[Monotone-devel] Re: cherrypicking in monotone (was: interesting thread...)

[William Uther]

Nathaniel Smith wrote:

> In case people are interested, there's a nice thread going on over on
> the VCS-neutral revctrl@ mailing list about cherrypicking in
> monotone-style DAG-of-snapshot systems, DaggyFixes, and more generally
> the connection between darcs-style and monotone-style merging:
>
>   http://thread.gmane.org/gmane.comp.version-control.revctrl/159

I was hoping to test my ideas and try to implement cherry-picking in  
monotone sometime soon.  But I got distracted and wrote up http:// 
revctrl.org/OperationalTransformation instead.

Cherry-picking for monotone:

Assume we're cherry-picking the changes in B-C and merging them into  
revision D.

a) Find the ancestor you'd use for three way merge, call it A.
b) Pluck B-C and make a new branch with this change from A.  Call the  
new revision C'.  If there are any conflicts then abort and tell the  
user what conflicted - the conflicting revision is a dependency on  
what they're trying to pull across and should probably be pulled  
across too.
c) either:
    i) Modify C so it has C' as a parent, but otherwise no change.
    ii) Merge C' and C to make C''.  C'' should be identical to C.   
Auto-resolve all conflicts in favour of C (there shouldn't be any,  
but there may be if you didn't abort in b).  Any descendants of C  
should then be merged with C'' - this should always be a clean  
merge.  I am wary of doing this (merging descendants with C'')  
automatically though.
d) Merge C' with D and allow the user to resolve conflicts.

The end result is something that looks like http://venge.net/monotone/ 
wiki/DaggyFixes .  But done post-hoc.  And it records the history  
correctly so that you shouldn't get future conflicts (I think).

Now for the monotone questions...  It seems you're using *-merge for  
merging (in roster_merge.cc).  When it gets a conflict on file  
contents then fall back to resolve_merge_conflicts() (in merge.cc).   
That first tries to do three-way-merging (in diff_patch.cc), and if  
that fails calls out to a lua hook for the user to fix things.

a) I haven't traced much of monotone's three-way-merge yet.  I'm  
guessing noone has thought about this, but does anyone know how  
monotone's merge behaves relative to property TP2 of http:// 
revctrl.org/OperationalTransformation ?  (I was going to have a think  
about it, but I thought I'd ask first :)

b) As far as implementing cherry-picking, it seems that the least  
common ancestor for merge is found about line 80 of merge.cc.  Is  
that the right place to be looking?

Cheers,

Will         :-}