On Sat, Mar 21, 2020 at 11:51:10AM +0100, Johannes Schindelin wrote: > My biggest caveat is that I had to force-exit the rebase at some stage > due to reasons I only vaguely remember. It had something to do with the > replacement cache not being updated when an `exec` is executed that adds a > replacement object via `git replace` [*1*]. This issue might have > _nothing_ to do with nested rebases, but as I said, my recollection is > vague. This is the sort of internal implementation gotcha I worry about. > There are a couple more concerns, of course, such as: what to do if the > user deletes the entire todo list (which is traditionally the only way to > abort a rebase)? My gut feeling is that it should go back to the > _previous_ version of the todo list. My assumption has been that, for simplicity, there would only be one commit in progress, and aborting it aborts everything. > Another big concern is what to do about `rebase.missingCommitsCheck`: with > nested rebases, this will get increasingly tricky. Like, imagine you are > rebasing 5 commits, the third of them results in merge conflicts, you > realize that it is obsolete and so is now the first, already rebased > commit. You do a nested rebase of the latest two commits to drop them, but > they don't have their original commit hashes any longer. So it gets a bit > finicky to keep track of what commit has been dropped on purpose and what > was forgotten to pick instead. This doesn't *seem* difficult, but I don't know how the current mechanism works. It just checks that all commits that were on the to-do list when the editor started are still listed (possibly marked "drop") when it exits. When you do a nested commit, additional commits are prepended to the to-do list, you're dropped into the editor, and the same check is made when the editor returns. If rebase.missingCommitsCheck = error is triggered, you end up with the <upstream> tree state with nothing applied and may either --continue to ignore the error or --edit-todo to put back the missing commits. Let me give an example. Suppose I have commits a-b-c-d-e, and start with rebase -i b. My to-do list starts out as c-d-e, but suppose I decide to cherry-pick z and add it to the list, so it's now z-c-d-e. So I start rebasing, and it turns out that d creates a merge conflict because I forgot a prerequisite patch y. And I really want y and z before b, anyway. So the tree state is currently a-b-z'-c', with d in progress and e yet to do. In my simple model, I have to resolve and commit d, so the tree state is a-b-z'-c'-d'. Then I can rebase -i a, and am presented with a to-do list of b-z'-c'-d'-e. If I delete any of those five commits, then rebase.missingCommitsCheck will trigger. If I put y in the list, save it, then change my mind and --edit-todo and delete y, it will also trigger. Now, it sould be nice if there were a way to say "screw this mess; just check out HEAD and put d back on the to-do list", but that could create a bit of a mess if I've split d and already committed half of it. If I used that after doung that, I'd have a to-do list of b-z'-c'-d'-d-e which might be awkward, but maybe it wouldn't be too bad.
