Backing out a merge - consequences?

[Matt Mackall]

On Thu, 2014-07-10 at 14:40 -0400, Tom Udale wrote:
> Hi Matt,
> 
> >> We have a situation where a feature branch was inappropriately merged
> >> (and pushed, of course) into the default branch.
> >>
> >> I have on my local machine run
> >>
> >> hg backout -r 1234 --parent 1233
> >>
> >> which has dutifully put my local repo back at the state of rev 1233.
> >
> > The "backing out a merge" behavior was so broken, we deprecated it in
> > 2011. Don't use it.
> >
> > While it has the desired effect on the _contents_ of your changeset, it
> > can't unrecord that a merge happened in your history graph. Since the
> > merge is still there, but with a "null" result, future merges will be
> > confused and dutifully undo the same changes.
> 
> I had a feeling something like this was afoot.
> 
> 
> 
> > To fix it, you need to commit a correct merge above the backed-out merge
> > and force all users to update past it so that future merges won't be
> > confused.
> 
> This is what I have:
> 
>    o           1234 default - "premature merge w\feature"
>    |\
>    o \         1233 default
>    |  \
>    |   o       1232 feature
>    |   |
> 
> 
> If I understand what you are suggesting, you are saying to do something 
> like this:
> 
> 
> |   o           1236 default - tag "DO NOT Use Me!!"
> o   |           1235 default - tag "Use Me!!"
>   \  |
>    \ o           1234 default - "premature merge w\feature"
>     \|\
>      o \         1233 default
>      |  \
>      |   o       1232 feature
>      |   |
> 
> 
> And push with a -f to handle the creation of two heads on default.
> 
> Then much later when we _actually_ do want to merge the feature branch 
> in, we can merge the two default branches together and then merge in the 
> new feature branch changesets like so:
> 
> 
> 
> o               1401 default "merge in rest of feature"
> |\
> | \
> |  \
> |   \
> |    \
> o     \         1400 default "merge in old premature merge".
> |\     \
> | \     o       1399 feature "final feature fixes"
> |  \    |
> ~~~~~~~~~
> 
> ...other changes
> 
> ~~~~~~~~~
> |   o   |       1236 default - tag "DO NOT Use Me!!"
> o   |   |       1235 default - tag "Use Me!!"
>   \  |   |
>    \ o   |       1234 default - "premature merge w\feature"
>     \|\  |
>      o \ |       1233 default
>      |  \|
>      |   o       1232 feature
>      |   |

You might want to instead mark the "do not use" branch head as closed
with:

hg commit --close-branch -m "do not use"

..which will avoid the aforementioned "confused merge" in the future.


-- 
Mathematics is the supreme nostalgia of our time.