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In a traditional VCS, I can understand why you would not commit unresolved files because you could break the build. However, I don't understand why you shouldn't commit unresolved files in a DVCS (some of them will actually prevent you from committing the files).

Instead, I think that your repository should be locked from pushing and pulling, but not committing.

Being able to commit during the merging process has several advantages (as I see it):

  • The actual merge changes are in history.
  • If the merge was very large, you could make periodic commits.
  • If you made a mistake, it would be much easier to roll back (without having to redo the entire merge).
  • The files could remain flagged as unresolved until they were marked as resolved. This would prevent pushing/pulling.

You could also potentially have a set of changesets act as the merge instead of just a single one. This would allow you to still use tools such as git rerere.

So why is committing with unresolved files frowned upon/prevented? Is there any reason other than tradition?

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By whom is it frowned upon or prevented? –  pdr Oct 10 '12 at 13:51
    
@pdr Some developers I worked with frowned upon it. At least in hg 1.6 after a merge, files are marked as unresolved. hg will not let you commit until you have marked them as resolved (doesn't necessarily mean you actually have to resolve them, but I would assume that's the idea). –  Explosion Pills Oct 10 '12 at 13:53
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So by "unresolved files", do you actually mean "unresolved merges"? –  pdr Oct 10 '12 at 13:56
    
@pdr no, hg actually maintains a list of files that have or have not been flagged as "resolved" (using hg resolve). If there are any U files on this list, it won't let you commit. –  Explosion Pills Oct 10 '12 at 13:57
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hg resolve is used specifically for merges with conflicts; see selenic.com/mercurial/hg.1.html#resolve. Note that Mercurial will not let you commit files with unresolved merge conflicts. You must use hg resolve -m ... before you can commit after a conflicting merge. –  Mike Partridge Oct 10 '12 at 14:30
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5 Answers

The biggest issue that I can see is that it creates a window of commits where things are half-merged and (probably) not working correctly. When you push the final set of local commits, all of those intermediate commits will also apear for everyone else. In ideal world, I should be able to pull any commit and the code should work. If you start committing in the middle of merges, the state of the code isn't well-defined.

One thing you could do is make local commits to your merge, and then bundle them into one big commit when you push (though I'm not sure how(if?) any vcs support this). While this might yield some of the benefits you mentioned, I'm not sure it's worth the extra complexity (we're already dealing with a fairly confusing and complex area).

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I don't know that I agree with the ability to pull any commit and have it working .. a lot of the point of committing in a DVCS is to commit your progress, so stuff is bound to be broken or incomplete much of the time. –  Explosion Pills Oct 10 '12 at 14:47
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Always having working commits has some nice benefits. For example, if you are trying to track down when a bug was introduced, it's useful to be able to traverse the history to see when something broke (vcs even have commands for doing binary search over the history). If you don't have working commits, you won't be able to track bugs down as effectively. –  Oleksi Oct 10 '12 at 15:03
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Git supports bundled commits. –  deltree Oct 10 '12 at 18:08
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I am most familiar with Git, so I will be answering for that perspective.

I don't see a reason why you or any good VCS would want to allow committing an unmerged file, particularly if it was code. You need to keep the repository in a consistent state, and what you are suggesting would violate commit atomicity. Many VCS's physically change the file to show where the conflicts are - Git, SVN, and CVS use >>>> <<<< type markers. In a VCS with atomic repository-level commits and merges, you would just have created a node that makes no sense to anyone but you. In software development, your project couldn't build. In a group's document, no one knows which changes are correct.

Now, Git provides some tools that could ease this, were the type of commit you suggest allowed. You could squash all you merge commits together before you pushed, for example. That winds up being the same as a typical merge commit.

Specific concerns about your list of benefits:

  1. The actual merge changes are in history. Why do you need extra information? DVCS's are very good about limiting conflicts to confined areas. Once you choose which changeset to keep, comparing the merge commit node's copy to the previous copy will give you exactly this.
  2. If the merge was very large, you could make periodic commits. This is a valid concern, but you shouldn't ever get here in the first place. Branches should constantly be pulling in upstream changes just so this won't ever happen. Tools like rebase or cherry-pickking one commit at a time can also assist you here in some situations.
  3. If you made a mistake, it would be much easier to roll back (without having to redo the entire merge). See above - your conflicts shouldn't become this unmanagable.

The only way this suggestion could work is if the branch was if the whole merge was atomic - you could see a series of commits, but they would be merely steps in a larger merge commit that had to be treated as one node in the commit tree. I don't think any current VCS has support for this type of workflow, and I don't think it is necessary.

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Conflicts probably shouldn't become this unmanageable, but often are (at least in my experience). I think that the whole merge should be atomic; that is what I'm suggesting. May not be necessary, I'm just wondering why it hasn't been done. You also can't necessarily choose one or the other option in a conflict; sometimes it's a combination of both changes. This is especially confusing. –  Explosion Pills Oct 10 '12 at 14:49
    
"You need to keep the repository in a consistent state". I don't think that's true. A local repository is a work space, not a shrine. If it helps to commit in the middle of a merge, do it IMHO. If you do this because you don't know any better, I would suggest not doing it. If you're an experienced programmer, however, you should do whatever works best for you. Don't get dogmatic about keeping your local repository pristine. –  Bryan Oakley Oct 10 '12 at 18:45
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I think it's best to push small changes and push often, when it's possible(and of course it isn't always), and don't commit code that doesn't build, or is half complete(we all make mistakes, but don't do it on purpose). I also come from git, and one of the best things I think is the fact that you can have a workable copy of the repo on your desktop...which you can then modify to your hearts content. When your big changes are done, send it away.

Doing a lot of open source with git the biggest frustration for me was getting half done code plopped into the repo, and trying to do a build, but couldn't, because the guy did half the work and said "I'm busy now, I'll finish it in a week". So I'd end up having to scrap it(which would annoy the guy), or take the time to finish and completely integrate it.

I guess from my perspective, keep the half ass stuff locally, send the good stuff over the wire.

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Don't be a slave to your tools.

The goal of a VCS is to enable you to do your job. Your job is not to keep a pristine local repository, your job is to write code. If committing early and often locally lets you work better, do it.

You shouldn't, however, push broken code upstream.

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My main experience lies with Mercurial, although I also use git sporadically.

Mercurial doesn't disallow you to commit unresolved files, it just discourages you. Same deal with pushing before pulling changes that you don't have.

All you need to do in Mercurial is, once you have the files the way you want to commit them:

hg resolve --mark --all
hg commit -m "I'm such a rebel"

--mark will... mark files as resolved without prompting you with the merge tool. --all will take care of selecting all files mark with conflicts.

If you want to push without pulling (and consequently having to merge other's changes) do like a Jedi:

hg push --force

Next guy who pulls will get +1 head (no pun intended)

I'm sure there is a way to do the same stuff with Git (although it probably is more convoluted).

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