing B into A, but it gives us the "raw" history. Instead we might rebase it like this: A+B---C topic / G---H master Now we can push "topic" to master, but as you've noted this loses the raw history, but now consider doing this instead: A---B---C A2+B2---C2 topic / \ / D---E---F---G---G master I.e. you could have started working on commit A/B/C, now you "git replace" them (which would be some fancy rebase alias), and what it'll do is create a merge commit that entirely resolves the conflict so that hte tree is equivalent to what "master" was already at. Then you rewrite them and re-apply them on top. If you run "git log" it will already ignore A,B,C unless you specify --full-history, so git already knows to ignore these sort of side histories that result in no changes on the branch they got merged into. I don't know about bisect, but if it's not doing something similar already it would be easy to make it do so. You could even add a new field to the commit object of A2+B2 & C2 which would be one or more of "replaces <sha1 of A/B/C>", commit objects support adding arbitrary new fields without anything breaking. But most importantly, while I think this gives you the same things from a UX level, it doesn't need any changes to fetch, push, gc or whatever, since it's all stuff we support today, someone just needs to hack "rebase" to create this sort of no-op merge commit to take advantage of it.
