How does Github for Mac synchronize?

Question

According to the Github for Mac blog announcement,

Once you're ready to share your commits, or pull in remote commits — just press the Sync Branch button. We'll perform a smarter version of pull --rebase && push that reduces merge commits but doesn't rewrite your merges.

What is "a smarter version" of pull --rebase && push? What exactly are they doing? Is it possible to do this "smarter" thing on the command line?

Answer 1

The blog post "Rebasing Merge Commits in Git" (from Glen Maddern) illustrates the danger of a git pull --rebase when you have made local merges:

If you do a git pull --rebase of master on top of origin/master now, you would delete your local merge.
See the next picture after the rebase: no more merge commit.

This is bad for a whole lot of reasons.

• For one, the feature commits are actually duplicated, when really I only wanted to rebase the merge. If you later merge the feature branch in again, both commits will be in the history of master.
• And origin/feature, which supposed to be finished and in master, is left dangling.
  Unlike the awesome history that you get from following a good branching/merging model, you’ve actually got misleading history.
  For example, if someone looks at the branches on origin, it’ll appear that origin/feature hasn’t been merged into master, even though it has! Which can cause all kinds of problems if that person then does a deploy. It’s just bad news all round.

That is what Github for (Mac|Windows) would detect and avoid.

If you didn't detect it in time, the same blog post mentions the following recover:

[master] git reset --hard origin/master
HEAD is now at 9f3e34d sneaky extra commit
[master] git merge --no-ff feature

Actual Solution:

You can achieve the desired result:

Instead of using git pull --rebase, use a git fetch origin and git rebase -p origin/master

I suppose the "smarter version" of pull --rebase is that combination of "fetch + rebase preserving the merge".

Glen also proposes the following aliases to counter the fact that this sequence of command would no longer use the tracking information associated to a local branch.

function git_current_branch() {
  git symbolic-ref HEAD 2> /dev/null | sed -e 's/refs\/heads\///'
}

alias gpthis='git push origin HEAD:$(git_current_branch)'
alias grb='git rebase -p'
alias gup='git fetch origin && grb origin/$(git_current_branch)'

Jason Weathered posted a "http://jasoncodes.com/posts/gup-git-rebase", but now refers to git-up, from Aanand Prasad.

Answer 2

Although @VonC’s answer is pretty darn useful, I wanted to mention of newer legit. Basically what legit does is a smart merge (much similar, if not equal, to what GitHub for Mac does):

1. Check the log for merge commits with git log --merges branch..from_branch
2. And if there’s any, then do the usual merge, otherwise do a rebase

git pull --rebase doesn’t work as per @VonC’s answer, whereas git rebase --preserve-merges is kinda better, but creates duplicate commits if you have merged other branches in; so you need to decide at some point if “rebase” is actually makes sense, or should you “merge” instead and that’s what legit does precisely (again: just like GitHub for Mac).