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Comment by usr1106 | Hacker News Reader

> the merged commit can actually be literally anything

That cannot be stressed enough. Years ago a searched for a weird bug and could not find it. Of course I did not look at merge conflicts because "they don't really introduce anything new". But it turned out that a merge commit had introduced an arbitrary line, not related to its parents.

Also the opposite is true: You can make a conflict free merge of two correct parents and the result is broken code. It does not seem to happen very often in real life, but it's a risk one should be aware of.

I wonder how many merge commits there are in the Linux source tree that introduce something new by conflict resolution or break something existing without conflict resolution involved.

At work we are a small enough organization that we don't really merge, all our merges are fast-forward. But that means you move the risks above to rebasing.

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> You can make a conflict free merge of two correct parents and the result is broken code. It does not seem to happen very often in real life, but it's a risk one should be aware of.

Correct and I would say even more than that; in a system more than minimal complexity you need to run your tests on your branch with main merged into it before you merge your branch into main. Too often CI is set up to only test HEAD of the new branch.

Yes! It is driving me insane that this is not the default setup. If you do the merge of main into the branch yourself, there is a race condition where by the time the tests are completed and you want to merge the branch into main, main has changed again, so you have to repeat. Exactly because of that it would be nice if CI guaranteed a correct sequencing of merging and testing.

You need to use a merge queue (aka merge train). It should be completely standard at this point (though it unfortunately isn't). Doesn't help that it's a Gitlab Premium feature.

Ah, that's what a merge train is. I always found the term confusing. We pay for Premium, but we don't use merge trains. We use only fast forward merging, so I guess it's a no-op in that case.

It's not a nop. Merge queues solve the following issue:

1. Author 0 makes branch_0 and tests it in CI. It passes.

2. Author 1 makes branch_1 and tests it in CI. It passes.

3. They both merge their branches.

This can break `master`, irrespective of the kind of merging you use. Fundamentally two people can make changes that are valid (pass CI) independently but not when combined.

That kind of breakage gets more common the more different people you have working on a project. If you just have a handful of people it's relatively rare so you can just accept `master` breaking occasionally. If you have a few dozen people or more then it's pretty much a requirement.

I don't think there are any serious downsides to enabling it.

If you are limited to fast forward merging only one of your authors can merge. The second one needs to rebase and their updated MR will go through CI again, now on top of the first author's commit that has already been merged.

I think that is unique to Gitlab though. For example Azure DevOps will rebase the branch for you when completing a pull request.

Ah I see what you mean. Yeah that is roughly equivalent to a manual merge queue. It's going to fall apart on medium/large teams because you'll constantly be rebasing and rerunning CI.

Oops, that was a slip:

> Of course I did not look at merge conflicts

Should of course been "merge commits".

Sorry, too late to edit.

lots; the tongue-in-cheek name is "evil merge" but it's honestly pretty common precisely because, as you say, the mere textual merge could be a broken result.

"conflict" is a bad name because it sounds like some kind of error; it's just a place where a simple algorithm can't make a judgment call so a human has to. and it's not the only place.

the only reason an "evil merge" is "evil" is because the tooling breaks down a little around it (however! if you look at diffs for commits, nearly all tooling will show you the useful information here, e.g. in gitk it's color coded bold black)

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