On Tue, Mar 26 2019, Denton Liu wrote: > Hi Ævar, > > On Tue, Mar 26, 2019 at 09:35:48PM +0100, Ævar Arnfjörð Bjarmason wrote: >> >> On Tue, Mar 26 2019, Denton Liu wrote: >> >> > Hi Ævar, >> > >> > On Tue, Mar 26, 2019 at 03:35:34PM +0100, Ævar Arnfjörð Bjarmason wrote: >> >> >> >> On Sat, Mar 23 2019, Denton Liu wrote: >> >> >> >> > This series teaches rebase the --keep-base option. >> >> > >> >> > 'git rebase --keep-base <upstream>' is equivalent to >> >> > 'git rebase --onto <upstream>... <upstream>' or >> >> > 'git rebase --onto $(git merge-base <upstream> HEAD) <upstream>' . >> >> > >> >> > This seems to be a common case that people (including myself!) run into; I was >> >> > able to find these StackOverflow posts about this use case: >> >> > >> >> > * https://stackoverflow.com/questions/53234798/can-i-rebase-on-a-branchs-fork-point-without-explicitly-specifying-the-parent >> >> > * https://stackoverflow.com/questions/41529128/how-do-you-rebase-only-changes-between-two-branches-into-another-branch >> >> > * https://stackoverflow.com/a/4207357 >> >> >> >> Like with another series of yours I think this would be best squashed >> >> into one patch. >> > >> > Will do. >> > >> >> >> >> Maybe I've misunderstood this but isn't this like --fork-point except >> >> with just plain "git merge-base" instead of "git merge-base >> >> --fork-point", but then again 2/3 shows multiple base aren't supported, >> >> but merge-base supports that. >> >> >> > >> > --fork-point gets used to determine the _set of_ commits which are to be >> > rebased, whereas --keep-base (and --onto) determine the base where that >> > set of commits will be spliced. As a result, these two options cover >> > orthogonal use-cases. >> >> Right. After playing with this a bit more though --fork-point is mostly >> there, it it does find the same fork point, as evidenced all your tests >> (that aren't asserting incompatibility with other options) passing with >> this: >> >> diff --git a/t/t3416-rebase-onto-threedots.sh b/t/t3416-rebase-onto-threedots.sh >> index 9c2548423b..ab2d50e69a 100755 >> --- a/t/t3416-rebase-onto-threedots.sh >> +++ b/t/t3416-rebase-onto-threedots.sh >> @@ -116,7 +116,7 @@ test_expect_success 'rebase --keep-base master from topic' ' >> git checkout topic && >> git reset --hard G && >> >> - git rebase --keep-base master && >> + git rebase $(git merge-base --fork-point master HEAD) && >> git rev-parse C >base.expect && >> git merge-base master HEAD >base.actual && >> test_cmp base.expect base.actual && >> @@ -140,7 +140,7 @@ test_expect_success 'rebase -i --keep-base master from topic' ' >> git reset --hard G && >> >> set_fake_editor && >> - EXPECT_COUNT=2 git rebase -i --keep-base master && >> + EXPECT_COUNT=2 git rebase -i $(git merge-base --fork-point master HEAD) && >> git rev-parse C >base.expect && >> git merge-base master HEAD >base.actual && >> test_cmp base.expect base.actual && >> >> I've poked at some of this recently in >> https://public-inbox.org/git/20190221214059.9195-3-avarab@xxxxxxxxx/ as >> noted in the feedback there (I haven't gotten around to v2 yet) it's >> entirely possible that I haven't understood this at all :) >> >> But it seems to me that this patch/implementation conflates two >> unrelated things. >> >> Once is that we use --fork-point to mean that we're going to find the >> divergence point with "merge-base --fork-point". This gets you halfway >> to where you want to be, i.e. AFAICT the --keep-base and --fork-point >> will always find the same commit for "git rebase" and "git rebase >> --keep-base". See the "options.restrict_revision = get_fork_point(...)" >> part of the code. > > I don't think this is true. The code that --keep-base uses to find the > merge base is get_oid_mb, see the relevant snippet > > if (strstr(options.onto_name, "...")) { > if (get_oid_mb(options.onto_name, &merge_base) < 0) > > whereas the --fork-point code uses get_fork_point, as you mentioned > above. As a result, they don't necessarily refer to the same commit in > the case where upstream is rewound. > >> >> The other, which you want to disable, is that --fork-point *also* says >> "OK, once we've found the divergence point, let's then rebase it on the >> latest upstream. Or in the example above the "master" part of "git >> merge-base --fork-point master HEAD". > > Correct, I guess in essence this is what I'm doing. > >> >> Shouldn't --keep-base just be implemented in terms of skipping *that* >> part, i.e. we find the fork point using the upstream info, but then >> don't rebase *on* upstream. >> >> The reason the distinction matters is because with your patch these two >> act differently: >> >> git rebase --keep-base >> git rebase $(git merge-base @{u} HEAD) >> >> The latter will skip work ("Current branch master is up to date"), but >> --keep-base will always re-rebase things. There's some cases where >> --fork-point does that, which I was trying to address with my linked WIP >> patch above. > > I believe this is desired behaviour. Suppose we have this (modified) > graph from the git-merge-base docs, where B3 was formerly part of > origin/master but it was then rewound: > > ---o---o---B2--o---o---o---B (origin/master) > \ > B3 > \ > Derived (local master) > > If we run "git rebase --keep-base", we'll get the following graph: > > ---o---o---B2--o---o---o---B (origin/master) > \ > Derived (local master) > > which I believe is the desired behaviour (we're abandoning B3 since > upstream abandoned it). > > I hope I'm understanding you correctly. Please let me know if I've > misinterpreted anything you've said or if anything I've said is unclear. Yeah. I'm still confused, but mainly because I haven't allocated enough brainpower to try to understand it :) So yeah, I can believe it's subtly different, would be great to have a v2 whose docs/tests cover those subtleties, right now (as seen in my discussion upthread) the tests that are there can identically use the fork point. I also wonder if we can holistically think about this UI / how we can expose different things. E.g. for the times I've needed this and have manually dug up the fork point I haven't wanted to handle the case of upstream rewinding, just re-rebase-i on some old base, while still having upstream tracking info, and for rebase to exit early if there's nothing to do (similar to if I feed it the fork point as a rev). >> >> Whereas the thing you actually want to work is: >> >> git rebase -i --keep-base >> git rebase -i $(git merge-base @{u} HEAD) >> >> I.e. to have both of those allow you to re-arrange/fixup whatever and >> still rebase on the same divergence point with @{u}, and won't run >> rebase when there's no work to do unless you give it --force-rebase. >> >> > reason that --onto already disallows multiple bases. If we have multiple >> > bases, how do we determine which one is the "true base" to use? It makes >> > more sense to error out and let the user manually specify it. >> >> Ah, makes sense. >> >> >> I'd find something like the "DISCUSSION ON FORK-POINT MODE" in >> >> git-merge-base helpful with examples of what we'd pick in the various >> >> scenarios, and also if whatever commit this picks was something you >> >> could have "git merge-base" spew out, so you could get what rebase would >> >> do here from other tooling (which maybe is possible, but I'm confused by >> >> the "no multiple bases"...). >> > >> > If I'm understanding you correctly then yes, this could be done with >> > other tooling. See the 0/3 for equivalent commands. >> > >> > Perhaps I should update the rebase documentation to mention that >> > --fork-point and --keep-base are orthogonal because it's unclear for >> > you, it's probably unclear for other users as well. >> > >> > Thanks, >> > >> > Denton
