Backing out a merge - consequences?

[Tom Udale]

Hi Matt,

> 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.


Misha mentioned this also.  I had this same idea and rejected it because 
I thought it closed out only named branched and not individual heads. 
But of course I keep forgetting that named branches are not special, and 
in fact can have multiple heads themselves.

I tried this and it looks good.

Thanks for your help.

All the best,

Tom