Re: Getting an actuallt useful merge base?

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, Feb 25, 2021 at 02:40:59AM +0000, brian m. carlson wrote:
> On 2021-02-24 at 17:58:34, Michal Suchánek wrote:
> > Hello,
> > 
> > I find the results of git merge-base A B quite useless.
> > 
> > Suppose you have a repository with file sets
> > 
> > S and T
> > 
> > where S are sources which are developed in mainline and number of stable
> > versions, and feature branches, and T are build tools (such as autoconf
> > tests or whatever) that are largely independent of the source version.
> > 
> > Because of the independence of T from S T are developed in a separate
> > branch t which is merged into all branches developing S as needed.
> > 
> > Fixes to S may affect more than one version, and depending on the
> > situation it might be useful to apply fixes to S to mutiple
> > stable/feature branche at once. For that one would need a merge base of
> > the branches in question.
> > 
> > However, merge-base almost always give a commit on branch t which is the
> > merge base of files in set T and does not contain files in set S at all.
> > In other words it is merge base only for files from set T and not set S.
> > Can I get merge base that is merge base for all files that have common
> > history between two branches?
> 
> The merge base is determined by the history.  In your case, I imagine
> you have a history like this:
> 
>  A -- B -- C -- D -- E -- F -- G (S)
>         _/        _/        _/
>  H -- I -- J -- K -- L -- M -- N (T)
> 
> Here, the merge base of N and G is M, and the merge base of F and M is
> K.  Those are the most recent common ancestors, which are typically
> chosen as the merge base.
> 
> In your case, you probably want to cherry-pick a commit, or maybe rebase
> a small set of commits onto another set.  That would probably work
> better than trying to merge.  It's possible that there's something about
> this case that I'm missing where it wouldn't work properly, but it's
> definitely the approach I would try.

It's like this

T
----o----o----o----o----o----o----o----o----o----o----o----o---(t)---o----o----
     \             \     \                                      \\\
      \             \     \                                      \\\
       \             \     \                                      \\\
        \        o----o----o\̶---o---(s)---o----o----o----o----o----o\̶\̶-(a)
         \      /            \      /                                \\
S+T  o----o----o----o----o----o----o----o----o----o----o----o----o----o\̶--(b)
    /                                       /                           \
---o----o----o----o----o----o----o----o----o----o----o----o----o----o----o---(m)

So (t) is common ancestor for (a) and (b) that merge-base reports but it is
only ancestor for files in set T, and does not have files from set S at all.
The common ancestor I am insterested in is (s) which is merge base for both
sets of files.

Thanks

Michal