From: Patrick Steinhardt <ps@pks.im>
Date: Tue, 06 Jan 2026 15:39:42 GMT
Subject: Re: [PATCH RFC 00/11] Introduce git-history(1) command for easy history editing
Message-ID: <aV0svlRpEZPoJmkO@pks.im>
In-Reply-To: <CABPp-BG9h1xxp9b2UfE00EaKozk5kGAUELe-Od139amZdAa47g@mail.gmail.com>

On Sat, Dec 20, 2025 at 08:51:23AM -0800, Elijah Newren wrote:
> On Fri, Dec 19, 2025 at 8:30 AM Elijah Newren <newren@gmail.com> wrote:
> >
> > On Fri, Dec 19, 2025 at 4:22 AM Patrick Steinhardt <ps@pks.im> wrote:
> > >
> > [...]
> > > Okay, so the majority of folks here seem to favor rewriting all
> > > dependent branches, which is also the default that JJ uses here, and
> > > git-replay(1) does it, too.
> > >
> > > There is one major difference between git-replay(1) and git-history(1)
> > > though: the former works with revision ranges, whereas the latter does
> > > not. By using revision ranges we avoid the problem I have mentioned in a
> > > different branch of this discussion, which is that we have no easy way
> > > to figure out which branches we'd have to touch in the first place. This
> > > is because we simply walk the revision range there and then look at
> > > which of our references point into that range. That's simple enough.
> > >
> > > But in our case we're not working with ranges, we are working with a
> > > singular commit.
> >
> > I don't understand the distinction at all.  `git replay edit` also
> > took a single commit, and then implemented the obvious (and jj-like)
> > behavior of rewriting all branches that descended from that commit.
> >
> > > In my head this meant that we'd have to basically do a
> > > revision walk that starts from all of our branches so that we can figure
> > > out which of them would eventually reach the commit that we are about to
> > > rewrite.
> >
> > Yes, and it's only a few lines of code, as I showed earlier.
> >
> > > And that of course doesn't scale.
> >
> > That's quite an assumption about scaling; I don't believe it.  Under
> > what conditions would this be slow enough for users to notice and be
> > bothered?  commit-graphs not enabled + weird local clone with
> > thousands of local branches?  Also, isn't jj specifically designed for
> > large repositories and with scaling in mind, and yet this is their
> > default behavior?
> >
> > More importantly, this is being used to justify a large principle of
> > least astonishment violation (disconnecting branches with shared
> > history), so we'd not only need to show that walking all branches was
> > slower enough for users to notice, but slower enough that the negative
> > user performance experience offsets the negative user experience from
> > the astonishing behavior.  Typically, spending extra cycles to provide
> > users with good warnings/errors is a good use of time, especially when
> > it'll take them far longer to discover and recover from negative
> > surprises.
> 
> A quick clarification in case I'm misunderstood above:
> 
> When I talk about rewriting branches descended from the commit, I am
> specifically talking about refs/heads/*, not refs/remotes/origin/* or
> refs/tags/* or anything else.  Because:
>   * I suspect we'll soon implement an "immutable branches" concept, so
> that e.g. a request to modify a commit in the history of "main" would
> result in an error (by default).  This leads to the idea that we're
> just rewriting the user's local stuff they have on top of the
> immutable history, i.e. their local branches.
>   * refs/tags/* are designed to be immutable, obviously.  In fact, we
> might want to automatically include tags in the set of "immutable
> branches" by default.
>   * refs/remotes/* are designed to match what the corresponding remote
> had, not to be independently rewritten.  We'd really mess people up if
> we changed that.
>   * The fact that users created local branches means they are marking
> those parts of history as a relevant area of interest
>   * While it could be that some of refs/remotes/origin/* have shared
> history with whatever commit is being reworded/split/edited, so too
> could branches that were never pushed.  I think focusing on the
> branches of interest to the user (i.e. their local branches) makes
> sense.

Fair. I'll send a new revision tomorrow that does this. Thanks!

Patrick