Confused about github rebasing

[Alex Rønne Petersen]

On 15-03-2012 22:37, H. S. Teoh wrote:
> On Thu, Mar 15, 2012 at 09:51:30PM +0100, Alex Rønne Petersen wrote:
>> On 15-03-2012 21:49, H. S. Teoh wrote:
> [...]
>>> Another question. How to I repair my current history, which is all
>>> messed up now? That is, my branch has a whole bunch of commits I didn't
>>> make; how do I clean it up? Or is it easier to start from scratch? :)
>>>
>>>
>>> T
>>>
>>
>> Well, it really depends on how the history looks... How many commits
>> do you have? If it's a small number, just branch off upstream and
>> cherry-pick each commit.
> [...]
>
> OK, so I finally figured out how to repair it.
>
> Actually, I discovered that my origin/master branch was also broken
> (probably due to running the wrong git command in it in the past),
> because it had a bunch of commits from upstream that for some reason had
> different hashes (maybe the result of attempting to merge from a messed
> up branch?). This is probably why my topic branch was messed up in the
> first place, I think.
>
> So I did git reset --hard to clean up the history back to the point
> where it first starting diverging, then pulled from upstream/master
> (which I confirmed were fast-forward). Then I used `git push -f master`
> to clean up the history on github. So that takes care of master.
>
> Then in my topic branch, which had a messed up history:
>
> 	(branchpoint)---(fast-forwards from upstream)<-- master
> 	     \
> 	      \---(merged)---(merged)---...---(my changes)<-- topic
>
> I did a `git rebase --onto master (my changes)`:
>
> 	(branchpoint)---(fast-forwards from upstream)<-- master
> 	    \                                      \
> 	     \---(now unreachable commits)          \--(my changes)<-- topic
>
> Then I used git push -f to cleanup the history on github.
>
> So now everything looks OK again. Phew!
>
> I did discover an interesting thing about git though... at one point in
> the cleanup process I accidentally reset 1 more commit than I had
> intended, and that commit had no other references to it (unreachable).
> But luckily I still had its hash available, so `git merge<hash>`
> managed to restore it. I guess old unreachable commits are still kept
> until you run git gc.
>
> So that's one of the times when you *don't* want to run git gc. :-)
>
>
> T
>

See also git reflog. It's a life-saver.

-- 
- Alex