git - How rebase result may differ from result of a merge?

Question

In one of GitHub's articles I read the following:

You aren't able to automatically rebase and merge on GitHub when: Rebasing the commits is considered "unsafe", such as when a rebase is possible without merge conflicts but would produce a different result than a merge would.

It isn't clear for me how a rebase may produce a different result than a merge.

Can anyone explain how is it possible?

---

Link to the original article: https://help.github.com/articles/about-pull-request-merges/

Answer 1

Here's a construction proof of a case where rebase and merge produce different results. I assume this is the case they are talking about. Edit: There is another case that can occur, when merging branches where the side branch to be rebased has-or-merged contains one commit that will be skipped (due to patch-ID matching) during a rebase, followed by a reversion of that commit (that will not be skipped). See Changes to a file are not retained by merge, why? If I have time later I will try to add a construction proof for that example as well.

The trick is that since rebase copies commits but omits merges, we need to drop a merge whose resolution is not simple composition of its predecessors. For this merge to have had no conflicts, I think it must be an "evil merge", so this is what I put into the script.

The graph we build looks like this:

  B   <-- master
 /
A--C--E   <-- branch
    /
   /
   D   <-- br2

If you are on master (your tip commit is B) and you git merge branch, this combines the changes from diffing A-vs-B with those from diffing A-vs-E. The resulting graph is:

  B-----F   <-- master
 /     /
A--C--E   <-- branch
    /
   /
   D   <-- br2

and the contents of commit F are determined by those of A, B, and E.

If you are on branch (your tip commit is E) and you git rebase master, this copies commits C and D, in some order (it's not clear which). It completely omits commit E. The resulting graph is:

  B   <-- master
 / 
A   C'-D'   <-- branch
 
  D   <-- br2

(the original C and E are only available through reflogs and ORIG_HEAD). Moving master in a fast-forward fashion, the tip of master becomes commit D'. The contents of commit D' are determined by adding the changes extracted from C and D to B.

Since we used an "evil merge" to make changes in E that appear in neither C nor D, those changes vanish.

Here is the script that creates the problem (note, it makes a temporary directory tt that it leaves in the current directory).

#! /bin/sh

fatal() {
    echo fatal: "$@" 1>&2; exit 1
}

[ -e tt ] && fatal tt already exists

mkdir tt && cd tt && git init -q || fatal failed to create tt repo

echo README > README && git add README && git commit -q -m A || fatal A
git branch branch || fatal unable to make branch
echo for master > bfile && git add bfile && git commit -q -m B || fatal B

git checkout -q -b br2 branch || fatal checkout -b br2 branch
echo file for C > cfile && git add cfile && git commit -q -m C || fatal C
git checkout -q branch || fatal checkout branch
echo file for D > dfile && git add dfile && git commit -q -m D || fatal D
git merge -q --no-commit br2 && git rm -q -f cfile && git commit -q -m E ||
    fatal E
git branch -D br2
git checkout -q master || fatal checkout master

echo merging branch
git merge --no-edit branch || fatal merge failed
echo result is: *

echo removing merge, replacing with rebase of branch onto master
git reset -q --hard HEAD^ || fatal reset failed
git checkout -q branch || fatal switch back to master failed
git rebase master || fatal rebase failed
echo result is: *

echo removing rebase as well so you can poke around
git reset --hard ORIG_HEAD