merge fast forward

[Masklinn]

On 2010-05-17, at 20:13 , Faheem Mitha wrote:
> 
> 
> Hi Everyone,
> 
> An extract from chatter on #mercurial on IRC. I thought I'd post in case anyone had any thoughts about this. A short addition - actually one could include in this case all cases where merges could be handled programatically, ie.
> 
> hg merge other --fast-forward --select-everything-from-other-branch
> 
> The only cases one really needs to use the wd are ones which cannot be handled programatically, ie. which require a merge editor.
> 
>                                                       Regards, Faheem.
> 
> 12:42 < faheem_> it would be nice if there was a merge in memory option for merges you know will work without manual intervention
> 12:44 < muggs> faheem_: bypassing the working copy?
> 12:45 < muggs> something like:  hg merge OTHER --fast-forward
> 12:45 < muggs> if any file merges are required, it aborts?
> 12:49 < muggs> it's probably not easy to divorce the merge algorithm from the dirstate
> 12:49 < faheem_> muggs: are they closely linked together?
> 12:50 < muggs> it's where it resolves renames, permissions, and such
> 13:00 < faheem_> muggs: one problem with that approach is that there could
> still be conflicts with working copy
> 13:00 < faheem_> which you wouldn't know about till you tried to update to the merge
> 13:01 < faheem_> since obviously such an operation wouldn't change the wd
> 13:01 < muggs> sure, it's equivalent to doing a quick clone, merging, commiting, pulling, then deleting the clone
> 13:02 < faheem_> muggs: yes
> 13:02  * muggs wonders if you could just allocate a fake dirstate to merge with
> 13:03 < muggs> btw: I use the term fast-forward for a reason; this is the technical term for conflict-less merges.  git has special code for them

Uh… fast-forward isn't a merge without conflicts, it's a merge where there is no divergence (though it's a subset of merges with no conflicts, they're not the same thing).

Example:

          A---B---C topic
         /
    D---E master

in this case, if you merge topic into master you get

          A---B---C topic
         /         \
    D---E-----------F master

but if you merge with fast forward enabled you end up with

    D---E---A---B---C master

Note how the final graphs are entirely different.

I don't know if you can do that easily with local branches in mercurial, with remotes it's the default behavior of `hg pull`.