[WorldMaker]

A PR is an atomic thing, but at a higher level. It's an integration point. Merge commits are a great representation of an integration point. `git log --first-parent` gives you an integration log. Without `--first-parent` you have a "conversation log" with details about how PRs were formed.

`git bisect --first-parent` lets you start with your integration points (your PR merge commits), and because presumably you have Continuous Integration and make sure integrations points build and test successfully that should be a rather quick discovery run, and then when you discover the PR that introduced the issue, you have an opportunity to drill down into the even smaller specific change inside that PR that introduced the issue.

Merge commits are a navigation tool and an integration log. It seems useful to me to prefer them over rebases and squashes.

[skydhash]

> you have an opportunity to drill down into the even smaller specific change inside that PR that introduced the issue.

But what would be the point?

Let’s say you found an issue in the first commit of the PR (assuming it’s curated and every commit can compile). But the PR is atomic, and later commits rely on the assumption made in the first one. You would need to replay the later changes as well to figure out the impact.

Squashing PR means you consider changes at an holistic level regardless of the workflow that created them. If a PR fails with a regression test, the whole thing is suspect, and I don’t really care when in the workflow it was introduced.

If I have a PR titled “Add support for flac files” that introduced a regression, I don’t really want to know if the bug is on the commit “extract sampling information” or the commit “support flac tags”, because what got released was the PR, not individual commits. Just like no one care if a typo was introduced in draft 5 or draft 8. The only things that matters is when it got published.

For me PR are releasable patches. Individual commits in them are the engineer’s workbench. Whether you want to curate the latter is up to you, as long as the PR is atomic.

[WorldMaker]

The point is your ability to find needles in a haystack increases. A PR often is bigger than a 10-line change, but is often made up of smaller 10-line changes.

The ability to drill down with a second `git bisect` run (now with a known base and end commit, and even the ability to again use `--first-parent` to ignore merges inside the PR commit range) into the original contents of the PR is the ability to automate finding your needle in a 10-line change with its own git commit message and its context in the original conversation flow in the original PR.

That's a powerful ability.

Sure, you can probably comb the complete 100 or 1000 or 10,000 line PR to find the exact lines that caused that regression, you've narrowed down already to one useful haystack, but it's nice to have an optional second layer to break your haystacks down further sometimes.

(Especially if it turns out to be a regression from a merge commit inside that PR. Accidental bad merges happen all the time. Spotting them is hard sometimes. Spotting them after a rebase/squash happened is sometimes impossible because there's no unique record of the conflict resolutions unlike with a merge commit.)

[skydhash]

> Sure, you can probably comb the complete 100 or 1000 or 10,000 line PR to find the exact lines that caused that regression, you've narrowed down already to one useful haystack

But the PR is one single atomic changes. even if it 100 or 1000 lines. This very measure makes it easy to review because there’s only one assumption change PR A (the good one) and PR B (the bad one and and also the current one).

Don’t forget that the codebase will also have several modules. With just one single patch, I can see which modules are affected and then reason where the bug may be. Using merge may not have helped as the 10 line changes in an individual commit may have been because that’s where I integrated stuff that was unused in the previous commits before the merge. That’s why an holistic view matters.

> Spotting them after a rebase/squash happened is sometimes impossible because there's no unique record of the conflict resolutions unlike with a merge commit

That’s something I never needed because the only thing that matters is codebase at state A, and codebase at state B, and the diff between those two states. Ideally, a single reason for the transition between the two.

[WorldMaker]

I've had to deep regression analysis and archeology to make sure that regressions aren't recurring or stop recurring, because regressions can recur. An engineer that maybe read the wrong advice and got too eager in using a rerere cache only for it to cache bad regressions. Another engineer that missed a memo somewhere and regularly mismerges a feature thinking that work in progress is actually legacy code. A third engineer that accidentally committed temporary code used for testing that was more obvious looking at the exact commit where it was made than the PR it was made in.

There are so many such scenarios where more information is better. If you've got a good PR tool it might save caches of those branches pre-squash some amount of time and you can do some of that sort of archeology in your PR tool, but even GitHub will sometimes garbage collect PR commits from deleted branches eventually.

The git DAG being a two-dimensional data structure is a useful tool. I find that I want to preserve as much information as possible, including using `git merge --no-ff` in additional scenarios that many use `git rebase` for because I don't know when I will need that (integration or testing or process change) information, but if I find that I need that information it is good to have it.

It's related to the same reason we don't throw out commit messages on ancient commits. In my experience, no matter how outdated that information gets, you are going to find surprising reasons to need it. Source control isn't just about recent history, even if that is most of your day-to-day needs. Sometimes you do need to revisit the past and you don't always know exactly what you will need from that past until you do need that information search.

[skydhash]

> I've had to deep regression analysis and archeology to make sure that regressions aren't recurring or stop recurring, because regressions can recur. An engineer that […] looking at the exact commit where it was made than the PR it was made in.

That mostly a staple of the merge workflows where people are crisscrossing merges all over the place. At the end you have those horrendous diffs.

A rebase (and squash) only considers the tip of the main branch (which is a working state) and add changes that bring it to the next working state. You’re always aware of the latest working model of the code because that’s the starting point of your work (not something from $days ago). There’s no bad merges in the history of the PR branch.

[WorldMaker]

I've seen some really bad trainwreck merges in rebase heavy workflows, too. (Unwinding them is awful.) Rebases create just as many merge conflicts as merge commits do [0], but rebases don't save the evidence for them. Just because the evidence was lost of them doesn't mean the merge conflict markers were never there.

Any time you integrate two branches, no matter how long running or short running, you have possible merge conflicts. Like I said, I prefer keeping that integration log as a tangible source control artifact. I understand how many people don't care for it. But don't mistake it for solely an aesthetic choice. Merge conflicts are a necessary part of source control and sweeping them under the rug is one way with dealing with them, but in my opinion not exactly the healthiest way.

[0] ETA: Merge conflicts are not just a technical issue, but a communications and coordination issue. Software development is a social activity and as long as it is a social activity it creates merge conflicts.