Martin von Zweigbergk wrote: > I'm working on a re-roll of > http://thread.gmane.org/gmane.comp.version-control.git/205796 > > and finally got around to including test cases for what you fixed in > this patch. I want to make sure I'm testing what you fixed here. See > questions below.
Thanks for that. I should have done this myself. > On Sat, Jan 12, 2013 at 12:46 PM, Phil Hord <ho...@cisco.com> wrote: >> Since 90e1818f9a (git-rebase: add keep_empty flag, 2012-04-20) >> 'git rebase --preserve-merges' fails to preserve empty merge commits >> unless --keep-empty is also specified. Merge commits should be >> preserved in order to preserve the structure of the rebased graph, >> even if the merge commit does not introduce changes to the parent. >> >> Teach rebase not to drop merge commits only because they are empty. > Consider a history like > > # a---b---c > # \ \ > # d---l > # \ > # e > # \ > # C > > where 'l' is tree-same with 'd' and 'C' introduces the same change as 'c'. > > My test case runs 'git rebase -p e l' and expects the result to look like > > # a---b---c > # \ \ > # d \ > # \ \ > # e---l > This is probably right, but it is not exactly the case that caused my itch. I think my branch looked like this: # a---b---c # \ # d---f # \ \ # e---g # \ # l where g is tree-same with f. That is, e merged with f, but all of e's changes were dropped in the merge. So when I ran 'git rebase -p c l', I expected to end up with this: # a---b---c # \ # d---f # \ \ # e---g # \ # l But instead, I got an error because git-rebase--interactive.sh decided that g was empty, so it dropped it by commenting it out of the todo list: pick d pick e pick f #pick g pick l At the end of this attempt, I got some odd error about a cherry-pick have incorrect parameters or somesuch. I bisected the problem to a commit that clued me in to one of my commits being silently dropped. And that is specifically what I fixed. This happened only because 'is_empty_commit' checks for tree-sameness with the first parent; it does not consider whether there are multiple parents. Perhaps it should. >> A special case which is not handled by this change is for a merge commit >> whose parents are now the same commit because all the previous different >> parents have been dropped as a result of this rebase or some previous >> operation. > And for this case, the test case runs 'git rebase -p C l'. Is that > what you meant here? > > Before your patch, git would just say "Nothing to do" Huh. That is worse than I thought. > and after your > patch, we get > > # a---b---c > # \ \ > # d \ > # \ \ > # e \ > # \ \ > # C---l > > As you say, your patch doesn't try to handle this case, but at least > the new behavior seems better. I think we would ideally want the > recreated 'l' to have only 'C' as parent in this case. Does that make > sense? This is not what I meant, but it is a very interesting corner case. I am not sure I have a solid opinion on what the result should be here. I feel like it should look the same as you show here, since neither 'c' nor 'C' is a candidate for collapsing during this rebase. But I may be missing some subtlety here. Here is the corner case I was thinking of. I did not test this to see if this will happen, but I conceived that it might. Suppose you have this tree where # a---b---c # \ # d---g---l # \ / # C where 'C' introduced the same changes as 'c'. When I execute 'git rebase -p l c', I expect that I will end up with this: # a---b---c---d--- # \ \ # ---g---l That is, 'C' gets skipped because it introduces the same changes already seen in 'c'. So 'g' now has two parents: 'd' and 'C^'. But 'C^' is 'd', so 'g' now has two parents, both of whom are 'd'. I think it should collapse to this instead: # a---b---c---d---g---l I don't think this occurs because of my patch, and I am not sure it occurs at all. It is something that I considered when I was thinking of failure scenarios for my patch. I expect it also may happen if 'C' is an already-empty commit, or if it is made empty after conflict resolution involving the user. I mentioned it because I thought my patch _could_ address this if my is_merge_commit test would also consider whether the parents are distinct from each other or not. I hope this is clear, but please let me know if I made it too confusing. Phil -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
