Re: master c6f03ed: Fix a problem in url.el without GnuTLS

[David Engster <deng@randomsample.de>]

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Eli Zaretskii writes:
>> From: David Engster <deng@randomsample.de>
>> Cc: emacs-devel@gnu.org
>> Date: Sat, 13 Dec 2014 20:44:18 +0100
>> 
>> When you rebase a commit, it becomes a new one. Therefore, you can only
>> safely rebase "local" commits (meaning: commits only *you* have).
>
> If "safely" here means "while preserving the commit's sha1", then it's
> quite obvious.  But why would that matter in this scenario?  And why
> does that cause the merged versions appear as if they were not merged
> at all, even when rebase=preserve was/is used?

I think we really have to discuss this on an actual example. I've
attached a small script which will create three directories underneath a
directory called "MERGE_REBASE_TEST": 'upstream', 'ted' and 'eli'. There
are two branches, 'master' and 'stable', which Ted sets up and pushes. I
put some sleeps in there just so that the commits have different time
stamps.

Ted merges 'stable' into 'master', but does not push yet. Then Eli makes
a new commit on 'master' and pushes. This is where the script ends.

Now go to 'ted' and do 'gitk --all' to see the situation. If you try to
push with 'git push origin master', then this will fail because there's
this new commit from Eli. Now do 'git pull --rebase=preserve', and then
again do 'gitk --all'. You'll see that the 'stable' branch was rebased
onto 'origin/master'. This changed the SHA1 of those two commits on
'stable', as they now descend from a different parent. You *have* a
merge, but it is *not* a merge from origin/stable, but from a new
(unnamed) branch that was created by rebasing origin/stable.

Simply delete the MERGE_REBASE_TEST directory and run the script again
to try the alternatives for Ted. 'git pull --rebase' will do the same as
'--rebase=preserve' but will simply drop the merge commit ("flatten the
history"). Again, this will *not* merge the two commits from
origin/stable (a 'git log stable ^master' will list all commits that are
in stable but not in master).

If you simply do 'git pull', this will merge origin/master into your
tree. If you push this, everything will be OK. The above log command
will show nothing, just as it should after a succesful merge.

> I didn't say I want to have the same merge.  All I want is to have
> _some_ indication there was a merge from the other branch, including
> when that branch is a public branch.  You seem to say it's not
> possible with Git.

You can't at the same time move a branch onto a new parent (which is
what rebase does) and then merge it, so that it looks like you've merged
the original one which had another parent.

> Sorry, I still don't understand.  Which commits from what branches
> does Git merge after 'rebase=preserve'?

It merges the branch that is implicitly created by moving
'origin/emacs-24' to a newer parent.

> And how do conflicts enter this picture?  Suppose there were no
> conflicts at all during the original merge -- would the merge still
> disappear after 'rebase=preserve'?

I guess it's better to forget conflicts. I was hoping it would make
clearer how rebase works, but it just complicates things.

-David


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#!/bin/sh

mkdir MERGE_REBASE_TEST
cd MERGE_REBASE_TEST

# Create bare upstream
mkdir upstream
cd upstream
git init --bare
cd ..
# Ted clones, creates two commits on master and pushes.
git clone upstream ted
cd ted
echo "bla" > foo
git add foo
sleep 2
git commit -a -m "first commit on master"
echo "bla2" >> foo
sleep 2
git commit -a -m "second commit on master"
git push origin master
# Ted now creates branch 'stable' with two commits and pushes.
git checkout -b stable
echo "bla" > bar
git add bar
sleep 2
git commit -a -m "commit to stable 1"
echo "bla2" >> bar
sleep 2
git commit -a -m "commit to stable 2"
git push origin stable
# Ted goes back to master and merges 'origin/stable', but does NOT push.
git checkout master
sleep 2
git merge --no-ff origin/stable -m "merge stable"
cd ..
# Eli clones, creates new commit on master and pushes
git clone upstream eli
cd eli
git checkout master
echo "bla3" >> foo
sleep 2
git commit -a -m "New commit on master"
git push origin master
cd ..