[1718627440]

The thing to keep in mind is that Git doesn't version the file system, it versions the index. This is because a file system guy like Torvalds knows that the file system is a shared resource and no program should think it can control its state. Therefore a Git repository doesn't consists out of all the files below a directory, it consists out of everything in the index.

Git does version everything that is in the repository and all these states occur in the reflog.

[steveklabnik]

> The thing to keep in mind is that Git doesn't version the file system, it versions the index.

Yes. I think that this difference is what introduces a lot of friction, both in the model, and how people use it. The divergence between the files that exist on disk inside your working copy and what's actually tracked means lots of opportunities for friction that go away once you decide that it should. That doesn't mean things are perfect, for example, by default jj only snapshots the filesystem when you run a `jj` command, so you can still lose changes from in between those, you need to enable Watchman to get truly full logging here.

> all these states occur in the reflog.

Well, let's go back to the documentation for reflog:

> Reference logs, or "reflogs", record when the tips of branches and other references were updated in the local repository.

It only tracks changes to refs. That is, the states that refs have been in. So, one big example is detatched HEADs: any changes you make to those, which still are contents of the repository, are not tracked in the reflog.

Even for refs, there's differences: the reflog says "ref was in state x and changed to state y" without any more details. jj's oplog keeps track of not only the state change, but the reason why: "rebased commit <sha> with these args: jj rebase -r <sha> -d trunk"

The reflog only tracks individual refs. Say we rebase multiple commits. The reflog still just says "the head of this branch was in state x and changed to state y" but the oplog says "a rebase happened, it affected all of these commits refs in these ways," that is, it's just inherently more rich in what it tracks, and does it across all relative commits, not only the refs.

This doesn't mean the reflog is bad! It's just a very specific thing. Git could have an operation log too, it's just a different feature.

[1718627440]

> So, one big example is detatched HEADs: any changes you make to those, which still are contents of the repository, are not tracked in the reflog.

    $ git checkout HEAD
    $ git commit --allow-empty -m "_"
    $ git checkout master
    $ git reflog
    a91 (HEAD -> master, origin/master, origin/HEAD) HEAD@{0}: checkout: moving from b94 to master
    b94 HEAD@{1}: commit: _
    28d (origin/feature, feature) HEAD@{2}: checkout: moving from feature to @

> Even for refs, there's differences: the reflog says "ref was in state x and changed to state y" without any more details. jj's oplog keeps track of not only the state change, but the reason why: "rebased commit <sha> with these args: jj rebase -r <sha> -d trunk"

> The reflog only tracks individual refs. Say we rebase multiple commits. The reflog still just says "the head of this branch was in state x and changed to state y" but the oplog says "a rebase happened, it affected all of these commits refs in these ways," that is, it's just inherently more rich in what it tracks, and does it across all relative commits, not only the refs.

    68e HEAD@{15}: rebase (finish): returning to refs/heads/feature
    68e HEAD@{16}: rebase (pick): message #6
    7ff HEAD@{17}: rebase (pick): message #5
    797 HEAD@{18}: rebase (pick): message #4
    776 HEAD@{19}: rebase (pick): message #3
    c7d HEAD@{20}: rebase (pick): message #2
    f10 HEAD@{21}: rebase (pick): message #1
    c0d HEAD@{22}: rebase (start): checkout @~6

    a7c HEAD@{100}: rebase (reword): message ...
    3b1 HEAD@{229}: rebase (reset): '3b1' message ...
    4a4 HEAD@{270}: rebase (continue): message ...

[steveklabnik]

Oh yeah I forgot HEAD Is a ref, whoops. Duh! (My git is getting rusty at this point...)

jj still ends up keeping information in here that the reflog doesnt, but you're right that these aren't the strongest points.

[martinvonz]

One little benefit of the op log is that you can use a single `jj undo` to undo all the rebased branches/bookmarks in one go. If you have rebased many branches with `git rebase --update-refs`, you need to reset each of the branches separately AFAIK.

[1718627440]

      --update-refs, --no-update-refs
           Automatically force-update any branches that point to commits that
           are being rebased. Any branches that are checked out in a worktree
           are not updated in this way.

           If the configuration variable rebase.updateRefs is set, then this
           option can be used to override and disable this setting.

[martinvonz]

Are you saying that that text implies that the you can undo the rebase with a single command or that all the reflogs get updated atomically? Or how is it related to the comment you replied to?

[1718627440]

Which, the stuff you said earlier is in the reflog?

I think Git will just gain a oplog. You just need to append a commit hash to a list before each command and implement undo as remove item and checkout. The hardest thing will be race conditions, but Git already knows how to be a database.